脂肪
Fat/ja
栄養学、生物学、化学において、脂肪とは通常、脂肪酸のエステル、またはそのような化合物の混合物を意味し、最も一般的には生物または食物に存在するものを指す。
この用語は、特に植物油や動物の脂肪組織の主成分であるトリグリセリド（グリセロールのトリプルエステル）を指すことが多く、より狭義には、室温で固体または半固体のトリグリセリドを指す。また、この用語は、脂質-炭素、水素、または酸素から構成され、水に溶けないが非極性溶媒に溶ける生物学的に重要な物質-の同義語としてより広く使用されることもある。この意味では、トリグリセリドの他に、モノやジグリセリド、リン脂質（レシチンなど）、ステロール（コレステロールなど）、ワックス（蜜蝋など）、遊離脂肪酸など、ヒトの食事に少量しか含まれない化合物も含まれる。
脂肪は、炭水化物、タンパク質と並んで、ヒトの食事における三大大栄養素の一つであり、牛乳、バター、獣脂、ラード、塩豚、食用油などの一般的な食品の主成分である。脂肪は多くの動物にとって食物エネルギーの主要かつ濃密な供給源であり、ほとんどの生物において、エネルギー貯蔵、防水、断熱などの重要な構造的・代謝的機能を担っている。人体は、食事に含まれなければならない少数の必須脂肪酸を除いて、他の食物成分から必要な脂肪を作り出すことができる。食用脂肪はまた、いくつかのフレーバーや香り成分、および水溶性ではないビタミンのキャリアでもある。
生物学的重要性
ヒトおよび多くの動物において、脂肪はエネルギー源としての役割と、身体がすぐに必要とするエネルギー以上のエネルギーを貯蔵する役割の両方を果たす。脂肪1グラムが燃焼または代謝されると、約9食物カロリーを放出する（37kJ=8.8kcal）。
脂肪はまた、必須脂肪酸の供給源でもある。ビタミンのA、D、E、Kは脂溶性である。 つまり、脂肪と一緒に消化・吸収・運搬されるだけである。
脂肪は、健康な皮膚や毛髪を維持し、身体の器官を衝撃から絶縁し、体温を維持し、健康な細胞機能を促進する上で重要な役割を果たしている。脂肪はまた、多くの病気に対する有用な緩衝材としても機能する。化学物質であれ生物学的物質であれ、特定の物質が血流中で危険なレベルに達すると、身体はそれを新しい脂肪組織に貯蔵することで、効果的に希釈するか、少なくとも問題のある物質の平衡を保つことができる。これは、排泄、排尿、偶発的または意図的な採血、皮脂の排泄、毛髪の成長などの手段によって、原因物質が代謝または体外に排出されるまで、重要な臓器を保護するのに役立つ。
脂肪組織
動物では、脂肪組織、または脂肪組織は、長期間にわたって代謝エネルギーを貯蔵する体の手段である。脂肪細胞は食事や肝臓代謝から得た脂肪を貯蔵する。エネルギーストレス下では、これらの細胞は貯蔵された脂肪を分解し、脂肪酸とグリセロールを循環に供給する。これらの代謝活動はいくつかのホルモン（例えば、インスリン、グルカゴン、エピネフリン）によって調節されている。脂肪組織はレプチンというホルモンも分泌する。
生産と加工
油脂の生産と加工には、工業的にも、家内や家庭でも、様々な化学的・物理的技術が用いられている。それらには以下が含まれる：
代謝
膵リパーゼはエステル結合に作用し、結合を加水分解して脂肪酸を「放出」する。トリグリセリドの形では、脂質は十二指腸で吸収されない。脂肪酸、モノグリセリド（1つのグリセロールと1つの脂肪酸）、および一部のジグリセリドは、トリグリセリドが分解されると十二指腸で吸収される。
腸では、リパーゼと胆汁の分泌に続いて、トリグリセリドは脂肪分解と呼ばれる過程でモノアシルグリセロールと遊離脂肪酸に分解される。それらはその後、腸を裏打ちする吸収性の腸細胞に移動する。トリグリセリドは腸管細胞内でその断片から再構築され、コレステロールやタンパク質とともにパッケージされてカイロミクロンを形成する。これらは細胞から排泄され、リンパ系に集められ、血液に混ざる前に心臓近くの大血管に運ばれる。様々な組織がカイロミクロンを捕捉し、トリグリセリドを放出してエネルギー源として利用することができる。肝細胞はトリグリセリドを合成し、貯蔵することができる。肝細胞はトリグリセリドを合成し貯蔵することができる。身体がエネルギー源として脂肪酸を必要とするとき、グルカゴンというホルモンがホルモン感受性リパーゼによるトリグリセリドの分解を促し、遊離脂肪酸を放出する。脳は（ケトン体に変換されない限り）脂肪酸をエネルギー源として利用することができないため、トリグリセリドのグリセロール成分は、分解されるとジヒドロキシアセトンリン酸に変換され、さらにグリセルアルデヒド3リン酸に変換されることで糖新生を経てグルコースに変換され、脳の燃料となる。脳の必要量が身体の必要量を上回ることがあれば、脂肪細胞もそのために分解されることがある。
トリグリセリドは細胞膜を自由に通過することができない。血管壁にあるリポ蛋白リパーゼと呼ばれる特殊な酵素が、トリグリセリドを遊離脂肪酸とグリセロールに分解しなければならない。脂肪酸はその後、脂肪酸輸送タンパク質（FATPs）を介して細胞に取り込まれる。
トリグリセリドは、超低比重リポタンパク質（VLDL）およびカイロミクロンの主要成分として、エネルギー源および食事脂肪の輸送体として代謝において重要な役割を果たしている。これらは炭水化物（約4 kcal/gまたは17 kJ/g）の2倍以上のエネルギー（約9 kcal/gまたは38 kJ/g）を含む。
栄養と健康の側面
脂肪の最も一般的なタイプは、ヒトの食事やほとんどの生物において、トリグリセリドであり、三重のアルコールグリセロールである。H(-CHOH-)
3Hと3つの脂肪酸のエステルである。トリグリセリドの分子は、グリセロールの各-OH基と各脂肪酸のカルボキシル基HO(Od)CsのHO-部分との間の縮合反応（具体的にはエステル化）から生じ、エステル橋を形成していると説明することができる。水分子H
2Oの除去とともにsOs(O)Csを形成する。
他のあまり一般的でないタイプの脂肪にはジグリセリドやモノグリセリドがあり、エステル化がグリセロールの-OH基の2つまたは1つに限定されている。他のアルコール、例えばセチルアルコール（スペルマセチでは優勢）はグリセロールに取って代わることがある。リン脂質では、脂肪酸の1つがリン酸またはそのモノエステルで置き換えられている。 様々な量と種類の食事脂肪の有益性と危険性は多くの研究の対象であり、現在でも非常に議論の多いテーマである。
必須脂肪酸
ヒトの栄養には2種類の必須脂肪酸（EFA）がある： α-リノレン酸（オメガ3脂肪酸）とリノール酸（オメガ6脂肪酸）である。成人の体は、この2つから必要な他の脂質を合成することができる。
食事からの摂取
| Type | Processing treatment |
Saturated fatty acids |
Monounsaturated fatty acids |
Polyunsaturated fatty acids |
Smoke point | ||||
|---|---|---|---|---|---|---|---|---|---|
| Total | Oleic acid (ω-9) |
Total | α-Linolenic acid (ω-3) |
Linoleic acid (ω-6) |
ω-6:3 ratio | ||||
| Avocado | 11.6 | 70.6 | 52–66 |
13.5 | 1 | 12.5 | 12.5:1 | 250 °C (482 °F) | |
| Brazil nut | 24.8 | 32.7 | 31.3 | 42.0 | 0.1 | 41.9 | 419:1 | 208 °C (406 °F) | |
| Canola | 7.4 | 63.3 | 61.8 | 28.1 | 9.1 | 18.6 | 2:1 | 204 °C (400 °F) | |
| Coconut | 82.5 | 6.3 | 6 | 1.7 | 175 °C (347 °F) | ||||
| Corn | 12.9 | 27.6 | 27.3 | 54.7 | 1 | 58 | 58:1 | 232 °C (450 °F) | |
| Cottonseed | 25.9 | 17.8 | 19 | 51.9 | 1 | 54 | 54:1 | 216 °C (420 °F) | |
| Cottonseed | hydrogenated | 93.6 | 1.5 | 0.6 | 0.2 | 0.3 | 1.5:1 | ||
| Flaxseed/linseed | 9.0 | 18.4 | 18 | 67.8 | 53 | 13 | 0.2:1 | 107 °C (225 °F) | |
| Grape seed | 10.4 | 14.8 | 14.3 | 74.9 | 0.15 | 74.7 | very high | 216 °C (421 °F) | |
| Hemp seed | 7.0 | 9.0 | 9.0 | 82.0 | 22.0 | 54.0 | 2.5:1 | 166 °C (330 °F) | |
| High-oleic safflower oil | 7.5 | 75.2 | 75.2 | 12.8 | 0 | 12.8 | very high | 212 °C (414 °F) | |
| Olive, Extra Virgin | 13.8 | 73.0 | 71.3 | 10.5 | 0.7 | 9.8 | 14:1 | 193 °C (380 °F) | |
| Palm | 49.3 | 37.0 | 40 | 9.3 | 0.2 | 9.1 | 45.5:1 | 235 °C (455 °F) | |
| Palm | hydrogenated | 88.2 | 5.7 | 0 | |||||
| Peanut | 16.2 | 57.1 | 55.4 | 19.9 | 0.318 | 19.6 | 61.6:1 | 232 °C (450 °F) | |
| Rice bran oil | 25 | 38.4 | 38.4 | 36.6 | 2.2 | 34.4 | 15.6:1 | 232 °C (450 °F) | |
| Sesame | 14.2 | 39.7 | 39.3 | 41.7 | 0.3 | 41.3 | 138:1 | ||
| Soybean | 15.6 | 22.8 | 22.6 | 57.7 | 7 | 51 | 7.3:1 | 238 °C (460 °F) | |
| Soybean | partially hydrogenated | 14.9 | 43.0 | 42.5 | 37.6 | 2.6 | 34.9 | 13.4:1 | |
| Sunflower | 8.99 | 63.4 | 62.9 | 20.7 | 0.16 | 20.5 | 128:1 | 227 °C (440 °F) | |
| Walnut oil | unrefined | 9.1 | 22.8 | 22.2 | 63.3 | 10.4 | 52.9 | 5:1 | 160 °C (320 °F) |
飽和脂肪と不飽和脂肪の比較
食品によって、飽和脂肪酸と不飽和脂肪酸の割合は異なり、含まれる脂肪の量も異なる。牛肉や、ヨーグルト、アイスクリーム、チーズ、バターのような全脂肪乳や低脂肪乳を使った乳製品のような動物性食品は、そのほとんどが飽和脂肪酸である（食事性コレステロールを多く含むものもある）。豚肉、鶏肉、egg as food/ja卵、魚介類などの他の動物性食品はほとんどが不飽和脂肪酸である。工業化された焼き菓子は特に部分水素添加油を含む不飽和脂肪含量の高い油脂を使用することがあり、水素添加油で揚げた加工食品は飽和脂肪含量が高い。
ココナッツオイルやパーム核油などの例外もあるが、植物や魚油は一般に不飽和酸の割合が高い。不飽和脂肪を含む食品には、アボカド、ナッツ、オリーブ油、キャノーラなどの植物油がある。
食事中の飽和脂肪をシス不飽和脂肪に置き換えることで、心血管系疾患（CVD）、糖尿病、または死亡のリスクが減少することが、多くの慎重な研究によって判明している。これらの研究により、世界保健機関（WHO）を含む多くの医薬品団体や公衆衛生局が公式にこのような勧告を出すようになった。このような勧告を出している国には次のようなものがある：
- 英国
- 米国
- インド
- カナダ
- オーストラリア
- シンガポール
- ニュージーランド
- 香港
2004年のレビューでは、「特定の飽和脂肪酸摂取量の安全下限値は特定されていない」と結論づけ、今後の研究では、個人のライフスタイルや遺伝的背景の違いを背景とした飽和脂肪酸摂取量の違いの影響を重視すべきであると勧告している。
このアドバイスは、2種類の脂肪をそれぞれ悪い脂肪と良い脂肪とラベリングすることによって、しばしば単純化されすぎている。しかし、ほとんどの天然食品や伝統的な加工食品に含まれる油脂には、不飽和脂肪酸と飽和脂肪酸の両方が含まれているため、飽和脂肪酸を完全に排除することは非現実的であり、賢明でない可能性もある。例えば、ココナッツオイルやパームオイルのような飽和脂肪を多く含む食品は、発展途上国の人口の大部分にとって、安価な食事カロリーの重要な供給源となっている。
また、2010年のアメリカ栄養士会の会議では、飽和脂肪酸を避けることを一律に推奨すると、健康上の利点があるかもしれない多価不飽和脂肪酸の量を減らしたり、肥満や心臓病のリスクが高い精製炭水化物に置き換えたりすることになるのではないかという懸念が表明された。
このような理由から、例えば米国食品医薬品局では、飽和脂肪からの摂取カロリーを少なくとも10％（高リスク群では7％）、全脂肪からの摂取カロリーを平均30％（以下）とすることを推奨している。2006年には米国心臓協会（AHA）も一般的に7％の制限を推奨している。
WHO/FAOの報告書はまた、特にミリスチン酸とパルミチン酸の含有量を減らすように脂肪を置き換えることを推奨している。
地中海地域の多くの国で普及している、いわゆる地中海食は、北欧諸国の食事よりも多くの総脂肪を含むが、そのほとんどはオリーブオイルや魚、野菜、ラム肉などの特定の肉から摂取される不飽和脂肪酸（具体的には一価不飽和脂肪酸とオメガ3）の形であり、飽和脂肪酸の消費はそれに比べて最小限である。 2017年のレビューでは、地中海食が心血管疾患、がん罹患率、神経変性疾患、糖尿病、死亡率のリスクを低下させるという証拠が見つかった。2018年のレビューでは、地中海風の食事は非伝染性疾患のリスク低減など、全体的な健康状態を改善する可能性が示された。また、食事に関連する病気の社会的・経済的コストを削減する可能性もある。
このような飽和脂肪に対する否定的な見方に対して、少数の現代的なレビューが異議を唱えている。例えば、飽和脂肪をリノール酸に置き換えた場合の健康への影響について、1966年から1973年までのエビデンスを評価したところ、すべての原因、冠動脈性心疾患、心血管疾患による死亡率が増加することが判明した。これらの研究は多くの科学者によって異論があり、医学界では飽和脂肪と心血管疾患は密接に関係しているというのがコンセンサスとなっている。それでもなお、これらの矛盾した研究は、飽和脂肪酸の代わりに多価不飽和脂肪酸を摂取することの是非をめぐる論争に拍車をかけた。
心血管疾患
飽和脂肪が心血管疾患に及ぼす影響については、広範に研究されている。一般的なコンセンサスは、飽和脂肪の摂取量、血中コレステロール値、および心血管疾患の発生率の間には、強く、一貫性があり、段階的な関係があるという中程度の質の証拠があるということである。この関係は、多くの政府機関や医療機関を含め、因果関係があるものとして受け入れられている。
AHAによる2017年のレビューでは、アメリカ人の食事において飽和脂肪を多価不飽和脂肪に置き換えることで、心血管疾患のリスクを30％減らすことができると推定されている。
飽和脂肪の摂取は一般に脂質異常症-総コレステロールの高値、トリグリセリドの高値、低比重リポ蛋白（LDL、「悪玉」コレステロール）の高値、または高比重リポ蛋白（HDL、「善玉」コレステロール）の低値などの血中脂質値の異常-の危険因子と考えられている。これらのパラメータは、ある種の心血管疾患のリスク指標になると考えられている。
いくつかのメタアナリシス（過去に発表された複数の実験的研究のレビューと統合）により、飽和脂肪と血清コレステロール高値との間に有意な関係があることが確認されており、その結果、心血管疾患のリスク増大との因果関係が主張されている（いわゆる脂質仮説）。しかし、高コレステロールは多くの要因によって引き起こされる可能性がある。高LDL/HDL比のような他の指標がより予測的であることが証明されている。52カ国で行われた心筋梗塞の研究では、ApoB/ApoA1（それぞれLDLとHDLに関連）比が、すべての危険因子の中で最も強いCVDの予測因子であった。CVDには、肥満、トリグリセリド値、インスリン感受性、内皮機能、血栓形成性などが関与する経路もあるが、血中脂質プロファイルに異常がなければ、他の既知の危険因子は弱いアテローム作用しか持たないようである。異なる飽和脂肪酸は、様々な脂質レベルに対して異なる影響を及ぼす。
癌
飽和脂肪の摂取量とがんの関係を示す証拠は著しく弱く、医学的に明確なコンセンサスは得られていないようである。
- A meta-analysis published in 2003 found a significant positive relationship between saturated fat and breast cancer. However two subsequent reviews have found weak or insignificant relation, and noted the prevalence of confounding factors.
- Another review found limited evidence for a positive relationship between consuming animal fat and incidence of colorectal cancer.
- Other meta-analyses found evidence for increased risk of ovarian cancer by high consumption of saturated fat.
- Some studies have indicated that serum myristic acid and palmitic acid and dietary myristic and palmitic saturated fatty acids and serum palmitic combined with alpha-tocopherol supplementation are associated with increased risk of prostate cancer in a dose-dependent manner. These associations may, however, reflect differences in intake or metabolism of these fatty acids between the precancer cases and controls, rather than being an actual cause.
Bones
Various animal studies have indicated that the intake of saturated fat has a negative effect on the mineral density of bones. One study suggested that men may be particularly vulnerable.
Disposition and overall health
Studies have shown that substituting monounsaturated fatty acids for saturated ones is associated with increased daily physical activity and resting energy expenditure. More physical activity, less anger, and less irritability were associated with a higher-oleic acid diet than one of a palmitic acid diet.
Monounsaturated vs. polyunsaturated fat
The most common fatty acids in human diet are unsaturated or mono-unsaturated. Monounsaturated fats are found in animal flesh such as red meat, whole milk products, nuts, and high fat fruits such as olives and avocados. Olive oil is about 75% monounsaturated fat. The high oleic variety sunflower oil contains at least 70% monounsaturated fat. Canola oil and cashews are both about 58% monounsaturated fat. Tallow (beef fat) is about 50% monounsaturated fat, and lard is about 40% monounsaturated fat. Other sources include hazelnut, avocado oil, macadamia nut oil, grapeseed oil, groundnut oil (peanut oil), sesame oil, corn oil, popcorn, whole grain wheat, cereal, oatmeal, almond oil, hemp oil, and tea-oil camellia.
Polyunsaturated fatty acids can be found mostly in nuts, seeds, fish, seed oils, and oysters.
Food sources of polyunsaturated fats include:
| Food source (100g) | Polyunsaturated fat (g) |
|---|---|
| Walnuts | 47 |
| Canola oil | 34 |
| Sunflower seeds | 33 |
| Sesame seeds | 26 |
| Chia seeds | 23.7 |
| Unsalted peanuts | 16 |
| Peanut butter | 14.2 |
| Avocado oil | 13.5 |
| Olive oil | 11 |
| Safflower oil | 12.82 |
| Seaweed | 11 |
| Sardines | 5 |
| Soybeans | 7 |
| Tuna | 14 |
| Wild salmon | 17.3 |
| Whole grain wheat | 9.7 |
Insulin resistance and sensitivity
MUFAs (especially oleic acid) have been found to lower the incidence of insulin resistance; PUFAs (especially large amounts of arachidonic acid) and SFAs (such as arachidic acid) increased it. These ratios can be indexed in the phospholipids of human skeletal muscle and in other tissues as well. This relationship between dietary fats and insulin resistance is presumed secondary to the relationship between insulin resistance and inflammation, which is partially modulated by dietary fat ratios (omega−3/6/9) with both omega−3 and −9 thought to be anti-inflammatory, and omega−6 pro-inflammatory (as well as by numerous other dietary components, particularly polyphenols and exercise, with both of these anti-inflammatory). Although both pro- and anti-inflammatory types of fat are biologically necessary, fat dietary ratios in most US diets are skewed towards omega−6, with subsequent disinhibition of inflammation and potentiation of insulin resistance. This is contrary to the suggestion that polyunsaturated fats are shown to be protective against insulin resistance.
The large scale KANWU study found that increasing MUFA and decreasing SFA intake could improve insulin sensitivity, but only when the overall fat intake of the diet was low. However, some MUFAs may promote insulin resistance (like the SFAs), whereas PUFAs may protect against it.
Cancer
Levels of oleic acid along with other MUFAs in red blood cell membranes were positively associated with breast cancer risk. The saturation index (SI) of the same membranes was inversely associated with breast cancer risk. MUFAs and low SI in erythrocyte membranes are predictors of postmenopausal breast cancer. Both of these variables depend on the activity of the enzyme delta-9 desaturase (Δ9-d).
Results from observational clinical trials on PUFA intake and cancer have been inconsistent and vary by numerous factors of cancer incidence, including gender and genetic risk. Some studies have shown associations between higher intakes and/or blood levels of omega-3 PUFAs and a decreased risk of certain cancers, including breast and colorectal cancer, while other studies found no associations with cancer risk.
Pregnancy disorders
Polyunsaturated fat supplementation was found to have no effect on the incidence of pregnancy-related disorders, such as hypertension or preeclampsia, but may increase the length of gestation slightly and decreased the incidence of early premature births.
Expert panels in the United States and Europe recommend that pregnant and lactating women consume higher amounts of polyunsaturated fats than the general population to enhance the DHA status of the fetus and newborn.
"Cis fat" vs. "trans fat"
In nature, unsaturated fatty acids generally have double bonds in cis configuration (with the adjacent C–C bonds on the same side) as opposed to trans. Nevertheless, trans fatty acids (TFAs) occur in small amounts in meat and milk of ruminants (such as cattle and sheep), typically 2–5% of total fat. Natural TFAs, which include conjugated linoleic acid (CLA) and vaccenic acid, originate in the rumen of these animals. CLA has two double bonds, one in the cis configuration and one in trans, which makes it simultaneously a cis- and a trans-fatty acid.
| Food type | Trans fat content |
|---|---|
| butter | 2 to 7 g |
| whole milk | 0.07 to 0.1 g |
| animal fat | 0 to 5 g |
| ground beef | 1 g |
Concerns about trans fatty acids in human diet were raised when they were found to be an unintentional byproduct of the partial hydrogenation of vegetable and fish oils. While these trans fatty acids (popularly called "trans fats") are edible, they have been implicated in many health problems.
The hydrogenation process, invented and patented by Wilhelm Normann in 1902, made it possible to turn relatively cheap liquid fats such as whale or fish oil into more solid fats and to extend their shelf-life by preventing rancidification. (The source fat and the process were initially kept secret to avoid consumer distaste.) This process was widely adopted by the food industry in the early 1900s; first for the production of margarine, a replacement for butter and shortening, and eventually for various other fats used in snack food, packaged baked goods, and deep fried products.
Full hydrogenation of a fat or oil produces a fully saturated fat. However, hydrogenation generally was interrupted before completion, to yield a fat product with specific melting point, hardness, and other properties. Partial hydrogenation turns some of the cis double bonds into trans bonds by an isomerization reaction. The trans configuration is favored because it is the lower energy form.
This side reaction accounts for most of the trans fatty acids consumed today, by far. An analysis of some industrialized foods in 2006 found up to 30% "trans fats" in artificial shortening, 10% in breads and cake products, 8% in cookies and crackers, 4% in salty snacks, 7% in cake frostings and sweets, and 26% in margarine and other processed spreads. Another 2010 analysis however found only 0.2% of trans fats in margarine and other processed spreads. Up to 45% of the total fat in those foods containing man-made trans fats formed by partially hydrogenating plant fats may be trans fat. Baking shortenings, unless reformulated, contain around 30% trans fats compared to their total fats. High-fat dairy products such as butter contain about 4%. Margarines not reformulated to reduce trans fats may contain up to 15% trans fat by weight, but some reformulated ones are less than 1% trans fat.
High levels of TFAs have been recorded in popular "fast food" meals. An analysis of samples of McDonald's French fries collected in 2004 and 2005 found that fries served in New York City contained twice as much trans fat as in Hungary, and 28 times as much as in Denmark, where trans fats are restricted. For Kentucky Fried Chicken products, the pattern was reversed: the Hungarian product containing twice the trans fat of the New York product. Even within the United States, there was variation, with fries in New York containing 30% more trans fat than those from Atlanta.
Cardiovascular disease
Numerous studies have found that consumption of TFAs increases risk of cardiovascular disease. The Harvard School of Public Health advises that replacing TFAs and saturated fats with cis monounsaturated and polyunsaturated fats is beneficial for health.
Consuming trans fats has been shown to increase the risk of coronary artery disease in part by raising levels of low-density lipoprotein (LDL, often termed "bad cholesterol"), lowering levels of high-density lipoprotein (HDL, often termed "good cholesterol"), increasing triglycerides in the bloodstream and promoting systemic inflammation.
The primary health risk identified for trans fat consumption is an elevated risk of coronary artery disease (CAD). A 1994 study estimated that over 30,000 cardiac deaths per year in the United States are attributable to the consumption of trans fats. By 2006 upper estimates of 100,000 deaths were suggested. A comprehensive review of studies of trans fats published in 2006 in the New England Journal of Medicine reports a strong and reliable connection between trans fat consumption and CAD, concluding that "On a per-calorie basis, trans fats appear to increase the risk of CAD more than any other macronutrient, conferring a substantially increased risk at low levels of consumption (1 to 3% of total energy intake)".
The major evidence for the effect of trans fat on CAD comes from the Nurses' Health Study – a cohort study that has been following 120,000 female nurses since its inception in 1976. In this study, Hu and colleagues analyzed data from 900 coronary events from the study's population during 14 years of followup. He determined that a nurse's CAD risk roughly doubled (relative risk of 1.93, CI: 1.43 to 2.61) for each 2% increase in trans fat calories consumed (instead of carbohydrate calories). By contrast, for each 5% increase in saturated fat calories (instead of carbohydrate calories) there was a 17% increase in risk (relative risk of 1.17, CI: 0.97 to 1.41). "The replacement of saturated fat or trans unsaturated fat by cis (unhydrogenated) unsaturated fats was associated with larger reductions in risk than an isocaloric replacement by carbohydrates." Hu also reports on the benefits of reducing trans fat consumption. Replacing 2% of food energy from trans fat with non-trans unsaturated fats more than halves the risk of CAD (53%). By comparison, replacing a larger 5% of food energy from saturated fat with non-trans unsaturated fats reduces the risk of CAD by 43%.
Another study considered deaths due to CAD, with consumption of trans fats being linked to an increase in mortality, and consumption of polyunsaturated fats being linked to a decrease in mortality.
Trans fat has been found to act like saturated in raising the blood level of LDL ("bad cholesterol"); but, unlike saturated fat, it also decreases levels of HDL ("good cholesterol"). The net increase in LDL/HDL ratio with trans fat, a widely accepted indicator of risk for coronary artery disease, is approximately double that due to saturated fat. One randomized crossover study published in 2003 comparing the effect of eating a meal on blood lipids of (relatively) cis and trans-fat-rich meals showed that cholesteryl ester transfer (CET) was 28% higher after the trans meal than after the cis meal and that lipoprotein concentrations were enriched in apolipoprotein(a) after the trans meals.
The citokyne test is a potentially more reliable indicator of CAD risk, although is still being studied. A study of over 700 nurses showed that those in the highest quartile of trans fat consumption had blood levels of C-reactive protein (CRP) that were 73% higher than those in the lowest quartile.
Breast feeding
It has been established that trans fats in human breast milk fluctuate with maternal consumption of trans fat, and that the amount of trans fats in the bloodstream of breastfed infants fluctuates with the amounts found in their milk. In 1999, reported percentages of trans fats (compared to total fats) in human milk ranged from 1% in Spain, 2% in France, 4% in Germany, and 7% in Canada and the United States.
Other health risks
There are suggestions that the negative consequences of trans fat consumption go beyond the cardiovascular risk. In general, there is much less scientific consensus asserting that eating trans fat specifically increases the risk of other chronic health problems:
- Alzheimer's disease: A study published in Archives of Neurology in February 2003 suggested that the intake of both trans fats and saturated fats promotes the development of Alzheimer disease, although not confirmed in an animal model. It has been found that trans fats impaired memory and learning in middle-age rats. The brains of rats that ate trans-fats had fewer proteins critical to healthy neurological function. Inflammation in and around the hippocampus, the part of the brain responsible for learning and memory. These are the exact types of changes normally seen at the onset of Alzheimer's, but seen after six weeks, even though the rats were still young.
- Cancer: There is no scientific consensus that consuming trans fats significantly increases cancer risks across the board. The American Cancer Society states that a relationship between trans fats and cancer "has not been determined." One study has found a positive connection between trans fat and prostate cancer. However, a larger study found a correlation between trans fats and a significant decrease in high-grade prostate cancer. An increased intake of trans fatty acids may raise the risk of breast cancer by 75%, suggest the results from the French part of the European Prospective Investigation into Cancer and Nutrition.
- Diabetes: There is a growing concern that the risk of type 2 diabetes increases with trans fat consumption. However, consensus has not been reached. For example, one study found that risk is higher for those in the highest quartile of trans fat consumption. Another study has found no diabetes risk once other factors such as total fat intake and BMI were accounted for.
- Obesity: Research indicates that trans fat may increase weight gain and abdominal fat, despite a similar caloric intake. A 6-year experiment revealed that monkeys fed a trans fat diet gained 7.2% of their body weight, as compared to 1.8% for monkeys on a mono-unsaturated fat diet. Although obesity is frequently linked to trans fat in the popular media, this is generally in the context of eating too many calories; there is not a strong scientific consensus connecting trans fat and obesity, although the 6-year experiment did find such a link, concluding that "under controlled feeding conditions, long-term TFA consumption was an independent factor in weight gain. TFAs enhanced intra-abdominal deposition of fat, even in the absence of caloric excess, and were associated with insulin resistance, with evidence that there is impaired post-insulin receptor binding signal transduction."
- Infertility in women: One 2007 study found, "Each 2% increase in the intake of energy from trans unsaturated fats, as opposed to that from carbohydrates, was associated with a 73% greater risk of ovulatory infertility...".
- Major depressive disorder: Spanish researchers analysed the diets of 12,059 people over six years and found that those who ate the most trans fats had a 48 per cent higher risk of depression than those who did not eat trans fats. One mechanism may be trans-fats' substitution for docosahexaenoic acid (DHA) levels in the orbitofrontal cortex (OFC). Very high intake of trans-fatty acids (43% of total fat) in mice from 2 to 16 months of age was associated with lowered DHA levels in the brain (p=0.001). When the brains of 15 major depressive subjects who had committed suicide were examined post-mortem and compared against 27 age-matched controls, the suicidal brains were found to have 16% less (male average) to 32% less (female average) DHA in the OFC. The OFC controls reward, reward expectation, and empathy (all of which are reduced in depressive mood disorders) and regulates the limbic system.
- Behavioral irritability and aggression: a 2012 observational analysis of subjects of an earlier study found a strong relation between dietary trans fat acids and self-reported behavioral aggression and irritability, suggesting but not establishing causality.
- Diminished memory: In a 2015 article, researchers re-analyzing results from the 1999-2005 UCSD Statin Study argue that "greater dietary trans fatty acid consumption is linked to worse word memory in adults during years of high productivity, adults age <45".
- Acne: According to a 2015 study, trans fats are one of several components of Western pattern diets which promote acne, along with carbohydrates with high glycemic load such as refined sugars or refined starches, milk and dairy products, and saturated fats, while omega-3 fatty acids, which reduce acne, are deficient in Western pattern diets.
Biochemical mechanisms
The exact biochemical process by which trans fats produce specific health problems are a topic of continuing research. Intake of dietary trans fat perturbs the body's ability to metabolize essential fatty acids (EFAs, including omega-3) leading to changes in the phospholipid fatty acid composition of the arterial walls, thereby raising risk of coronary artery disease.
Trans double bonds are claimed to induce a linear conformation to the molecule, favoring its rigid packing as in plaque formation. The geometry of the cis double bond, in contrast, is claimed to create a bend in the molecule, thereby precluding rigid formations.
While the mechanisms through which trans fatty acids contribute to coronary artery disease are fairly well understood, the mechanism for their effects on diabetes is still under investigation. They may impair the metabolism of long-chain polyunsaturated fatty acids (LCPUFAs). However, maternal pregnancy trans fatty acid intake has been inversely associated with LCPUFAs levels in infants at birth thought to underlie the positive association between breastfeeding and intelligence.
Trans fats are processed by the liver differently than other fats. They may cause liver dysfunction by interfering with delta 6 desaturase, an enzyme involved in converting essential fatty acids to arachidonic acid and prostaglandins, both of which are important to the functioning of cells.
Natural "trans fats" in dairy products
Some trans fatty acids occur in natural fats and traditionally processed foods. Vaccenic acid occurs in breast milk, and some isomers of conjugated linoleic acid (CLA) are found in meat and dairy products from ruminants. Butter, for example, contains about 3% trans fat.
The U.S. National Dairy Council has asserted that the trans fats present in animal foods are of a different type than those in partially hydrogenated oils, and do not appear to exhibit the same negative effects. A review agrees with the conclusion (stating that "the sum of the current evidence suggests that the Public health implications of consuming trans fats from ruminant products are relatively limited") but cautions that this may be due to the low consumption of trans fats from animal sources compared to artificial ones.
In 2008 a meta-analysis found that all trans fats, regardless of natural or artificial origin equally raise LDL and lower HDL levels. Other studies though have shown different results when it comes to animal-based trans fats like conjugated linoleic acid (CLA). Although CLA is known for its anticancer properties, researchers have also found that the cis-9, trans-11 form of CLA can reduce the risk for cardiovascular disease and help fight inflammation.
Two Canadian studies have shown that vaccenic acid, a TFA that naturally occurs in dairy products, could be beneficial compared to hydrogenated vegetable shortening, or a mixture of pork lard and soy fat, by lowering total LDL and triglyceride levels. A study by the US Department of Agriculture showed that vaccenic acid raises both HDL and LDL cholesterol, whereas industrial trans fats only raise LDL with no beneficial effect on HDL.
Official recommendations
In light of recognized evidence and scientific agreement, nutritional authorities consider all trans fats equally harmful for health and recommend that their consumption be reduced to trace amounts. In 2003, the WHO recommended that trans fats make up no more than 0.9% of a person's diet and, in 2018, introduced a 6-step guide to eliminate industrially-produced trans-fatty acids from the global food supply.
The National Academy of Sciences (NAS) advises the U.S. and Canadian governments on nutritional science for use in public policy and product labeling programs. Their 2002 Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids contains their findings and recommendations regarding consumption of trans fat.
Their recommendations are based on two key facts. First, "trans fatty acids are not essential and provide no known benefit to human health", whether of animal or plant origin. Second, given their documented effects on the LDL/HDL ratio, the NAS concluded "that dietary trans fatty acids are more deleterious with respect to coronary artery disease than saturated fatty acids". A 2006 review published in the New England Journal of Medicine (NEJM) that states "from a nutritional standpoint, the consumption of trans fatty acids results in considerable potential harm but no apparent benefit."
Because of these facts and concerns, the NAS has concluded there is no safe level of trans fat consumption. There is no adequate level, recommended daily amount or tolerable upper limit for trans fats. This is because any incremental increase in trans fat intake increases the risk of coronary artery disease.
Despite this concern, the NAS dietary recommendations have not included eliminating trans fat from the diet. This is because trans fat is naturally present in many animal foods in trace quantities, and thus its removal from ordinary diets might introduce undesirable side effects and nutritional imbalances. The NAS has, thus, "recommended that trans fatty acid consumption be as low as possible while consuming a nutritionally adequate diet". Like the NAS, the WHO has tried to balance public health goals with a practical level of trans fat consumption, recommending in 2003 that trans fats be limited to less than 1% of overall energy intake.
Regulatory action
In the last few decades, there has been substantial amount of regulation in many countries, limiting trans fat contents of industrialized and commercial food products.
Alternatives to hydrogenation
The negative public image and strict regulations has led to interest in replacing partial hydrogenation. In fat interesterification, the fatty acids are among a mix of triglycerides. When applied to a suitable blend of oils and saturated fats, possibly followed by separation of unwanted solid or liquid triglycerides, this process could conceivably achieve results similar to those of partial hydrogenation without affecting the fatty acids themselves; in particular, without creating any new "trans fat".
Hydrogenation can be achieved with only small production of trans fat. The high-pressure methods produced margarine containing 5 to 6% trans fat. Based on current U.S. labeling requirements (see below), the manufacturer could claim the product was free of trans fat. The level of trans fat may also be altered by modification of the temperature and the length of time during hydrogenation.
One can mix oils (such as olive, soybean, and canola), water, monoglycerides, and fatty acids to form a "cooking fat" that acts the same way as trans and saturated fats.
Omega-three and omega-six fatty acids
The ω−3 fatty acids have received substantial attention. Among omega-3 fatty acids, neither long-chain nor short-chain forms were consistently associated with breast cancer risk. High levels of docosahexaenoic acid (DHA), however, the most abundant omega-3 polyunsaturated fatty acid in erythrocyte (red blood cell) membranes, were associated with a reduced risk of breast cancer. The DHA obtained through the consumption of polyunsaturated fatty acids is positively associated with cognitive and behavioral performance. In addition, DHA is vital for the grey matter structure of the human brain, as well as retinal stimulation and neurotransmission.
Interesterification
Some studies have investigated the health effects of interesterified (IE) fats, by comparing diets with IE and non-IE fats with the same overall fatty acid composition.
Several experimental studies in humans found no statistical difference on fasting blood lipids between a diet with large amounts of IE fat, having 25-40% C16:0 or C18:0 on the 2-position, and a similar diet with non-IE fat, having only 3-9% C16:0 or C18:0 on the 2-position. A negative result was obtained also in a study that compared the effects on blood cholesterol levels of an IE fat product mimicking cocoa butter and the real non-IE product.
A 2007 study funded by the Malaysian Palm Oil Board claimed that replacing natural palm oil by other interesterified or partially hydrogenated fats caused adverse health effects, such as higher LDL/HDL ratio and plasma glucose levels. However, these effects could be attributed to the higher percentage of saturated acids in the IE and partially hydrogenated fats, rather than to the IE process itself.
Role in disease
In the human body, high levels of triglycerides in the bloodstream have been linked to atherosclerosis, heart disease and stroke. However, the relative negative impact of raised levels of triglycerides compared to that of LDL:HDL ratios is as yet unknown. The risk can be partly accounted for by a strong inverse relationship between triglyceride level and HDL-cholesterol level. But the risk is also due to high triglyceride levels increasing the quantity of small, dense LDL particles.
Guidelines
The National Cholesterol Education Program has set guidelines for triglyceride levels:
| Level | Interpretation | |
|---|---|---|
| (mg/dL) | (mmol/L) | |
| < 150 | < 1.70 | Normal range – low risk |
| 150–199 | 1.70–2.25 | Slightly above normal |
| 200–499 | 2.26–5.65 | Some risk |
| 500 or higher | > 5.65 | Very high – high risk |
These levels are tested after fasting 8 to 12 hours. Triglyceride levels remain temporarily higher for a period after eating.
The AHA recommends an optimal triglyceride level of 100 mg/dL (1.1 mmol/L) or lower to improve heart health.
Reducing triglyceride levels
Weight loss and dietary modification are effective first-line lifestyle modification treatments for hypertriglyceridemia. For people with mildly or moderately high levels of triglycerides, lifestyle changes, including weight loss, moderate exercise and dietary modification, are recommended. This may include restriction of carbohydrates (specifically fructose) and fat in the diet and the consumption of omega-3 fatty acids Medications are recommended in those with high levels of triglycerides that are not corrected with the aforementioned lifestyle modifications, with fibrates being recommended first. Omega-3-carboxylic acids is another prescription drug used to treat very high levels of blood triglycerides.
The decision to treat hypertriglyceridemia with medication depends on the levels and on the presence of other risk factors for cardiovascular disease. Very high levels that would increase the risk of pancreatitis are treated with a drug from the fibrate class. Niacin and omega-3 fatty acids as well as drugs from the statin class may be used in conjunction, with statins being the main medication for moderate hypertriglyceridemia when reduction of cardiovascular risk is required.
脂質の消化と代謝
脂肪は健康な体内で分解され、その成分であるグリセロールと脂肪酸を放出する。グリセロール自体は肝臓でグルコースに変換され、エネルギー源となる。脂肪やその他の脂質は、膵臓で産生されるリパーゼと呼ばれる酵素によって体内で分解される。
多くの種類の細胞は、代謝のエネルギー源としてグルコースと脂肪酸のどちらかを使うことができる。特に心臓と骨格筋は脂肪酸を好む。長年の反対主張にもかかわらず、脂肪酸はミトコンドリアの酸化を通して脳細胞の燃料源としても使われる。
こちらも参照
- Animal fat/ja
- Monounsaturated fat/ja
- Diet and heart disease/ja
- Fatty acid synthesis/ja
- Food composition data/ja
- Western pattern diet/ja
- Oil/ja
- 脂質
