Place · Level 3
Fat Types
不是脂肪都一样 · 饱和/不饱和/反式脂肪决定膜、脂蛋白和长期风险
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Chapter 1
Double bonds bend
Double bonds bend
Fatty acids differ by chain length and number of double bonds. Saturated fats have no double bonds and the chain stays straight; unsaturated fats have double bonds — cis double bonds in particular bend the chain.
This small shape difference decides whether a fat is solid or liquid at room temperature, and also affects cell membrane fluidity.
This small shape difference decides whether a fat is solid or liquid at room temperature, and also affects cell membrane fluidity.
Three geometries
Same 18 carbons, three completely different structural fates:Saturated (stearic acid 18:0): all single bonds, chain straight like a pencil, packs tightly, solid at room temperature (butter, beef tallow, coconut oil)Cis-monounsaturated (oleic acid 18:1): one cis double bond bends the chain by 30°, liquid at room temperature (olive oil, avocado, nuts)Cis-polyunsaturated (linoleic 18:2 / α-linolenic 18:3 / DHA 22:6): multiple cis double bonds twist the chain like a spring; stays liquid even refrigerated, but easily oxidized (fish oil, flaxseed oil, canola oil)Trans (artificial trans-18:1 elaidic): trans-configured double bond straightens the chain again, semi-solid at room temperature, but biologically behaves like saturated and can't enter normal fatty-acid metabolic pathways — the molecular root of trans fat's harm
So 'saturated vs unsaturated' is essentially about whether the chain bends: straight chains pack tightly, bent chains move more freely. Membrane fluidity, lipoprotein behavior, cardiovascular effects all flow from this geometric difference.
Chapter 2
Unsaturated fats
Unsaturated fats
Monounsaturated fats are common in olive oil, avocado, and nuts; polyunsaturated fats include omega-6 and omega-3, common in seeds, nuts, fish, and some plant oils.
The practical focus isn't 'as low-fat as possible' — it's replacing trans fats and excess saturated fat with unsaturated fats.
The practical focus isn't 'as low-fat as possible' — it's replacing trans fats and excess saturated fat with unsaturated fats.
Omega ratio
ω-6 (linoleic acid, LA) and ω-3 (ALA / EPA / DHA) are both essential fatty acids the body can't produce. The issue is the ratio:Pre-historic / hunter-gatherer diet: ω-6 : ω-3 ~1:1 to 4:1Modern Western diet: ~15–25:1. The absolute amount is high, mainly because ω-6 exploded (soybean, corn, sunflower oils) while ω-3 didn't keep up
Mechanistically, ω-6 (arachidonic acid) and ω-3 (EPA) share the same enzymes (COX, LOX) to produce eicosanoids: ω-6 produces pro-inflammatory mediators (PGE2 / LTB4), ω-3 produces anti-inflammatory mediators (PGE3 / LTB5 / resolvins). The two pathways compete for the same enzyme substrates, so adding ω-3 is more effective than just cutting ω-6 — though absolute amount is still bounded by EPA/DHA ceilings.
Avoiding extremes — a few rules:
ω-6 isn't a toxin in itself; linoleic acid usually lowers low-density lipoprotein cholesterol: The so-called 'bad cholesterol' — the higher it is, the more plaque tends to build in artery walls., and adding ω-3 doesn't require zeroing out ω-6Trans fats stacked with refined sugar are much more harmful than ω-6DHA / EPA from fish or algae oil is more reliable; ALA → EPA conversion is under 10%
So practically: 2 fatty-fish meals per week or an algae-oil supplement is more effective than blanket-avoiding seed oils.
Chapter 3
Saturated is not one switch
Saturated is not one switch
Saturated fat is common in fatty meat, butter, cheese, coconut oil, and palm oil. It does affect low-density lipoprotein cholesterol: The so-called 'bad cholesterol' — the higher it is, the more plaque tends to build in artery walls., but the risk depends on what replaces it — using whole grains, nuts, and olive oil for replacement is not the same as using refined sugar.
Nutrition judgments always have to ask: what's replacing what.
Nutrition judgments always have to ask: what's replacing what.
LDL particles
'LDL cholesterol' on your physical-exam report is in mg/dL — it measures total cholesterol within LDL particles, not the particle count. Modern evidence points to particle count (apolipoprotein B: One sits on every artery-clogging particle, so counting it counts the harmful particles directly. / LDL-P) as a better predictor of cardiovascular risk than low-density lipoprotein cholesterol: The so-called 'bad cholesterol' — the higher it is, the more plaque tends to build in artery walls..Why does this matter? Because saturated fat's effects go beyond LDL-C:
When saturated fat rises, LDL-C usually rises, and particle count rises too, but the LDL skews 'large and fluffy', relatively benignWhen refined sugar or trans fats rise, LDL-C may not move much, but particle count rises with 'small dense' LDL (sdLDL) — which more easily penetrates vessel walls
So 'my LDL-C is normal but I eat lots of sugar' isn't safe — sdLDL is usually invisible on standard physical exams, while vascular damage continues.
When possible, get an extra ApoB test — it directly counts particles, more stable than LDL-C. Some Chinese hospitals already include it; <80 mg/dL is a low-risk reference.
On 'is coconut oil a good oil?' Coconut oil is naturally saturated (~90% SFA, primarily lauric acid 12:0); short-term RCTs show it raises LDL-C more than butter. AHA 2017 explicitly does not recommend using it as a cardiovascular-healthy oil. This doesn't contradict the anthropological fact that coconut oil has been used long-term in traditional diets — the risk is relative to what it replaces, not absolute.
Chapter 4
Trans fats are the clear avoid
Trans fats are the clear avoid
Industrial trans fats come from partially hydrogenated oils — they raise low-density lipoprotein cholesterol: The so-called 'bad cholesterol' — the higher it is, the more plaque tends to build in artery walls. and lower high-density lipoprotein cholesterol: The so-called 'good cholesterol' — it helps ferry excess cholesterol back to the liver., the least-worth-keeping position in the fat world.
If the ingredient list shows 'partially hydrogenated oils', this isn't an 'eating a little is fine' kind of fat — it should be avoided as much as possible.
If the ingredient list shows 'partially hydrogenated oils', this isn't an 'eating a little is fine' kind of fat — it should be avoided as much as possible.
Global ban
Trans fats are the only food ingredient in nutrition history officially banned by multiple countries:Denmark 2003: first national ban (industrial trans fats <2% of fats)US FDA 2018: revoked 'GRAS (generally recognized as safe)' status, effectively banningWHO 2018 REPLACE program: global phase-out goal by 2023China from 2013: mandatory labeling; since 2024, partial hydrogenated oils banned in some categories
Risk data: for each additional 2% of total energy from trans fats, coronary heart disease risk rises 23% (Mozaffarian NEJM 2006), and mortality rises over 20%. This is the clearest, most consistent dose-response harm in nutrition science — sharper than any saturated fat controversy.
It may still hide in these places:
Baked goods made with partially hydrogenated palm oil (cakes, pastries, shortenings)Cheap margarineRepeatedly high-temperature frying (natural oils can produce some trans at >200°C over time)Certain heavily processed creamers / coffee whiteners
Note that naturally-occurring ruminant trans fats (vaccenic acid + CLA) from cow and sheep milk and meat are small in amount, and epidemiology hasn't shown them to cause the same harm as industrial trans fats — there's no need to avoid cheese on this account.
Chapter 5
Food matrix
Food matrix
Same fat amount, but the food matrix in nuts, fish, yogurt, chips, and pastries is completely different. Fiber, protein, sodium, sugar, and processing method all combine to influence the result.
So the more reliable framing is: eat more fat from whole foods, less from trans fats and ultra-processed high-fat combinations.
So the more reliable framing is: eat more fat from whole foods, less from trans fats and ultra-processed high-fat combinations.
Three high-fat foods
'50 g fat per 100 g' can refer to three completely different things.A. A handful of mixed nuts (50 g almonds + walnuts)
About 30 g fat, mostly MUFA + PUFA + ALAFood matrix: fiber + protein + vitamin E + magnesium + polyphenols; raw nut cell walls leave about 15% of the fat unabsorbedEpidemiology: 30 g of nuts daily lowers all-cause mortality ~20% (PREDIMED + meta-analyses)
B. A salmon fillet (150 g raw)
About 15 g fat, mostly EPA + DHA + MUFAFood matrix: high-quality protein + vitamin D + B12 + seleniumRCT: 2 fatty-fish meals per week lower cardiovascular death ~25% (Mozaffarian 2011)
C. A bag of chips (100 g)
About 35 g fat, high ω-6 seed oil plus trace trans (formed by repeated frying)Food matrix: high sodium + refined starch + nearly zero fiber + flavorings; ultra-processed (NOVA-4)Epidemiology: each 10% increase in daily energy from ultra-processed foods raises all-cause mortality ~15% (Monteiro et al.)
All three are 'high-fat', but their metabolic fates are entirely different. 'Is fat good or bad?' is the wrong question. 'What food matrix is this meal?' is the right question. So fat grams on a nutrition label have no meaning divorced from the food matrix.