而现代饮食里，亚油酸那一队长得多。亚油酸大量存在于常见的植物籽油，以及用这些油炸出来、拌出来的加工食品里，日常摄入量远大于 ALA。于是窗口大半被亚油酸占着，排在后面的 ALA 只能零星通过。按这个机制推，低转化率里有一部分来自这台机器被占用了，不全是人体天生做不好；这台酶的活性还受基因影响，人和人差别很大。
所以你手上有两个杠杆，别混为一谈：
让队伍变短：亚油酸吃得少一些，按机制推，ALA 的转化会好一点。但这不等于该去躲植物籽油：亚油酸替代饱和脂肪时能降 ，也没有人体证据显示把 omega-6 压低对健康有好处（背景见站内 种子油）。绕过这台机器：直接吃已经是成品的 EPA 和 （深海鱼、藻油），它们根本不用过 Δ6 这一关。 注意这两个杠杆的力量并不对等。让队伍变短最多改善比例，改不了上限：就算窗口全部空出来，ALA 走到 DHA 那一段路本身仍然又长又低效。所以对确实需要 DHA 的人（孕期、完全不吃鱼的人），正确动作依然是绕过去，而不是指望把队排空。这就是核桃不能替代深海鱼或藻油的机制原因。
U.S. Department of Agriculture, Agricultural Research Service. (2024). FoodData Central: Nuts, walnuts, English (SR Legacy). Composition reference for walnuts (~65% fat, rich in the plant omega-3 ALA). fdc.nal.usda.gov/food-search?query=walnuts%20english
Burdge, G. C., & Calder, P. C. (2005). Conversion of alpha-linolenic acid to longer-chain polyunsaturated fatty acids in human adults. Reproduction, Nutrition, Development, 45(5), 581-597. Conversion of dietary ALA to EPA is limited (~5-10%) and conversion to DHA is very low (often <1%), so plant ALA cannot substitute for the EPA/DHA in fish. 10.1051/rnd:2005047
Estruch, R., Ros, E., Salas-Salvadó, J., Covas, M.-I., Corella, D., Arós, F., et al. (2018). Primary prevention of cardiovascular disease with a Mediterranean diet supplemented with extra-virgin olive oil or nuts. New England Journal of Medicine, 378(25), e34. PREDIMED RCT (~7,447 high-risk adults); a Mediterranean diet supplemented with mixed nuts (or EVOO) reduced major cardiovascular events versus a control low-fat diet. (First published 2013; retracted in 2018 over randomization issues and republished the same year with corrected data.). 7,447 adults aged 55-80 (57% women) at high cardiovascular risk; median 4.8 years. Events 3.8% (olive oil), 3.4% (nuts), 4.4% (control); adjusted HR 0.69 (0.53-0.91) and 0.72 (0.54-0.95). Results similar after omitting 1,588 participants whose assignments departed from protocol (household members enrolled without randomisation, some participants at 1 of 11 sites assigned without randomisation, inconsistent use of randomisation tables at another site) (abstract, PMID 29897866). 10.1056/NEJMoa1800389
Bao, Y., Han, J., Hu, F. B., Giovannucci, E. L., Stampfer, M. J., Willett, W. C., & Fuchs, C. S. (2013). Association of nut consumption with total and cause-specific mortality. New England Journal of Medicine, 369(21), 2001-2011. Two prospective cohorts (>118,000 people, 30-year follow-up); nut intake frequency — analyzed separately for tree nuts and peanuts — was inversely associated with total and cause-specific mortality. 10.1056/NEJMoa1307352
Estruch, R., Martínez-González, M. A., Corella, D., Salas-Salvadó, J., Fitó, M., Chiva-Blanch, G., et al. (2019). Effect of a high-fat Mediterranean diet on bodyweight and waist circumference: a prespecified secondary outcomes analysis of the PREDIMED randomised controlled trial. Lancet Diabetes & Endocrinology, 7(5), e6-e17. Replaces the authors' 2016 report, withdrawn after the randomisation deviations (866 participants, 11.6%) came to light; the estimates do not rely only on the assumption that everyone was randomised. 7,447 older adults at high cardiovascular risk, diets unrestricted in energy and no physical-activity advice, median 4.8 years: all three groups had marginally reduced bodyweight and increased waist circumference. Adjusted 5-year difference versus the low-fat-advice control: bodyweight -0.410 kg (p = 0.056) with olive oil and -0.016 kg (-0.453 to 0.421; p = 0.942) with nuts; waist -0.466 cm (p = 0.154) and -0.923 cm (-1.604 to -0.241; p = 0.008). Funders included the California Walnut Commission and nut companies (abstract, PMID 31003626). 10.1016/S2213-8587(19)30074-9
