A free radical pulls the weakest hydrogen, the bis-allylic H, off a polyunsaturated fatty acid in the membrane, leaving a lipid radical.Cell membranes contain abundant polyunsaturated fatty acids () — especially , with 6 double bonds. The bis-allylic H between two double bonds is the weakest bond on the membrane; any free radical (•OH / ONOO⁻ / O₂•⁻) can easily abstract it, leaving a carbon-centred lipid radical L•.
2 · Chain peroxidation
The lipid radical takes up oxygen and grabs hydrogen from the adjacent , making a new radical, so one initiating radical can oxidise hundreds of PUFAs.L• + O₂ → lipid peroxyl radical LOO• — this new radical grabs H from adjacent , producing LOOH + a new L•, and the chain continues. One initiating radical can oxidise hundreds of PUFAs, forming end-products like 4-HNE / MDA — the common molecular signature of atherosclerosis, neurodegeneration, and retinopathy.
3 · α-tocopherol breaks chain
In the membrane, α-tocopherol hands an H to LOO• and stops the chain; vitamin C then restores it, and GSH restores the oxidised C.α-tocopherol stands in the membrane's hydrophobic layer as the membrane's only lipid-soluble antioxidant. It gives the H on its phenolic -OH to LOO• → producing LOOH (stable) + tocopheroxyl radical TO•. TO• reactivity is ~ 10⁴ × weaker than LOO• — it no longer propagates. Then aqueous-phase vitamin C donates an electron, reducing TO• back to α-tocopherol, and GSH then reduces oxidised C — forming the C·E·GSH antioxidant network.
4 · Why supplements failed
The mechanism is elegant, yet in several large randomized trials vitamin E did not reduce cardiovascular events, and some saw more heart failure, haemorrhagic stroke or prostate cancer.· HOPE (Yusuf 2000 NEJM): 9,541 high-risk adults aged 55 or over, 400 of natural-source vitamin E daily for a mean 4.5 years; the composite of heart attack, stroke and cardiovascular death was 16.2% vs 15.5%, no reduction. The extension, HOPE-TOO (Lonn 2005 JAMA), median 7 years: primary endpoint still null, all heart-failure events 1.13, heart-failure hospitalisation RR 1.21 · Heart Protection Study (2002): 20,536 adults with coronary disease, other arterial disease or diabetes, given 600 mg vitamin E + 250 mg vitamin C + 20 mg beta-carotene daily; major vascular events 22.5% vs 22.5%, stroke 5.0% vs 5.0% · PHS-II (Sesso 2008 JAMA): male physicians, 400 IU every other day; major cardiovascular events null ( 1.01), haemorrhagic stroke HR 1.74 (P = 0.04) · SELECT (Klein 2011 JAMA): 35,533 middle-aged and older men with no sign of prostate cancer, 400 IU daily; prostate cancer about 17% more common (HR 1.17), not yet significant at the 2009 interim report (Lippman 2009) · Miller 2005 (Annals) : slightly higher all-cause mortality in trials of 400 IU a day or more; the authors themselves noted that most of those high-dose trials were small and enrolled people with chronic disease, so whether it applies to healthy adults is uncertain
Why the mechanism failed to predict the outcome — several explanations, all still hypotheses:
· Antioxidant activity in a test tube is not the body's overall redox balance · Some reactive oxygen is part of normal cell signalling, and suppressing it wholesale may not help · One antioxidant cannot stand in for a whole network — but missing vitamin C to recycle it cannot be the whole story: HPS gave E and C together and still found nothing · Heart attacks and cancers have many causes, and a single-point intervention does not move them
The lesson is not that vitamin E is useless; it is that a plausible mechanism and fewer illnesses are separated by the whole human body. Food (nuts, seeds, plant oils) is enough for everyday intake.