Nutrition basicsPhytochemicalsAntioxidants
Phytochemicals ("phyto" = plant) are bioactive compounds plants make for colour, flavour and defence. Unlike vitamins and minerals, they are not essential — you won't get a deficiency disease without them — but diets rich in them are consistently linked to lower rates of heart disease and some cancers. There are thousands; the families below matter most in food.
The honest headline. The strongest evidence is for whole foods and dietary patterns rich in these compounds — not for isolated high-dose supplements, which have often disappointed or even backfired (see the beta-carotene warning below). "Eat the rainbow" is the practical takeaway, and a better summary of the science than any single extract.
Polyphenols & flavonoids (incl. bioflavonoids)
What they are: the biggest family of plant antioxidants. Flavonoids are the main subgroup — once marketed as "vitamin P" (bioflavonoids), though they are not a true vitamin. Key types and sources:
• Flavonols (quercetin) — onions, apples, kale • Flavan-3-ols / catechins (incl. EGCG) — green tea, cocoa • Anthocyanins — the blue/red/purple of berries, red cabbage • Flavanones (hesperidin, rutin) — citrus • Isoflavones — soya. Plus non-flavonoid polyphenols like the olive's hydroxytyrosol and coffee's chlorogenic acid.
Evidence — graded honestly: diets high in flavonoids track with better heart health in large observational studies. A few specific claims are strong enough to be officially authorised: cocoa flavanols (200 mg/day) to help maintain the elasticity of blood vessels (EFSA),1 and olive polyphenols to protect blood lipids from oxidation. Beyond those, most "antioxidant" marketing claims are not authorised — the benefit lives in the whole food, not a megadose.
Carotenoids
What they are: fat-soluble pigments giving the yellow, orange and red of plants. The main dietary ones:
• Beta-carotene — carrots, sweet potato, squash; a provitamin A (the body converts it to vitamin A as needed) • Lutein & zeaxanthin — leafy greens, egg yolk; concentrate in the retina and support eye health (studied in age-related macular degeneration) • Lycopene — tomatoes (more available when cooked), watermelon.
Good for: eyes (lutein/zeaxanthin), vitamin A status (beta-carotene), and they travel with the colourful vegetables a healthy diet is built on. Absorbed better with a little fat.
Important caveat: from food, carotenoids are safe and beneficial. But high-dose beta-carotene supplements were found to increase lung cancer risk in smokers in two large trials (ATBC and CARET) — a landmark example of an isolated antioxidant pill backfiring.2 Get carotenoids from your plate, not from high-dose capsules — especially if you smoke.
Glucosinolates & isothiocyanates (sulforaphane)
What they are: sulphur compounds in cruciferous vegetables — broccoli, Brussels sprouts, cabbage, kale, rocket. Chopping or chewing converts them into active isothiocyanates such as sulforaphane (richest in broccoli sprouts).
Evidence: they switch on the body's own detox/antioxidant defences and are studied for cancer prevention — mechanistically compelling and supported by population studies, though human trial proof of clinical outcomes is still limited.
Tip: overcooking destroys the enzyme that makes sulforaphane. Lightly steam, or chop raw and let it stand a few minutes before cooking, to keep more of it.
Organosulfur compounds (allicin)
What they are: the pungent sulphur compounds of the allium family — garlic, onion, leek. Crushing garlic forms allicin, its signature active compound.
Evidence: garlic has modest, real effects — meta-analyses show small reductions in blood pressure and cholesterol — but it's a helpful addition, not a treatment. Let crushed garlic rest ~10 minutes before heating to preserve allicin.
Phytoestrogens (isoflavones & lignans)
What they are: plant compounds shaped a little like the hormone estrogen, so they can bind estrogen receptors — but far more weakly than the body's own estrogen, and they favour the "beta" receptor. The main groups: isoflavones (soya beans, tofu, tempeh, edamame, red clover), lignans (flaxseed is by far the richest, plus sesame and whole grains — gut bacteria convert them to active enterolactone), and coumestans (sprouts, some beans).
Menopause: the best-studied use. Meta-analyses find soy isoflavones give a modest reduction in hot-flush frequency and severity — real, but smaller and less consistent than hormone therapy, and strongest in people whose gut bacteria make "equol" from daidzein.4
Two myths worth correcting: (1) Soy does not "feminise" men — meta-analyses show normal soy or isoflavone intake has no effect on testosterone or estrogen in men.5 (2) Soy foods do not raise breast-cancer risk — human evidence shows soy foods are neutral or modestly protective, and they're considered safe even for survivors.6 The one caution is high-dose isolated isoflavone supplements, which aren't well proven and are best avoided in hormone-sensitive cancer without medical advice.
Bottom line: whole soy foods and ground flaxseed are a safe, gentle part of the diet — a possible mild help for menopausal symptoms, not a hormone drug.
So should you take a supplement?
Usually no. The pattern across the research is consistent: phytochemical-rich foods help; isolated high-dose phytochemical pills mostly don't — and occasionally harm. That's probably because these compounds work together, in small amounts, alongside the fibre and nutrients of the whole food. The practical, evidence-based move is simply variety and colour:
Eat the rainbow. Blue/purple (berries, red cabbage), red (tomato, pepper), orange/yellow (carrot, squash, citrus), green (leafy and cruciferous veg, green tea), white (garlic, onion). Different colours = different phytochemical families. That single habit delivers the whole orchestra instead of one loud instrument.
See also: what "superfood" really means, vitamin C (often sold with bioflavonoids), and nuts.
References
- EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific opinion on cocoa flavanols and maintenance of normal endothelium-dependent vasodilation (basis of the authorised 200 mg/day claim). EFSA Journal, 2012/2014. efsa.europa.eu
- National Cancer Institute / NIH. Beta-carotene and lung cancer risk in smokers — the ATBC and CARET trials. cancer.gov — Antioxidants & Cancer Prevention
- Kapoor B, et al. Dietary phytochemicals in health and disease: mechanisms, clinical evidence, and applications — a comprehensive review. 2024. PMC11922683
- Taku K, Melby MK, Kronenberg F, Kurzer MS, Messina M. Extracted or synthesized soybean isoflavones reduce menopausal hot flash frequency and severity: systematic review and meta-analysis of randomized controlled trials. Menopause, 2012;19(7):776–90. pubmed.ncbi.nlm.nih.gov
- Reed KE, et al. Neither soy nor isoflavone intake affects male reproductive hormones: an expanded and updated meta-analysis of clinical studies. Reproductive Toxicology, 2021. pubmed.ncbi.nlm.nih.gov
- Fritz H, et al. Soy, red clover, and isoflavones and breast cancer: a systematic review. PLoS One, 2013. PMC3842968
This guide summarizes nutrition evidence for education. It is not a substitute for individual medical advice.