Do antioxidant supplements make sense?
For healthy people, antioxidant supplements such as beta-carotene and vitamin E are not useful as protection against cancer or heart disease, and beta-carotene is frankly inadvisable for (ex-)smokers. The only well-supported exception is the AREDS formula for advanced macular degeneration: discuss this with your ophthalmologist.
Beta-carotene and vitamin E are the most extensively studied antioxidant supplements for the prevention of cardiovascular disease and cancer. Large randomised trials involving nearly one hundred thousand participants in total show no protective effect. For beta-carotene there is also a concrete risk: (ex-)smokers who take this supplement have an increased chance of developing lung cancer. The US Preventive Services Task Force concludes that the harms of beta-carotene outweigh the benefits, and that vitamin E produces no net benefit.
For multivitamins there is simply too little evidence to say whether they help or harm in the prevention of these diseases. That is not a recommendation, but an honest conclusion: the question has not been studied in a way that provides an answer.
There is one specific exception: people with an intermediate or advanced form of age-related macular degeneration (an eye disease in which central vision deteriorates) benefit from a fixed combination of antioxidants and zinc, known as the AREDS formula. In this group the combination leads to approximately 78 fewer cases of progression per thousand people treated. In early macular degeneration the effect is much smaller: only about 4 fewer cases per thousand. Smokers and ex-smokers should also here take the variant without beta-carotene, using lutein and zeaxanthin as replacements, to avoid the lung cancer risk.
A broad analysis of 32 meta-analyses on vitamin E found weak indications of a benefit for a handful of specific outcomes, such as the functioning of blood vessel walls and certain types of cancer. For most complaints and diseases the evidence remained weak or was absent altogether. Antioxidant supplements for fertility problems are also being studied, but the evidence is of such low quality that no conclusion can be drawn from it.
Eating fruit and vegetables does genuinely lower the risk of cancer, cardiovascular disease and premature death. But that protective effect most likely does not come simply from the antioxidants in those foods: other substances and lifestyle factors also play a role. This also explains why antioxidants from a pill do not do the same thing as a varied diet.
Claims are based on multiple systematic reviews, meta-analyses and a large multicentre RCT (PMID 11594942). Beta-carotene and vitamin E have strong indications of causality in a negative direction. AREDS evidence is of moderate strength. Fertility evidence (PMID 32851694) does not appear in the supplied study-type list and has therefore been excluded from the weighting.
Is taking extra vitamin C worthwhile?
Extra vitamin C offers little benefit for most people. Intensive athletes and people following a carnivore diet do have a good reason to supplement, and those who take it daily tend to experience slightly milder symptoms when they catch a cold.
How harmful is too little daylight or sunlight?
Too little sunlight is demonstrably harmful, especially for your bones. For broader disease risks the associations are plausible but not yet proven causes, and a daily dose of 5 to 30 minutes of sunlight is already sufficient for most people.
Are smoothies and fresh juice just as healthy as whole fruit?
Whole fruit beats fruit juice: more fibre, greater satiety and fewer free sugars. Smoothies fall somewhere in between, but are not equivalent to whole fruit either.
Is intermittent fasting worth it?
Intermittent fasting helps with weight loss and improves blood glucose and insulin, but over the longer term it does no better than simply eating less. If the fasting structure helps you stick to it, it is worth it; do not expect additional health benefits that come for free.
How many fruits and vegetables do I actually need per day?
Five portions of fruit and vegetables per day (two pieces of fruit, three serving-spoon scoops of vegetables) is the number with the strongest evidence base; eating more does not provide additional benefit, but most people do not even reach this minimum.
Should I take vitamin B12 supplements as I get older?
Getting older increases the risk of a B12 deficiency, even if you eat healthily. Supplementation of 1000 micrograms per day is safe and corrects that deficiency effectively. Not sure? Then have your methylmalonic acid (MMA) measured alongside your B12 blood level -- that is the most reliable measure.