Does taking vitamin D help against cancer?
If you already have enough vitamin D in your blood, extra supplementation probably does not help against cancer. If you have a demonstrable deficiency, have it corrected, but do not count on that significantly lowering your cancer risk.
Low blood levels of vitamin D are associated in observational research with a higher risk of various cancers, including colorectal, breast, bladder and ovarian cancer. But an association is not proof that supplementation actually helps: people with a deficiency may be less healthy for other reasons, and those confounding factors are difficult to rule out.
Large clinical trials in humans give a clearer answer, and it is a disappointing one for supplementation. A Finnish study in nearly 2,500 older adults over five years, in which participants received up to 3,200 IU per day, found no significant reduction in cancer incidence. The Women's Health Initiative, one of the largest trials ever conducted in women, likewise found no evidence that calcium plus vitamin D prevents breast or colorectal cancer in postmenopausal women.
There is, however, an important caveat to those negative results. In both large trials, participants already had, on average, sufficient vitamin D in their blood at the start. The idea is that supplementation adds little if you do not have a deficiency. One systematic review concludes that studies focusing specifically on people with a demonstrable deficiency (blood levels below roughly 40 ng/mL) do show a reduction in cancer risk and cancer mortality. But that conclusion comes from a single author and rests on a selection of studies, so the strength of that evidence is limited.
In short: if your vitamin D level is normal, extra supplementation probably offers no protection against cancer. Whether supplementation does anything in the case of a genuine deficiency has not yet been established with certainty. Have your level tested if you are unsure; if you are well below the norm, it is certainly worthwhile to address your deficiency, even though the cancer-protective effect of doing so has not yet been confirmed.
Based on multiple observational studies, two large RCTs (FinVit and WHI) and one systematic review. The observational studies show an association but not causality. The RCTs are predominantly negative, but may have had a selection bias because participants already had sufficient vitamin D.
Does fasting or intermittent fasting help prevent cancer?
There is no evidence that fasting prevents cancer in humans; the animal results are promising but contradictory. If you want to address excess weight as a way of reducing risk, intermittent fasting can be a tool for that, but do not expect a direct cancer-preventive effect.
Does eating a high-fibre diet help protect against colorectal cancer?
Eating more fibre is consistently associated with a lower risk of colorectal cancer, with whole grains having the strongest supporting evidence. Try to eat wholegrain bread, oats or legumes every day as a concrete step.
What can I do myself to lower my risk of cancer?
By not smoking, limiting alcohol, maintaining a healthy weight and exercising enough, you can demonstrably lower your risk of several common cancers, even if cancer runs in your family.
Does green tea protect against cancer?
Drinking green tea is not a proven protection against cancer: cell and animal research is promising, but studies in humans contradict each other too often to draw a firm conclusion. A few cups a day is probably harmless; concentrated supplements are less certainly so.
Can you get too much vitamin D?
Too much vitamin D is genuinely harmful, but that risk only arises at blood levels far above the recommended values. Stick to the dosage on the packaging, and unsure about your level? Have it measured by your GP.
How much vitamin D should you take per day?
For healthy adults under 75, extra vitamin D is probably not necessary, but for children, people aged 75 and older, pregnant women, and those at high risk of diabetes, supplementation is worthwhile. If you are unsure whether you belong to a risk group, it is worth discussing this with your general practitioner.