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.
Bone health is the most solid link: without enough sunlight, your skin produces hardly any vitamin D. In children, this leads to rickets, a serious bone disease accompanied by growth disorders. In adults, a deficiency accelerates bone loss and increases the risk of fractures. This link is well established and is considered causal.
Multiple studies also connect low sunlight exposure to a higher risk of conditions such as certain cancers (breast, colorectal cancer), cardiovascular disease, multiple sclerosis, rheumatoid arthritis and type 1 diabetes. Estimates run to well over 800,000 additional deaths per year in the US and Europe that may be related to this. However, note that these are associations, not proven causes. The mechanisms are not yet fully understood.
Notably, vitamin D supplements do not solve this problem. Oral supplementation has not convincingly been shown to prevent the diseases associated with low sunlight exposure. This suggests that sunlight does more than simply produce vitamin D, for example through the release of nitric oxide by the skin. A pill therefore does not replace sunlight on a one-to-one basis.
There are also weaker signals pointing to a possible link with nearsightedness, Alzheimer's disease, asthma and autism. These are early, epidemiological indications for which causality is far from established. Treat these with caution.
The good news: sunscreen barely lowers your vitamin D levels in practice, even if laboratory conditions suggest otherwise. Ordinary people who use sunscreen daily maintain a normal vitamin D status. Exceptions are people with a skin condition that forces them into rigorous sun protection: they face a genuine risk of deficiency and are advised to have their levels checked and to supplement if necessary. On the other hand, people with albinism need to be considerably more careful with sunlight, as they lack the protection of melanin and face an elevated risk of skin cancer early in life.
For most people, 5 to 30 minutes of exposure per day, depending on skin type and the UV level of the season, is sufficient to produce enough vitamin D. Think of arms, legs or the face, with gradual build-up. Avoiding sunburn remains the guiding principle throughout.
Claims used are based on PMIDs 18400738, 15585788, 32918212, 32668607, 31069788, 30945275 and 31777350. Strength of evidence varies by outcome: strong for bone health, moderate for cardiovascular and autoimmune diseases, limited for the remaining associations.
Is too much zinc harmful?
Too much zinc is certainly harmful: as an adult, stay below 25 mg per day (the EFSA limit) and be cautious with high-dose zinc supplements, because even moderate over-supplementation can disrupt copper balance.
Are diet sodas and sweeteners bad for you, and if so, why?
For the occasional glass of diet soda, the risks have not yet been clearly demonstrated. If you drink large amounts every day, several large studies, especially those concerning aspartame and acesulfame-K, give reason for caution.
How many vegetables do you really need per day?
Five servings of vegetables and fruit per day offers the best protection, with non-starchy vegetables counting and juice and potatoes not. More than five servings adds nothing.
Does eating less help you live longer?
In laboratory animals the effect on lifespan is well established, but in healthy humans direct evidence is lacking. A moderate and sustainable eating pattern is wiser than extreme restriction; discuss major changes to your diet with a dietitian or doctor.
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.
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.