The one thing that is clear about micronutrient supplements is that there can't be any general answers about whether they help. There are three different variables:

- individual variations in metabolism
- nutrient density of the diet
- cofactors that affect bioavailability.

We now know a lot about how people's metabolic pathways can vary. Thousands of variations are catalogued as "inborn errors of metabolism", some diagnosable by DNA sequencing and some not, with consequences ranging from death in the womb to early dementia. None is all that common but cumulatively they add up. And you can catch nutrient malabsorption at any age - many quite common diseases can cause it. (Worldwide, tropical sprue may be the most important).

Human diets are far more variable now than before. The nutrient densities of foods can depend on the soil and fertilizer use where they're grown, the crop variety, storage and transport methods, processing, packaging and light exposure in the shop. Nutrient tables like the USDA's or McCance and Widdowson can't take account of all that.

And whether a nutrient gets through to do its thing may depend on what else you eat, when, and who you're sharing your body with. Eggs can block biotin absorption: a biotin level in the diet above the RDA may be nowhere near enough if eggs are a major protein source for you. Eat lots of cabbage (of some varieties but NOT all) and you may need extra iodine. Alcohol can block thiamine absorption badly enough to cause irreversible brain damage. Intestinal worms can eat up your pyridoxine and folate before you get a chance.

No population-wide policy can address all of this, and doctors rarely know about it in useful detail. If you can identify why your nutritional requirements might be idiosyncratic, and spot the right symptoms, you can do better.

(I take a high dose of vitamin A every two or three weeks. The reasons are straight out of a medical textbook, and I know it's worked within hours, but I haven't the faintest idea why my diet isn't providing enough - should be well above RDA).