When to take supplements: a GP says the morning has no advantage for most vitamins, but iron and coffee don't mix

Magnesium for better sleep, vitamin D for the bones, omega-3 for the heart, iron when the blood test says so. Supplements now sit on kitchen shelves like spices. And with them comes the eternal question: morning or night? GP Vicente Pascual, who works with a Spanish supplement maker, gives an answer that isn't quite what the marketing department would want: for most vitamins and minerals, the time of day isn't decisive.

Morning matters mainly for stimulants. Products with caffeine are best taken earlier in the day, because a late dose can delay sleep. For most vitamins and minerals there's no proven advantage to taking them in the morning - you can take them whenever suits your daily rhythm best. The exception is sleep products. "Products intended for sleep, such as those containing melatonin, are recommended shortly before going to bed," Pascual says. Other sleep aids depend on their ingredients and on the instructions of the manufacturer or a health professional.

What matters more than the time is what you swallow them with. Vitamin D, which is fat-soluble, and omega-3 are absorbed better with a meal that contains fat. Iron is usually absorbed better on an empty stomach, but it irritates many people's stomachs, so it can be taken with food too. Vitamin C helps iron, while calcium, coffee and tea get in its way. So an iron tablet with your morning coffee is a bad combination. For multivitamins with no special instructions, the doctor advises taking them with a main meal, "preferably breakfast, so you don't forget".

Then come the mistakes. "Natural doesn't necessarily mean harmless," Pascual warns. People often take two products containing the same nutrients without realising they're doubling the dose, or don't tell their doctor they're taking supplements while on treatment. Calcium and iron, for example, can interfere with the absorption of levothyroxine, the thyroid medication, if the doses aren't spaced apart. His advice is to check all products with a doctor or pharmacist.

Even storage has rules. Most supplements keep best in their original closed packaging, in a dry, cool place away from light and heat. The fridge, unless the box specifically says so, can do more harm than good - moisture undermines the product's stability.

And the most important part, which the doctor himself admits: a varied diet, such as the Mediterranean one, good sleep and exercise come first. Needs depend on age, diet, stage of life, symptoms, illnesses and medication, and some deficiencies need a check-up and tests first. When a doctor who works with a supplement maker tells you the plate comes first and the pill only after - it's worth listening.