The anti-nutritional effects of coffee

Yes, but this effect mainly concerns certain nutrients and certain conditions of consumption. The best-documented interaction involves non-haem iron, found in particular in plant foods and some fortified products. Human studies show that coffee drunk with a meal can reduce its absorption, mainly because of its polyphenolic compounds. That does not mean, however, that habitual coffee consumption systematically causes nutritional deficiencies.

Yes, coffee can significantly reduce the absorption of non-haem iron when it is drunk with a meal. In a clinical study, a cup of coffee taken with a meal containing a hamburger reduced iron absorption by around 39%. Other experiments have confirmed a dose-dependent effect linked in particular to the polyphenols in coffee. This effect mainly concerns non-haem iron and deserves particular attention in people with high requirements or an inadequate iron status.

Yes, it is preferable to avoid taking coffee at the same time as an iron-rich meal or an iron supplement when optimising absorption matters. A trial in women with iron deficiency showed that drinking coffee reduced the absorption of an iron supplement by 54% under the conditions studied. The effect nevertheless depends on the meal, the amount of coffee and individual iron status.

Caffeine can temporarily increase the urinary excretion of calcium and magnesium, but that does not mean habitual coffee consumption automatically causes a deficiency. In a small metabolic study, caffeine led to a clear but modest rise in the urinary losses of these two minerals over 24 hours. These findings argue more for attention to overall dietary balance than for the claim that coffee systematically « depletes » magnesium or calcium stores.

The data do not allow us to state simply that coffee blocks the absorption of vitamins B1 and B6. A large observational study in more than 10,000 adults found lower circulating concentrations of vitamin B6, folate and riboflavin among heavy coffee drinkers. An association, however, demonstrates neither reduced intestinal absorption nor a deficiency caused by coffee. The specific relationship between coffee and vitamin B1 likewise calls for more clinical data.

It makes more sense to think in milligrams of caffeine than in numbers of cups, as the content varies widely with the method of preparation. For healthy adults, EFSA considers intakes of up to 400 mg of caffeine per day to raise no safety concern. As a guide, EFSA estimates around 90 mg for 200 ml of filter coffee and 80 mg for a 60 ml espresso. Individual sensitivity can nevertheless vary a great deal.

Yes. When you drink it matters as much as how much you drink in total. EFSA indicates that a single dose of around 100 mg of caffeine can alter the duration or the structure of sleep in some adults, particularly when it is consumed close to bedtime. A total intake that stays within the 400 mg per day limit can therefore still disrupt sleep when a large share of the caffeine is taken late in the day.

During pregnancy, EFSA considers that a total intake of up to 200 mg of caffeine per day, from all sources combined, raises no safety concern for the foetus according to the data assessed. It is therefore better to speak in milligrams rather than in « numbers of cups », since the amount of caffeine varies considerably with the size, the bean and the method of preparation. Tea, chocolate, energy drinks and certain medicines can also contribute to daily intake.