Which form of iron is best absorbed?
Haem iron from meat, fish and poultry is absorbed best. If you choose a supplement, divalent iron (ferrous sulfate) works better than trivalent iron, but have your iron status checked first because too much iron is harmful.
Haem iron, the iron found in meat, fish and poultry, is absorbed best: 15% to 35% reaches your bloodstream. The emptier your stores, the more you absorb. Non-haem iron, the form found in plant-based foods and most supplements, achieves only 2% to 20%. What you eat alongside it makes a big difference: vitamin C and meat, fish or poultry at the same meal can increase absorption up to fourfold. Tea, coffee, grains and legumes, on the other hand, strongly inhibit absorption.
If you need a supplement, pay attention to the form of iron. Look for 'ferrous sulfate' on the label: this is divalent iron and it is better absorbed than trivalent iron. In a study of people with iron deficiency, after 12 weeks 80% of the ferrous sulfate group had normal haemoglobin, compared with 50% in a group receiving a lower dose of trivalent iron. Brand does not matter. For conditions such as anaemia or fatigue caused by iron deficiency, discuss this with your doctor: in research, participants received up to 200 mg of elemental iron per day for at least 3 to 12 weeks, but a doctor determines what you personally need.
Pay attention to safety. Ferrous sulfate is considerably more toxic in cases of large overdose than complex-bound forms such as iron polymaltose. This makes safe storage especially important if you have children in the household. If you use thyroid medication such as levothyroxine, never take iron at the same time: it demonstrably reduces the effectiveness of the medication. The same applies to fluoroquinolone antibiotics, such as ciprofloxacin. In both cases, consult your doctor about timing.
Do you want iron for better immunity or sports performance? Then it is not worth your money: there is no good evidence that iron supplementation improves fitness or immunity in people whose iron levels are already adequate. Finally, too much iron is also harmful. Excessive intake can cause oxidative stress and has been linked to neurodegenerative conditions. If you have a family history of haemochromatosis, a hereditary condition in which the body stores too much iron, be especially careful. Have your iron status tested before you start supplementing.
This answer is based on several specialist reviews and one randomised study in humans, alongside some laboratory research in a worm model. The reviews on haem iron versus non-haem iron and the role of dietary components are well documented and consistent. The comparison between ferrous sulphate and ferric polymaltose in humans rests on a single randomised study; further confirmation is desirable. The toxicity data on overdose come largely from animal studies and cannot yet be directly quantified for humans.
How much iron do you need per day?
For healthy adults without a deficiency, 1 to 2 mg of iron per day through food is sufficient. When a deficiency or anaemia is confirmed, requirements are much higher; discuss that with your doctor.
Is it better not to take iron and calcium together?
For healthy people without medication, strictly separating intake is probably unnecessary, as long-term research shows no deterioration in iron levels. If you use thyroid medication or antibiotics, spacing them throughout the day is important.
Which form of intermittent fasting works best?
Alternate-day fasting has the strongest short-term evidence for weight loss, but the advantage over simply eating less is small and disappears after six months. Which approach suits you best depends on what you can sustain the longest.
Does the form of omega-3 you choose matter?
The form matters, but the meal you take omega-3 with matters at least as much: always take ethyl esters with a fatty meal, otherwise a large proportion is poorly absorbed.
Which fermented foods are the healthiest?
Yoghurt and kefir have the most human evidence, so if you want to make one choice, start there. For kimchi, kombucha and miso, human evidence is not yet available, although they show promise in the laboratory.
How much magnesium do you need per day?
Research suggests that your body reaches balance at around 165 mg per day, but official recommendations are higher and there is considerable individual variation. In any case, make sure you get enough magnesium-rich foods, as a large part of the population falls short.