Why do you get sick more often when you are on a diet?

Many diets temporarily provide too few micronutrients: vitamins, minerals and other nutrients that immune cells need to function properly. Magnesium is a concrete example of this. Without sufficient magnesium, killer T-cells, the immune cells that directly attack pathogens, cannot properly use an activating protein on their surface. As a result, they recognise infections more slowly and respond less quickly. This is not an indirect effect: the link has been demonstrated to be causal.

Another mechanism runs through your gut. Any serious change in what you eat disrupts the composition of your gut bacteria. Such a disruption goes hand in hand with lower immunity and poorer metabolism. Your gut bacteria partly train your immune system; when that balance is lost, you feel the consequences.

Caloric restriction also lowers inflammatory markers in your blood, such as CRP and certain signalling molecules that steer the immune response. That sounds positive, but the same system that suppresses chronic low-grade inflammation also governs the active defence against a new virus or bacterium. Less chronic inflammation therefore does not automatically mean better protection.

Intermittent fasting has mixed effects. It can improve blood sugar regulation and dampen certain inflammatory processes, but at the same time has time- and tissue-dependent effects on immune cells that are not unambiguously positive. Whether you become more or less ill depends on the type of fasting, how long you maintain it, and what you eat during those periods.

Some dietary patterns, such as the Mediterranean diet and certain forms of caloric restriction, appear to strengthen the gut barrier and improve the part of your immune system that learns from previous infections. However, this evidence is still preliminary, largely derived from review studies, and not yet strong enough for firm recommendations.