How does being underweight lead to a weakened immune system?

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Read somewhere that a BMI of under 18.5 can lead to getting ill more often due to the immune system being less strong but how does weight contribute to your body being able to fight illnesses?

In: Biology
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That depends on many factors like body composition, general fitness, age etc. But generally the less fat stores you have the less energy your body has to fight off any illnesses that are thrown your way. If your body is using up all and any nutrients just to keep itself ticking over it won’t have much energy left to ramp up the immune system when you’re attacked by a virus or whatever else.

This is a true statement in most cases. It primarily comes from the assumption that a low BMI (below 18.5 is classified as anorexic) indicates a poor nutrient statues of the individual. Because one typically is consuming less calories than their bodies daily minimum requirement they lose weight and reach this BMI. Anytime you calorie consumption is below your daily minimum needs or Basal Metabolic Rate the body begins to break down fat stores and other sources to supply vital organs with the nutrients they need. The immune system requires nutrients and as a result of a lack in nutrient supply it begins to weaken. You may produce less immune cells. These immune cells may be less functional because they can’t produce the compounds they need to perform optimally. Poor nutrients could weaken barriers that may keep infection out.

The causality is running the other direction. In general, you have to be malnourished or have some sort of illness in order to be that much underweight. The same thing that is making you underweight is compromising your immune system.