The immune system doesn't just chase down pathogens and destroy errant cells. It is also involved in regeneration, tissue maintenance, the workings of synaptic connections in the brain, and many other processes. When the immune system runs down with age, becoming overly inflammatory and less competent. This is disruptive of many processes. The research here should be viewed in that context; if the immune system is involved in health-related metabolic adaptations to dietary intake, how does that interaction run awry with age?
Until recently, it was believed that the immune system was mostly dormant unless the body was under attack in connection with infections. However, it now turns out that the immune system most likely also plays an important role for perfectly healthy people and can affect the body's production of vital energy sources. Specifically, the immune system causes the liver of the healthy body to produce an energy source called ketone bodies. This takes place by letting the liver burn fat during fasting.
When we're fasting - that is, we haven't eaten anything for maybe half a day or a full day - we start drawing on our fat deposits, but not all of our body cells are capable of burning fat. This applies, among other things, to the brain, which instead depends on the production of ketone bodies, which the liver forms by metabolising fats. The ketone bodies thereby energise the body, allowing us to function even if we don't eat anything.
Ketone bodies are also the focal point of many popular weight loss diets focusing on cutting carbohydrates from our food, so the body begins burning fat instead. Other research also suggests that the ketone bodies may have a positive impact on, among other things, risk factors for the development of cardiovascular disease. Researchers now believe that the immune system affects the production of ketone bodies in fit and healthy individuals and given the beneficial effects of ketone bodies in various common metabolic disorders, this knowledge can hopefully also be applied to understand how the immune system is trying to keep the body in equilibrium when we're sick.