The fatigue, pessimism, and lack of motivation we experience when sick is an evolved program called "sickness behavior." It's not just a passive result of the illness but an active, adaptive strategy to conserve energy, hide from predators, and focus bodily resources on fighting infection.

Life operates on a finite energy budget divided between vital functions, stress responses, and growth/maintenance/repair (GMR). Energy allocated to stress is directly diverted from GMR, meaning chronic stress actively prevents your body from healing, repairing, and growing.

Feeling energetic isn't about consuming more calories. The limiting factor is how efficiently mitochondria transform and distribute energy to different systems. This reframes the problem of fatigue from insufficient energy production to inefficient energy allocation.

The evolutionary "fight or flight" response floods the body with hormones like cortisol to handle immediate threats. This life-saving mechanism comes at a cost: it diverts resources away from non-essential functions like digestion. Chronic stress therefore leads to a chronically sacrificed and weakened gut.

The composition of proteins in blood changes so dramatically with age that it can accurately predict a person's age. Crucially, these blood-borne factors are not just passive markers; they actively influence how cells and organs function, acting as a form of internal medicine.

It's a misconception that taking in more air equals more oxygen for the body. In fact, breathing too much or too fast constricts blood vessels, cutting off circulation and reducing oxygen absorption in cells. Slower, lighter breathing increases circulation and allows the body to do more with less effort.

Over 95% of the body's serotonin originates in the gut, not the brain. Its primary role is not just mood regulation but managing gravity's physical toll by stabilizing blood pressure when standing, coordinating muscles for balance, and supporting lymphatic flow, making it a key 'gravity management molecule.'

A human born and raised in Mars's one-third gravity would likely not develop the bone density and muscular strength required to withstand Earth's gravity. The physical stress would be painful and potentially debilitating, effectively trapping them on their home planet for life.

The fatigue, apathy, and loss of appetite you feel when sick are not just passive symptoms. They are an evolved, intelligent response to conserve energy by shutting down non-essential processes (like digestion and motivation) to redirect that energy budget to fight infection.