The gut contains approximately 100 trillion bacteria — more than the total number of cells in your body — comprising somewhere between 500 and 1,000 distinct species. This microbiome influences everything from immune function and mental health to metabolic rate and inflammation.
Microbiome diversity — the number of different bacterial species — is strongly associated with health outcomes. Higher diversity correlates with lower rates of obesity, type 2 diabetes, inflammatory bowel disease and even depression. Diversity naturally declines with age, but diet is a far more powerful determinant than chronological age alone.
The gut produces approximately 90% of the body's serotonin and contains around 100 million neurons — more than the spinal cord. The gut-brain axis means that gut health directly influences mood, anxiety, stress response and cognitive function.
Basal metabolic rate (BMR) — the number of calories your body burns at rest — peaks in your mid-20s and declines at roughly 2–3% per decade thereafter. Much of this is due to gradual muscle mass loss rather than the metabolic rate of individual cells changing. Maintaining muscle mass through resistance training is the most effective way to prevent this decline.