Hunger Hormones
Gut Hormones and Hunger: Why You Cannot Just Eat Less
Hunger is controlled by a complex system of gut hormones — ghrelin, GLP-1, PYY, CCK. When these are dysregulated by obes...
"My metabolism has slowed down" is one of the most common explanations for age-related weight gain — and it is largely true, though the mechanism is different from what most people assume. Metabolism does not slow primarily because of some intrinsic age-dependent cellular change. It slows because of what typically happens to body composition with age: lean mass (muscle) decreases and fat mass increases.
Understanding the actual mechanism points directly to what can be done about it.
What Resting Metabolic Rate Is
Resting metabolic rate (RMR) is the number of calories the body burns at complete rest — maintaining organ function, body temperature, cell repair, and basic physiological activity. For most adults, RMR accounts for 60 to 75% of total daily calorie expenditure.
Lean mass (muscle, organs, bone) is metabolically expensive — it burns significantly more calories per kilogram at rest than fat tissue. The liver alone accounts for approximately 26% of resting metabolic rate. Skeletal muscle accounts for approximately 22% but is highly variable depending on muscle mass.
Fat tissue burns very few calories at rest — approximately 4 to 5 kcal per kilogram per day, compared to 20 to 30 kcal per kilogram per day for lean mass.
Why Lean Mass Decreases With Age
Sarcopenia — age-related muscle loss — begins in the late 30s to early 40s and progresses at approximately 1 to 2% of lean mass per year in sedentary individuals. By age 65, a sedentary person may have lost 20 to 30% of their peak lean muscle mass.
The causes include:
- Reduced anabolic hormones: Testosterone, growth hormone, and IGF-1 all decline with age, reducing the body's muscle-building stimulus
- Anabolic resistance: Muscle tissue becomes less responsive to the protein synthesis stimulus of exercise and dietary protein
- Reduced physical activity: Life changes in the 40s and 50s typically involve less physical activity, reducing the mechanical stimulus that maintains muscle
- Reduced protein intake: Appetite often decreases with age; protein intake may fall below the threshold for muscle maintenance
Each kilogram of lean mass lost reduces RMR by approximately 13 to 15 kcal per day. Losing 10 kg of lean mass over 20 years reduces RMR by 130 to 150 kcal per day — equivalent to approximately one to two kg of fat gain per year even without any increase in food intake.
The Real Metabolic Data
A 2021 study published in Science (Pontzer et al.) directly measured metabolic rates across the lifespan in over 6,400 participants. Key findings:
- Metabolic rate adjusted for lean mass is actually stable from age 20 to 60
- The decline in absolute RMR during these decades is explained by lean mass loss, not intrinsic metabolic slowdown
- A genuine age-related metabolic decline (independent of lean mass) only begins after age 60
- Women do not have meaningfully lower metabolic rates than men when lean mass is accounted for
This suggests that for people between 20 and 60, "metabolic slowdown" is actually a lean mass problem — and lean mass is largely modifiable.
What Actually Slows: Total Energy Expenditure
Beyond RMR, non-exercise activity thermogenesis (NEAT) — the calories burned through all movement that is not formal exercise (fidgeting, walking, standing, daily tasks) — also tends to decrease with age. Sedentary occupations, more driving, less incidental movement, and the cumulative effect of life becoming less physically active all reduce total daily energy expenditure.
How GLP-1 and Weight Loss Interact With Age-Related Metabolism
GLP-1 pathway support on a weight loss program produces lean mass loss alongside fat loss (approximately 30 to 40% of total weight lost). This lean mass loss further reduces RMR. In older patients (50+), where lean mass is already reduced, protecting lean mass during semaglutide treatment is particularly important.
The mitigation is consistent with the broader picture: resistance training and adequate protein (1.4 to 1.6 g/kg/day). METASLIM's physician-guided program includes guidance on preserving metabolic health during the weight loss phase.