21 - Your Body Changes - Age, Hormones, and Stages

Andoni Arroyo opens with the question almost every reader past 35 has asked: why could you eat anything before and now you can't? Before you could have a whole pizza, go to bed, and be exactly the same. A week of holiday excess left no trace. You could skip the gym for months and the body held. That doesn't work the same now. Not imagination. Not lost willpower. The body you have now isn't the body you had then. The principles in this book don't expire — energy balance, protein satiety, fiber for microbiota, insulin response to carbs stay the same at 25 and at 55. What changes is the context: muscle mass, hormones, repair speed, stress and sleep response.

Muscle leaves slowly but it leaves. Sarcopenia — progressive muscle loss with aging. From 30 on, the body loses 3-5% per decade if nothing is done. Accelerates after 60, more after 70. Between 40 and 80, a sedentary person can lose 30-40% of total muscle. Why it matters: muscle is the biggest glucose consumer at rest, the biggest contributor to BMR, the tissue regulating much of insulin sensitivity. Less muscle = lower BMR. Strength is one of the most robust predictors of longevity and quality of life in the second half of life — more than BMI, more than cholesterol.

Why sarcopenia happens: hormonal (testosterone, growth hormone, IGF-1 decline), reduced activity, anabolic resistance (older muscle needs more protein for the same synthesis). The good news: it isn't inevitable. Strength training and adequate protein slow it considerably. Studies in people 65, 70, even 80 starting resistance training show gains comparable in percentage to young adults. Strength training matters more as you age, not less. Protein 1.2-1.6 g/kg/day for active older adults — the 0.8 baseline is insufficient.

Menopause concentrates many changes for women. Estrogen has receptors throughout the body — bone, muscle, liver, adipose tissue, brain, cardiovascular. When it falls (45-55), effects show across all systems. Fat distribution shifts from gynoid toward visceral (abdominal — metabolically active, higher cardiovascular risk). Insulin sensitivity worsens. Bone mineral density: 2-3% per year loss in the first years after menopause, raising osteoporosis and fracture risk. Same principles fix it: strength and impact training maintain bone density; calcium and vitamin D essential; adequate protein protects muscle that protects against falls. HRT is a legitimate medical option to discuss with a specialist.

Cortisol — the silent cost of chronic stress. Useful acutely, harmful chronically. We live in low-intensity but permanent stress. The body doesn't distinguish "tiger" from "urgent email" — both fire the HPA axis. Chronic effects: visceral fat accumulation, increased appetite for calorie-dense food, insulin resistance, disrupted circadian rhythm. The remedy: address causes when possible, build recovery via exercise, meditation, nature, disconnection.

Sleep is the invisible pillar of nutrition. If you could pick one health habit with biggest impact on metabolism, composition, appetite and decision-making, it wouldn't be diet or exercise — it would be sleep. Van Cauter and Spiegel (2004) restricted sleep to 4 hours for two nights in healthy young people. Ghrelin up 28%, leptin down 18%, appetite up 24%, desire for calorie-dense foods up 33-45%. Just two nights. A week of 5-6 hours produces an insulin sensitivity reduction equivalent to gaining 8-10 kg of visceral fat. Growth hormone — the main anabolic stimulus — releases mainly during deep sleep. Most adults need 7-9 hours. People who "function on five" are statistically rare (under 3% have the genetic variant).

What changes. What doesn't. The body changes, the principles don't. At 25 you could ignore protein and sleep six hours; metabolism cushioned the neglects. At 45 or 55 that margin narrows. What was ignorable becomes necessity. More protein, more strength training, more attention to sleep, more stress management. The body you care for at 50 is the body you'll have at 70. Plasticity is real throughout life. The best time to start was earlier. The second-best is now.

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