Calculations about human biology, calories, sleep, exercise, and medicine
50% of your body swaps to new material in about three weeks — mostly water and blood. Reaching 90% takes 10–18 years, because muscle and bone are slow. An exponential tissue-renewal model calculates the timeline tissue by tissue, then counts the burgers. The non-renewable 2.8% (neurons, cardiac muscle, ocular lens) stays original forever.
That viral claim — you eat a credit card's worth of plastic every week — already has a very aggressive assumption baked in. Change it, and the lifetime total swings from 76 cards to over 4,000. One variable, a 50-fold range. Here's how the math actually works.
Over a lifetime, a human produces roughly 5,249 kg of stool — about the weight of an adult African elephant. We calculate what fraction of your daily calories exits unused, why a vegan diet scores 74% digestive efficiency while a low-fiber diet hits 93%, and what Kleiber's Law tells us about why mice produce more waste per kilogram than elephants.