What is 90% of the human body made of?

Approximately 90% of the human body's atoms are hydrogen, though by mass, the body is roughly 60% water, with the remaining composition including proteins, fats, minerals, and carbohydrates. The answer varies depending on whether measuring by atom count, mass, or cellular composition.

By atom count, approximately 60% are hydrogen atoms and 26% are oxygen atoms (together comprising over 85%), with carbon accounting for about 11%, and nitrogen, calcium, phosphorus and other elements making up the remainder. However, measuring by mass provides different proportions since different atoms have different weights. By mass, water (H₂O) comprises about 60% of total body weight in adults—this percentage varies with age, sex, and body composition (infants are about 75% water, while elderly individuals may be 50% water due to decreased muscle mass and increased fat, which contains less water). The remaining 40% of body mass includes proteins (approximately 16%), fats (varies widely, averaging 15-20%), minerals (primarily calcium and phosphorus in bones, about 5%), and carbohydrates (less than 1%, primarily as glycogen stores). At the cellular level, the body contains approximately 37 trillion human cells plus a roughly equal number of bacterial cells (primarily in the gut), though bacterial cells contribute minimally to total mass due to their small size. Red blood cells represent the most numerous human cell type. Understanding body composition has practical implications for health—proper hydration maintains the water percentage necessary for cellular function, adequate protein intake supports tissue maintenance and repair, appropriate fat levels provide energy reserves without excessive health risks, and sufficient mineral intake ensures bone strength and numerous metabolic processes. Body composition changes with age, illness, and lifestyle, with optimal health requiring balance across all components rather than maximizing or minimizing any single element.