German physiologist whose definitive measurements of gross metabolism in mammals, including humans, helped establish the study of the physiology of metabolism and laid much of the foundation for modern nutritional science.

A pupil of the German chemists Justus von Liebig and Friedrich Wöhler at the University of Munich, where he later served as professor of physiology (1863–1908), Voit became involved in experiments designed to determine the utilization and disposition of proteins, fats, and carbohydrates in animals under varying conditions.

In 1862 he began a collaboration with the German chemist Max von Pettenkofer that led to his most productive investigations. After building a “respiration chamber” capable of supporting human subjects, they proceeded to study animal metabolism during states of activity, rest, and fasting by measuring accurately the ingestion and excretion of foodstuffs, the consumption of oxygen, and the production of carbon dioxide and heat.

Through 11 years of intensive experimentation, they made the first accurate determination of human energy requirements (in terms of caloric intake), demonstrated the validity of the laws of conservation of energy in living animals, and did much to establish the concept that the basis of metabolism lies in the cells rather than in the blood. Voit also showed that an animal's oxygen requirement is the result, not the cause, of metabolism, that carbon dioxide production is proportional to the rate of muscular activity, and that the body's protein requirement is determined by the organized mass of the tissues, whereas its fat and carbohydrate requirements are determined by the amount of mechanical work performed.