Energy is used for basal metabolic processes, body heat, physical activity, tissue maintenance and growth, fat deposition, and lactation. Excess energy is stored as fat. Energy in feedstuffs is contained primarily in the carbohydrate and fat fractions. Proteins can also supply energy when fed in excess or in times of severe malnutrition. Remember the old physics rule: Energy can neither be created nor destroyed, merely changed in form. Long ago, plants must have studied physics because that s exactly what they do – capture solar energy and through the process of photosynthesis, make carbohydrates.