Obesity, or excess stored energy, is the most pressing modern health concern; however discussions on obesity are inconsistent. Obesity stems from a mismatch between energy consumption and usage. When energy is balanced, a person maintains their weight, and if it is not, they gain or lose weight. Negative balance, or weight loss, hinders reproduction, so humans evolved to avoid weight loss through resting and eating.

When a person eats, food is digested into component parts—proteins, fats, and carbohydrates. Excess fats are stored as subcutaneous or visceral fat, with subcutaneous fat sitting under the skin and visceral fat lying between abdominal organs. Carbohydrates are broken down into either glucose or fructose sugars. Glucose prevents cell death, but excess glucose is toxic, so it is regulated by insulin. Some is used for energy, some is processed into glycogen in the liver, and the rest is stored as fat. Fructose is metabolized in the liver, and excess fructose is stored in the liver or transferred into the bloodstream, both of which have health consequences.

Bodies store and withdraw energy as needed, allowing people to function when they have a negative energy balance. However, positive energy balances are now more common. Low fiber levels in processed food speeds digestion, and the excess energy is often stored as visceral fat, which can cause metabolic syndrome, a medical condition characterized by digestive and cardiovascular symptoms.