Since the days of the hunter-gatherer, food intake has been limited by its availability, and significant physical effort was involved in bringing it home. For the first time in our history, the society suffers from an excess of food, coupled with an increasing lack of physical activity. Thus, human will power is called for to limit intake, eat the right kind of foods, and do voluntary exercise in order to avoid obesity. Consequently, many commercial diets have proliferated, but maintaining a healthy weight and diet remains a daily struggle for many. A better understanding of the basic aspects of human diet vis-a-vis energy balance would help in this regard. Although chemical engineering science has so far been applied mainly to industrial problems to the great benefit of society, it can also inform many problems of day-to-day existence. Here, we provide an example of a simple analysis of diet and energy balance, which could help in improved understanding of personal nutrition and diet. With the use of a single fitted parameter, the model can adequately describe weight change in men, women, and in mice. (C) 2011 Elsevier Ltd. All rights reserved.