If enough weight is gained due to increased body fat deposits, one may become overweight or obese, generally defined as having more body fat (adipose tissue) than is considered good for health. The Body Mass Index (BMI) measures body weight in proportion to the square of height and defines optimal, insufficient, and excessive weight based on the ratio.
Weight gain has a latency period. The effect that eating has on weight gain can vary greatly depending on the following factors: energy (calorie) density of foods, exercise regimen, amount of water intake, amount of salt contained in the food, time of day eaten, age of individual, individual’s country of origin, individual’s overall stress level, and amount of water retention in ankles/feet. Typical latency periods vary from three days to two weeks after ingestion.
Having excess adipose tissue (fat) is a common condition, especially where food supplies are plentiful and lifestyles are sedentary. As much as 64% of the United States adult population is considered either overweight or obese, and this percentage has increased over the last four decades.
A commonly asserted “rule” for weight gain or loss is based on the assumption that one pound of human fat tissue contains about 3,500 kilocalories (often simply called calories in the field of nutrition).Thus, eating 500 fewer calories than one needs per day should result in a loss of about a pound per week. Similarly, for every 3500 calories consumed above the amount one needs, a pound will be gained.
The assumption that a pound of human fat tissue represents about 3500 calories in the context of weight loss or gain is based on a review of previous observations and experiments by Max Wishnofsky published in 1958. He notes that previous research suggested that a pound of human adipose tissue is 87% fat, which equals 395 grams of fat. He further assumes that animal fat contains 9.5 calories per gram. Thus one pound of human fat tissue should contain 3750 calories. He then critically analyzes the relevant literature and applies a number of additional assumptions, including that the diet contains sufficient protein and that the person is in glycogen and nitrogen (protein) equilibrium, leading to most weight loss stemming from the catabolism of fat. He concludes that a 3500 calorie excess or deficit for a person meeting his assumptions, would lead to the gain or loss, respectively, of one pound of body weight. He notes that if the assumptions he makes are not met, a deficit of 3500 calories would not necessarily equate to a pound of weight loss.
In any case, Wishnofsky did not take into account numerous aspects of human physiology and biochemistry which refute this simple equivalence. Unfortunately, the claim has achieved the status of a rule of thumb and is repeated in numerous sources, used for diet planning by dietitians and misapplied at the population level as well.