Weight gain is an increase in body weight. This can involve an increase in muscle mass, fat deposits, excess fluids such as water or other factors. Weight gain can be a symptom of a serious medical condition. Weight gain occurs when more energy (as calories from food and beverage consumption) is gained than the energy expended by life activities, including normal physiological processes and physical exercise. 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 height, and defines optimal, insufficient, and excessive weight based on the ratio. Having excess adipose tissue (fat) is a common condition, especially where food supplies are plentiful and lifestyles are sedentary. Overweight and obesity may increase the risk of several diseases, such as diabetes, heart disease, and some cancers, and may lead to short- and long-term health problems during pregnancy. Rates of obesity worldwide tripled from 1975 to 2016 to involve some 1.8 billion people and 39% of the world adult population. A commonly asserted "rule" (the Wishnofsky Rule, aka Wishnofsky's Rule) for weight gain or loss, is based on the research of Max Wishnofsky (December 17, 1899 – August 2, 1965), a Russian-born physician who had a medical practice in Brooklyn, New York. The Wishnofsky Rule states that one pound of human fat tissue contains about 3,500 kilocalories (often simply called calories in the field of nutrition). Wishnofsky conducted a review of previous observations and experiments on weight loss and weight gain, and stated his conclusions in a paper he published in 1958. Thus, according to the Wishnofsky Rule, 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.