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.
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 noted that previous research suggested that a pound of human adipose tissue is 87% fat, which equals 395 grams of fat. He further assumed that animal fat contains 9.5 calories per gram. Thus one pound of human fat tissue should contain 3750 calories. He then critically analyzed the relevant literature and applied 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 concluded 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 noted that if the assumptions he made are not met, a deficit of 3500 calories would not necessarily equate to a pound of weight loss.
Wishnofsky did not take into account numerous aspects of human physiology and biochemistry which were unknown at the time. 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.
In regard to adipose tissue increases, a person generally gains weight by increasing food consumption, becoming physically inactive, or both. When energy intake exceeds energy expenditure (when the body is in positive energy balance), the body can store the excess energy as fat. However, the physiology of weight gain and loss is complex involving numerous hormones, body systems and environmental factors. Other factors beside energy balance that may contribute to gaining weight include:
A study, involving more than 12,000 people tracked over 32 years, found that social networks play a surprisingly powerful role in determining an individual's chances of gaining weight, transmitting an increased risk of becoming obese from wives to husbands, from brothers to brothers and from friends to friends. 
The human microbiota facilitates fermentation of indigestible carbohydrates to short-chain fatty acids, SCFAs, contributing to weight gain. A change in the proportion of Bacteroidetes and Firmicutes may determine host’s risk of obesity.
Sleep and stress
Lack of sufficient sleep has been suggested as a cause for weight gain or the difficulty in maintaining a healthy weight. Two hormones responsible for regulating hunger and metabolism are leptin, which inhibits appetite and increases energy expenditure, and ghrelin, which increases appetite and reduces energy expenditure. Studies have shown that chronic sleep deprivation is associated with reduced levels of leptin and elevated levels of ghrelin, which together result in increased appetite, especially for high fat and high carbohydrate foods. As a result, sleep deprivation over time may contribute to increased caloric intake and decreased self-control over food cravings, leading to weight gain.
Hormone and neurotransmitter imbalances
Weight gain is a common side-effect of certain psychiatric medications.
Pathological causes of weight gain include Cushing's syndrome, hypothyroidism, insulinoma, and craniopharyngioma. Genetic reasons can relate to Prader–Willi syndrome, Bardet–Biedl syndrome, Alström syndrome, Cohen syndrome, and Carpenter syndrome.
Excess adipose tissue can lead to medical problems; however, a round or large figure does not necessarily imply a medical problem, and is sometimes not primarily caused by adipose tissue. If too much weight is gained, serious health side-effects may follow. A large number of medical conditions have been associated with obesity. Health consequences are categorised as being the result of either increased fat mass (osteoarthritis, obstructive sleep apnea, social stigma) or increased number of fat cells (diabetes, some forms of cancer, cardiovascular disease, non-alcoholic fatty liver disease).  There are alterations in the body's response to insulin (insulin resistance), a proinflammatory state and an increased tendency to thrombosis (prothrombotic state).
In centuries past, a degree of plumpness has been seen as indicative of personal or family prosperity: "Calories were scarce, physical labor was hard, and most people were as lean as greyhounds." Only in the early 20th Century did fatness lose this appeal. The connection of fatness with financial well-being persists today in some less-developed countries. Indeed, it may be on the rise.
Despite the connotations that excess weight had in the past, it has for some time been seen as "unacceptable" in contemporary Western society. An expansive market has taken root since the mid-20th century, focusing on weight loss regimens, products and surgeries. This market has been aided by the rising number of overweight and obese citizens in the United States. Data from the CDC's National Health and Nutrition Examination Survey, indicates that the average weight of women between ages 30 and 60 has increased by 20 pounds, or 14%, since 1976. Among women who weigh 300 pounds or more, the increase was 18%.
However, some research has indicated the opposite pattern. It has been suggested that obesity among women residing in the U.S. has become more socially acceptable. According to a study published in the July issue of Economic Inquiry, this is likely because more than one-third of women aged 20 and older are obese in the United States. The study found that the average woman weighed 147 pounds in 1994, but stated that she wanted to weigh 132 pounds. By 2002, the average women weighed 153 pounds, but said that she wanted to weigh 135 pounds. "The fact that even the desired weight of women has increased suggests there is less social pressure to lose weight," the researchers noted. However, the difference between women's average weight and desired weight had increased as well.
In any case, weight gain and weight loss are still charged topics. The ever-present social stigma concerning weight gain, can have lasting and harmful effects on individuals, especially among young women. These are thought to include eating disorders and body dysmorphia.
- Katherine M. Flegal; Margaret D. Carroll, MS; Cynthia L. Ogden; Clifford L. Johnson, MSPH (2002). "Prevalence and Trends in Obesity Among US Adults, 1999–2000". JAMA. 288 (14): 1723–1727. doi:10.1001/jama.288.14.1723. PMID 12365955..
- Wishnofsky, M (1958). "Caloric equivalents of gained or lost weight". Am J Clin Nutr. 6 (5): 542–546. doi:10.1093/ajcn/6.5.542. PMID 13594881.
- Hall, Kevin; Chow, CC (June 18, 2013). "Why is the 3500 kcal per pound weight loss rule wrong?". International Journal of Obesity. 37 (12): 1614. doi:10.1038/ijo.2013.112. PMC 3859816. PMID 23774459.
- Hall, Kevin; Sacks, Gary; Chandramohan, Dhruva; Chow, Carson; Wang, Y Claire; Gortmaker, Steven; Swinburn, Boyd (2011). "Quantification of the effect of energy imbalance on bodyweight" (PDF). Lancet. 378 (9793): 826–37. doi:10.1016/s0140-6736(11)60812-x. PMC 3880593. PMID 21872751. Retrieved 9 January 2016.
- Stein, Rob (2007-07-26). "Obesity Spreads In Social Circles As Trends Do, Study Indicates". Washington Post. p. A01.
- Nicholas A. Christakis, M.D., M.P.H., and James H. Fowler (2007-07-26). "The Spread of Obesity in a Large Social Network over 32 Years". NEJM. 357 (4): 370–379. CiteSeerX 10.1.1.581.4893. doi:10.1056/NEJMsa066082. PMID 17652652.CS1 maint: Multiple names: authors list (link)
- Arora, Tulika; Sharma, Rajkumar (2011). "Fermentation Potential Of The Gut Microbiome: Implications For Energy Homeostasis And Weight Management". Nutrition Reviews. 69 (2): 99–106. doi:10.1111/j.1753-4887.2010.00365.x. PMID 21294743.
- Patel, Sanjay R.; Hu, Frank B. (January 17, 2008). "Short sleep duration and weight gain: a systematic review". Obesity (Silver Spring). 16 (3): 643–653. doi:10.1038/oby.2007.118. PMC 2723045. PMID 18239586.
- Newcomer JW (2005). "Second-generation (atypical) antipsychotics and metabolic effects: a comprehensive literature review". CNS Drugs. 19 Suppl 1: 1–93. doi:10.2165/00023210-200519001-00001. PMID 15998156.
- Haslam D, James WP (2005). "Obesity". The Lancet. 366 (9492): 1197–1209. doi:10.1016/S0140-6736(05)67483-1. PMID 16198769.
- Bray GA (2004). "Medical consequences of obesity". J. Clin. Endocrinol. Metab. 89 (6): 2583–9. doi:10.1210/jc.2004-0535. PMID 15181027.
- Natalie Angier, "Who Is Fat? It Depends on Culture." The History and Art of Being Fat. Accessed 2010.04.01.
- Fat Women: A Painter's Inspiration. Accessed 2010.04.01.
- Alex Duval Smith, Girls being force-fed for marriage as fattening farms revived. The Observer, Sunday 1 March 2009.
- Obesity Among Women In U.S. Becoming More Socially Acceptable, Study Says
- Phu Tang; Frank Heiland (2007). "Social Dynamics of Obesity". Economic Inquiry. 45 (3): 571–591. CiteSeerX 10.1.1.624.4917. doi:10.1111/j.1465-7295.2007.00025.x.
- Eisenberg, ME; Neumark-Sztainer, D; Story, M; Perry, C (2005). "The role of social norms and friends' influences on unhealthy weight-control behaviors among adolescent girls". Social Science & Medicine. 60 (6): 1165–73. doi:10.1016/j.socscimed.2004.06.055. PMID 15626514.
- Garner, DM; Garfinkel, PE (1980). "Socio-cultural factors in the development of anorexia nervosa". Psychological Medicine. 10 (4): 647–56. doi:10.1017/S0033291700054945. PMID 7208724.
- Media related to Human weight gain at Wikimedia Commons