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 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.
Gaining weight can cause the following effects, dependent on the variable listed above, but are generally limited to:
- Increase in body fat percentage
- Increase in muscle mass
- Increase in body hydration levels
- Increase in breast size
In more extreme cases:
- A noticeably larger stomach
- The abdomen will bulge outward and upward, creating a distended midsection
A simple formula for weight gain (or loss) by increase (or decrease) in body fat is given based on the simple fact that it takes 3,500 calories of stored energy to make one pound of fat.
where wf is your weight, in pounds, after gain/loss, wi is your weight, in pounds, before gain/loss, ei is your caloric intake during the time period in question, and eb is your caloric burn during the time period in question. The constant 3500 represents the aforementioned conversion factor: 3,500 kcal = 1 lb. of fat.
Assume for a moment that in the above formula, wi = 125, ei = 4000, and eb = 1500. Thus the equation becomes wf = 125 + 4000 - 1500⁄3500. By order of operations, we first solve inside the grouping symbol (fraction bar acts as a grouping symbol). 4000 - 1500 = 2500, so the numerator becomes 2500. The next step is to calculate multiplication and division (fraction is another form of division). 2500 ÷ 3500 = 5⁄7, so now, having solved half the equation, we are left with wf = 125 + 5⁄7. From here we add the two remaining terms on the right side of the equation, so wf = 880⁄7 = 125 5⁄7. Thus, a 125-pound person who consumes 4000 calories in a given period of time, while burning 1500 calories in same time period, gains 5⁄7 of a pound, for an ending weight of 125 5⁄7 lbs.
- Problems with above
Obviously this is purely hypothetical and excludes factors such as cortisol production and metabolic rate, but the formula provides a rough estimate of how many calories one has burned given starting weight, ending weight, and caloric intake (since caloric burn is the most difficult value to observe directly, and thus is the most likely variable to be solved for). Anecdotally, solving the above equation for eb produces the equation .
In regards to adipose tissue increases, a person generally gains fat-related 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 in a dense, high-energy form as fat. One pound of fat stores 3500 calories of energy, so over time, excessive energy intake and/or lack of exercise can contribute to fat gain and obesity. 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.
Weight gain is a common side-effect of certain psychiatric medications. Pathological causes of weight gain may be Cushing's syndrome, hypothyroidism, insulinoma, craniopharyngioma. Genetic reasons can relate to Prader–Willi syndrome, Bardet–Biedl syndrome, Alström syndrome, Cohen syndrome, Carpenter syndrome.
Excess adipose tissue on a human can lead to medical problems; however, a round or large figure does not of itself 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." In particular, a married woman who was thin was pitied, as her shape showed that her husband could not afford to feed her properly; conversely, having a fat wife was a status symbol: there was plenty to eat, and she did not need to work hard. 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 ages 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 weight 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, putting their conclusions into question.
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.
Weight gain is seen in professional sports most notably in combat sports because of their weight divisions. It occurs mostly in boxing, mixed martial arts, puroresu and professional wrestling.
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