Several studies have shown that obese men tend to have a lower sperm count, fewer rapidly mobile sperm and fewer progressively motile sperm compared to normal-weight men.

Obesity in Germany has created a cholesterol problem. High cholesterol is known to cause premature death, angina, heart disease and strokes.

There has been an increase of children with Type 1 diabetes between 1996 and 2011. Diabetics are at higher risk for complications such as heart attack and stroke. In Germany, 600,000 people suffered from diabetes near the end of World War II compared to eight million now.

Obesity can increased risk for secondary diseases such as diabetes, cardiovascular disease, certain cancers and Alzheimer's. Children who get diabetes can expect to lose 10 to 15 years off of their lives. Diabetes also affect the eyes, kidneys and nerves in the legs.

Obesity is a "very strong promoter of cancer." Obesity causes an increased risk for colon cancer and breast cancer.

Genes partly play a role in obesity. Scientists at the German Institute of Human Nutrition and the University Hospital of Leipzig stated that identified two genes that promote fat accumulation in the abdominal cavity. The increased activity of the genes also promotes the release of an enzyme that is responsible for the formation of cortisol. A permanent increase in cortisol levels contribute to obesity.

Various developmental factors may affect rates of obesity. Breast-feeding for example may protect against obesity in later life with the duration of breast-feeding inversely associated with the risk of being overweight later on. A child's body growth pattern may influence the tendency to gain weight. Researchers measured the standard deviation (SD [weight and length]) scores in a cohort study of 848 babies. They found that infants who had an SD score above 0.67 had catch up growth (they were less likely to be overweight) compared to infants who had less than a 0.67 SD score (they were more likely to gain weight).

A child's weight may be influenced when he/she is only an infant. Researchers also did a cohort study on 19,397 babies, from their birth until age seven and discovered that fat babies at four months were 1.38 times more likely to be overweight at seven years old compared to normal weight babies. Fat babies at the age of one were 1.17 times more likely to be overweight at age seven compared to normal weight babies.

Children's food choices are also influenced by family meals. Researchers provided a household eating questionnaire to 18,177 children, ranging in ages 11–21, and discovered that four out of five parents let their children make their own food decisions. They also discovered that compared to adolescents who ate three or fewer meals per week, those who ate four to five family meals per week were 19% less likely to report poor consumption of vegetables, 22% less likely to report poor consumption of fruits, and 19% less likely to report poor consumption of dairy foods. Adolescents who ate six to seven family meals per week, compared to those who ate three or fewer family meals per week, were 38% less likely to report poor consumption of vegetables, 31% less likely to report poor consumption of fruits, and 27% less likely to report poor consumption of dairy foods. The results of a survey in the UK published in 2010 imply that children raised by their grandparents are more likely to be obese as adults than those raised by their parents. An American study released in 2011 found the more mothers work the more children are more likely to be overweight or obese.

The causes of childhood obesity can be based on both a combination of individual choices and socio-environmental adaptions with genetic factors playing an important role also.

Genetics, according to the Australian Health Survey plays a primary role in determining obesity. In 2011/2012 it was recorded that 90% of the Australian population had inherited their obese tendencies due to the epigenetic modifications of their mothers during pregnancy. Interchanging closely with the genetic factor, the environment and individual temptations are also highlighted as contributing causes to the worldwide epidemic. The genetic configuration influences the tendencies to become overweight, and diet and physical activity determines to what extent those tendencies are elaborated.

Obesity is a chronic health problem. It is one of the biggest factors for a type II diabetes, and cardiovascular disease. It is also associated with cancer (e.g. colorectal cancer), osteoarthritis, liver disease, sleep apnea, depression and other medical conditions that affect mortality and morbidity.

According to the NHANES data, African American and Mexican American adolescents between 12 and 19 years old are more likely to be overweight than non-Hispanic White adolescents. The prevalence is 21%, 23% and 14% respectively. Also, in a national survey of American Indian children 5–18 years old, 39 percent were found to be overweight or at risk for being overweight. As per national survey data, these trends indicate that by 2030, 86.3% of adults will be overweight or obese and 51.1% obese.

A 2007 study found that receiving Food Stamps long term (24 months) was associated with a 50% increased obesity rate among female adults.

Looking at the long-term consequences, overweight adolescents have a 70 percent chance of becoming overweight or obese adults, which increases to 80 percent if one or more parent is overweight or obese. In 2000, the total cost of obesity for children and adults in the United States was estimated to be US$117 billion (US$61 billion in direct medical costs). Given existing trends, this amount is projected to range from US$860.7-956.9 billion in healthcare costs by 2030.

Food consumption has increased with time. For example, annual per capita consumption of cheese was in 1909; in 2000; the average person consumed of carbohydrates daily in 1970; in 2000; of fats and oils in 1909; in 2000. In 1977, 18% of an average person's food was consumed outside the home; in 1996, this had risen to 32%.

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.

According to Cleveland Clinic, cultural, social, and environmental factors, among others, all affect eating behaviors.

While the negative health outcomes associated with obesity are accepted within the medical community, the health implications of the overweight category are more controversial. The generally accepted view is that being overweight causes similar health problems to obesity, but to a lesser degree. A 2016 review estimated that the risk of death increases by seven percent among overweight people with a BMI of 25 to 27.5 and 20 percent among overweight people with a BMI of 27.5 to 30. The Framingham heart study found that being overweight at age 40 reduced life expectancy by three years. Being overweight also increases the risk of oligospermia and azoospermia in men.

Katherine Flegal et al., however, found that the mortality rate for individuals who are classified as overweight (BMI 25 to 30) may actually be lower than for those with an "ideal" weight (BMI 18.5 to 25).

Being overweight has been identified as a cause of cancer, and is projected to overtake smoking as the primary cause of cancer in developed countries as cases of cancer linked to smoking dwindle.

Psychological well-being is also at risk in the overweight individual due to social discrimination. However, children under the age of eight are normally not affected.

Being overweight has been shown not to increase mortality in older people: in a study of 70 to 75-year old Australians, mortality was lowest for "overweight" individuals (BMI 25 to 30), while a study of Koreans found that, among those initially aged 65 or more, an increase in BMI to above 25 was not associated with increased risk of death.

A sedentary lifestyle plays a significant role in obesity. Worldwide there has been a large shift towards less physically demanding work, and currently at least 30% of the world's population gets insufficient exercise. This is primarily due to increasing use of mechanized transportation and a greater prevalence of labor-saving technology in the home. In children, there appear to be declines in levels of physical activity due to less walking and physical education. World trends in active leisure time physical activity are less clear. The World Health Organization indicates people worldwide are taking up less active recreational pursuits, while a study from Finland found an increase and a study from the United States found leisure-time physical activity has not changed significantly. A 2011 review of physical activity in children found that it may not be a significant contributor.

In both children and adults, there is an association between television viewing time and the risk of obesity. A review found 63 of 73 studies (86%) showed an increased rate of childhood obesity with increased media exposure, with rates increasing proportionally to time spent watching television.

While genetic influences are important to understanding obesity, they cannot explain the current dramatic increase seen within specific countries or globally. Though it is accepted that energy consumption in excess of energy expenditure leads to obesity on an individual basis, the cause of the shifts in these two factors on the societal scale is much debated. There are a number of theories as to the cause but most believe it is a combination of various factors.

The correlation between social class and BMI varies globally. A review in 1989 found that in developed countries women of a high social class were less likely to be obese. No significant differences were seen among men of different social classes. In the developing world, women, men, and children from high social classes had greater rates of obesity. An update of this review carried out in 2007 found the same relationships, but they were weaker. The decrease in strength of correlation was felt to be due to the effects of globalization. Among developed countries, levels of adult obesity, and percentage of teenage children who are overweight, are correlated with income inequality. A similar relationship is seen among US states: more adults, even in higher social classes, are obese in more unequal states.

Many explanations have been put forth for associations between BMI and social class. It is thought that in developed countries, the wealthy are able to afford more nutritious food, they are under greater social pressure to remain slim, and have more opportunities along with greater expectations for physical fitness. In undeveloped countries the ability to afford food, high energy expenditure with physical labor, and cultural values favoring a larger body size are believed to contribute to the observed patterns. Attitudes toward body weight held by people in one's life may also play a role in obesity. A correlation in BMI changes over time has been found among friends, siblings, and spouses. Stress and perceived low social status appear to increase risk of obesity.

Smoking has a significant effect on an individual's weight. Those who quit smoking gain an average of 4.4 kilograms (9.7 lb) for men and 5.0 kilograms (11.0 lb) for women over ten years. However, changing rates of smoking have had little effect on the overall rates of obesity.

In the United States the number of children a person has is related to their risk of obesity. A woman's risk increases by 7% per child, while a man's risk increases by 4% per child. This could be partly explained by the fact that having dependent children decreases physical activity in Western parents.

In the developing world urbanization is playing a role in increasing rate of obesity. In China overall rates of obesity are below 5%; however, in some cities rates of obesity are greater than 20%.

Malnutrition in early life is believed to play a role in the rising rates of obesity in the developing world. Endocrine changes that occur during periods of malnutrition may promote the storage of fat once more food energy becomes available.

Consistent with cognitive epidemiological data, numerous studies confirm that obesity is associated with cognitive deficits. Whether obesity causes cognitive deficits, or vice versa is unclear at present.

Researchers from the NIH's National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) conducted a study and found that early-onset paternal obesity is connected with an increased risk of liver disease in their kin. Researchers found that obese fathers had an elevated level of serum alanine aminotransferase (ALT), a liver enzyme, compared to fathers who were not obese. They did a secondary analysis that excluded obese offspring. Children who were a normal weight but had obese fathers still had elevated ALT levels, which indicated that a child's ALT levels are not dependent upon the child's own BMI.

Several studies have shown that obese men tend to have a lower sperm count, fewer rapidly mobile sperm and fewer progressively motile sperm compared to normal-weight men. Researchers in France have said that poor children were up to three times more likely to be obese compared with wealthier children.

In French society, the main economic subgroup that is affected by obesity is the lower class so they perceive obesity as a problem related to social inequality.

Causes cited for the growing rates of obesity in the United Kingdom are multiple and the degree of influence of any one factor is often a source of debate. At an individual level, a combination of excessive food energy intake and a lack of physical activity is thought to explain most cases of obesity. Reduced levels of physical activity due to increased use of private cars, desk bound employment, a decline in home cooking skills and the ready availability of processed foods high in sugar, salt and saturated fats, are variously cited as contributing factors.

Obesity in the United Kingdom is a significant contemporary health concern, with officials stating that it is one of the leading preventable causes of death. In February 2016, Health Secretary Jeremy Hunt described rising rates of childhood obesity as a "national emergency".

Data published as a part of the World Health Organisation (WHO) study in 2014 indicated that 28.1% of adults in the United Kingdom were recognised as clinically obese with a Body Mass Index (BMI) greater than 30. In 2014 62% of adults in England were classified as overweight (a body mass index of 25 or above) or obese, compared to 53% 20 years earlier. More than two-thirds of men and almost six in 10 women are overweight or obese.

Experts have predicted that by the year 2020 one third of the United Kingdom population could be obese. Rising levels of obesity are a major challenge to public health. There are expected to be 11 million more obese adults in the UK by 2030, accruing up to 668,000 additional cases of diabetes mellitus, 461,000 cases of heart disease and stroke, 130,000 cases of cancer, with associated medical costs set to increase by £1.9–2.0B per year by 2030. Adult obesity rates have almost quadrupled in the last 25 years.

For children, data from the Health Survey for England (HSE) conducted in 2014 and examining patterns of overweight and obesity among children aged 2–15, showed that 17% of children were obese and an additional 14% of children were overweight.

Combing three years of data (2012, 2013 and 2014) Public Health England identified Barnsley, South Yorkshire as the local authority with the highest incidence of adult obesity (BMI greater than 30) with 35.1%. Data from the same study revealed that Doncaster, South Yorkshire was the local authority with the highest overall excess weight with 74.8% of adults (16 years and over) with a BMI greater than 25. In previous Public Health England studies based on 2012 data, Tamworth in Staffordshire had been identified as the fattest town in England with a 30.7% obesity rate.

Obese women have an increased risk of pregnancy-related complications, including hypertension, gestational diabetes, and blood clots. Also, the mother is at risk of going into preterm labor. Maternal obesity is also known to be associated with increased rates of complications in late pregnancy such as cesarean delivery, and shoulder dystocia. A meta-analysis estimated that Cesarean delivery rates increased with odds ratios of 1.5 among overweight, 2 among obese, and 3 among severely obese women, compared with normal weight pregnant women. In addition, morbidly obese women who have not had children before are at increased risk of all–cause preterm deliveries. It is well recognized that obese women are at increased risk of preeclampsia and that women who have never been pregnant are at higher risk of preeclampsia than women who have had children in the past.

1 in 4 children are overweight (25%) and 2 in 3 adults are overweight (63%)

The medical condition of being overweight or obesity is defined as "abnormal or excessive fat accumulation that may impair health". It is measured through the Body Mass Index (BMI), defined as a person's weight, in kilograms, divided by the square of the person's height, in meters. If an individual has a BMI of 25–29, he or she is overweight. Having a BMI of 30 or more means an individual is obese. The greater the BMI, the greater the risk of chronic diseases as a result of obesity. These diseases include cardiovascular diseases, diabetes, musculoskeletal disorders, cancer, and premature death.

In the United States, the prevalence of obese or overweight adult dogs is 23–53%, of which about 5% are obese; the incidence in adult cats is 55%, of which about 8% are obese.

In Australia, obesity is the most common nutritional disease of pets; the prevalence of obesity in dogs in Australia is approximately 40%.

The MONICA Project, sponsored by the World Health Organization, discovered that 30% of the population in the Arab World is overweight or obese, including adolescents and adults. This percentage is smaller for North African countries than oil-producing countries, but is still a prevalent trend. The spread of the Western lifestyle, defined as "the intake of attractive energy dense food with undesirable composition, increased consumption of animal fats and sugars and reduced consumption of dietary fiber, along with a lack of sufficient physical activity", is one of the leading causes. Specifically in the Arabian Peninsula, "physical activity of the population has significantly diminished with the availability of housemaids, private cars, television, and sophisticated household appliances. In addition, the types of food and fat intake have changed...".

Other common factors, besides a sedentary lifestyle and unhealthy food choices, across the Arab world include urbanization, technology, and a cultural appreciation of female plumpness and the practice of "leblouh" ("fattening").

This is a list of the states of India ranked in order of percentage of people who are overweight or obese, based on data from the 2007 National Family Health Survey.

Statistics Canada, a sub-organization of the government of Canada, reports that 14,222,521 Canadians (54% of the population) over the age of 18 self-reported as overweight or obese in 2014. According to Forbes, Canada ranks 33 on a 2007 list of fattest countries, with 61.1% of its citizens having a body mass index (BMI) of at least 25. In children, obesity has substantially increased between 1989 and 2004, with rates in boys increasing from 2% to 13% and rates among girls increasing from 2% to 11%.

A study has shown that alcohol consumption is directly associated with waist circumference and with a higher risk of abdominal obesity in men, but not in women. Excluding energy under-reporters slightly attenuated these associations. After controlling for energy under-reporting, it was observed that increasing alcohol consumption significantly increased the risk of exceeding recommended energy intakes in male participants – but not in the small number of female participants (2.13%) with elevated alcohol consumption, even after establishing a lower number of drinks per day to characterize women as consuming a high quantity of alcohol. Further study is needed to determine whether a significant relationship between alcohol consumption and abdominal obesity exists among women who consume higher amounts of alcohol.

The French, along with the Italians and the Swiss, were previously considered to be among the slimmest people in Europe on average.

World Health Organisation (WHO) data published in 2014 showed however that adult obesity rates in France were significantly ahead of the Netherlands at 19.8%, Germany at 20.1% and Italy at 21.0%.

In 2010 38.5% of men and 26% of women in France were considered overweight while 60% of men and 43% of women in Germany were considered overweight. Estimates show that 16% of French five-to-11-year-olds are obese. Obesity in that age group is set to increase to more than 20% by 2010.

Obesity in children is growing at a faster rate than obesity in adults. Obesity in children is growing at a rate of 17% while obesity in adults is growing at a rate of 6%. Recent studies show a slowdown in the growth of obesity in France especially for children.