Obesity is one of the leading preventable causes of death worldwide. A number of reviews have found that mortality risk is lowest at a BMI of 20–25 kg/m in non-smokers and at 24–27 kg/m in current smokers, with risk increasing along with changes in either direction. This appears to apply in at least four continents. In contrast, a 2013 review found that grade 1 obesity (BMI 30-35) was not associated with higher mortality than normal weight, and that overweight (BMI 25-30) was associated with "lower" mortality than was normal weight (BMI 18.5-25). Other evidence suggests that the association of BMI and waist circumference with mortality is U- or J-shaped, while the association between waist-to-hip ratio and waist-to-height ratio with mortality is more positive. In Asians the risk of negative health effects begins to increase between 22–25 kg/m. A BMI above 32 kg/m has been associated with a doubled mortality rate among women over a 16-year period. In the United States, obesity is estimated to cause 111,909 to 365,000 deaths per year, while 1 million (7.7%) of deaths in Europe are attributed to excess weight. On average, obesity reduces life expectancy by six to seven years, a BMI of 30–35 kg/m reduces life expectancy by two to four years, while severe obesity (BMI > 40 kg/m) reduces life expectancy by ten years.

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.

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.

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.

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.

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 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.

Developing asthma due to abdominal obesity is also a main concern. As a result of breathing at low lung volume, the muscles are tighter and the airway is narrower. It is commonly seen that people who are obese breathe quickly and often, while inhaling small volumes of air. People with obesity are also more likely to be hospitalized for asthma. A study has stated that 75% of patients treated for asthma in the emergency room were either overweight or obese.

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.

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.

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%.

Compared to non-obese animals, obese dogs and cats have a higher incidence of osteoarthritis (joint disease) and diabetes mellitus, which also occur earlier in the life of the animal. Obese animals are also at increased risk of complications following anesthesia or surgery.

Obese dogs are more likely to develop urinary incontinence, may have difficulty breathing, and overall have a poorer quality of life compared to non-obese dogs, as well as having a lower life expectancy. Obese cats have an increased risk of diseases affecting the mouth and urinary tract. Obese cats which have difficulty grooming themselves are predisposed to dry, flaky skin and feline acne.

Although some genetic links to obesity have been made, relatively little is known about how genes or loci might influence dynamic measures of weight change over time.

The physical implications of obesity in children include sleep apnoea, breathlessness, a reduced tolerance to exercise and orthopaedic and gastrointestinal problems including non-alcoholic fatty liver disease. Children who reciprocate these physical health disadvantages tend to struggle to concentrate more in- school and find it harder to fit in, being marginalised due to the inability to partake in physical exercise. According to the Dieticians Association of Australia 25-50% of overweight or obese children with turn out to be obese as adults. Long-term effects of obesity, therefore, include cardiovascular disease (hypertension and high blood pressure) and particular types of cancers in particular colon, kidney and breast cancer. Non-alcoholic fatty liver disease (NAFLD) is one of the most common risk factors associated with obesity being characterised as a buildup of fat within the liver cells. Musculoskeletal defects such as osteoarthritis are also said to have a strong link with obesity due to excessive amounts of weight being exerted on the joints. Individuals who have a Body Mass Index (BMI) that is equal to or greater than 25 kg/m2 are also said to have an increased chance of premature morality.

Obesity increases health risks, including diabetes, cancer, cardiovascular disease, high blood pressure, and non-alcoholic fatty liver disease, to name a few. Reduction of obesity lowers those risks.

A 1-kg loss of body weight has been associated with an approximate 1-mm Hg drop in blood pressure.

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%.

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.

Social conditions such as poverty, social isolation and inability to get or prepare preferred foods can cause unintentional weight loss, and this may be particularly common in older people. Nutrient intake can also be affected by culture, family and belief systems. Ill-fitting dentures and other dental or oral health problems can also affect adequacy of nutrition.

Loss of hope, status or social contact and spiritual distress can cause depression, which may be associated with reduced nutrition, as can fatigue.

The body mass index (BMI) does not capture information about percentage body fat (PBF), which is a better predictor of risk due to obesity.

The term "metabolically obese normal weight" (MONW) refers to people with normal weight and BMI, but who display some metabolic characteristics that may increase the risk of developing metabolic syndrome, in the same way as obesity. MONW subjects have excess visceral fat, and are predisposed to insulin resistance, hypertension and cardiovascular disease. These people show benefits from energy restriction and weight loss, for example a 4- to 12-week period of diet and exercise.

Some studies have suggested that the main factor which explains the metabolic abnormalities in MONW individuals is fat distribution. On the basis of these studies, a scoring method has been proposed to identify MONW individuals, based on the presence of associated diseases or biochemical abnormalities related to insulin resistance.

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.

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.

Gestational diabetes affects 3–10% of pregnancies, depending on the population studied.

Although genetic deficiencies are currently considered rare, variations in these genes may predispose to common obesity. Many candidate genes are highly expressed in the central nervous system.

Several additional loci have been identified. Also, several quantitative trait loci for BMI have been identified.

Confirmed and hypothesized associations include:

Some studies have focused upon inheritance patterns without focusing upon specific genes. One study found that 80% of the offspring of two obese parents were obese, in contrast to less than 10% of the offspring of two parents who were of normal weight.

The thrifty gene hypothesis postulates that due to dietary scarcity during human evolution people are prone to obesity. Their ability to take advantage of rare periods of abundance by storing energy as fat would be advantageous during times of varying food availability, and individuals with greater adipose reserves would more likely survive famine. This tendency to store fat, however, would be maladaptive in societies with stable food supplies. This is the presumed reason that Pima Indians, who evolved in a desert ecosystem, developed some of the highest rates of obesity when exposed to a Western lifestyle.

Numerous studies of laboratory rodents provide strong evidence that genetics plays an important role in obesity.

The risk of obesity is determined by not only specific genotypes but also gene-gene interactions. However, there are still challenges associated with detecting gene-gene interactions for obesity.