Complications:

General Adverse Effects of Obesity. Obesity, defined as a BMI of 30 or over, accounts for nearly 300,000 deaths in the U.S. each year. It is associated with more chronic health problems than smoking, heavy drinking, or poverty. Furthermore, given the current increase in obesity, it will surpass smoking as the most important preventable cause of death in America.

Some studies indicate the following health risks by body mass:

• The lowest risks for heart disease, diabetes, and some cancers are in people with BMI values of 21 - 25.
• The risks increase slightly when BMI values are 25 - 27.
• The risks are significant in BMIs 27 - 30.
• The same risks are dramatic at BMIs over 30.

Anyone with chronic health problems, such as heart or lung disease, stroke, or arthritis, should be concerned about extra weight. This same concern also applies to people with known risk factors for these conditions.

• Metabolic Changes. As fat stores increase, the fat cells themselves swell and produce chemicals that increase the risk for several diseases, including diabetes, high blood pressure, gallbladder disease, and some cancers.
• Increased Mass. The increased body weight itself causes problems that result in injury and diseases, including osteoarthritis and sleep apnea.
• Harmful Fat Cell Types. Weight concentrated around the abdomen and in the upper part of the body (the apple shape) poses a higher health risk than fat that settles around the hips and flank (the pear shape). Fat cells in the upper part of the body appear to have different qualities from those found in the lower parts. In fact, studies suggest a higher risk for diabetes in people with the "apple shape" and lower risk in those who are "pear shaped."

Weight gain in the area around the waist (apple type) is more dangerous than weight gained around the hips and flank area (pear type). Fat cells in the upper body have different qualities than those found in hips and thighs.

General Adverse Effects of Being Overweight (Not Obese). It is still not clear if being overweight (a BMI of 25 - 29.9) hurts healthy people with no risk factors for serious illnesses.

The risk for developing diabetes, gallstones, hypertension, heart disease, stroke, and colon cancer seems to rise according to how much the individuals are overweight. In any case, adults who are overweight in middle age face a poor quality of life as they age, with the quality declining the more they weigh.

Some argue, in fact, that unhealthy diet and sedentary lifestyle cause the harm -- not weight per se -- in anyone who is not severely obese. In support of this argument, a British study found that overweight fit individuals had half the death rate of unfit trim individuals.

Being somewhat overweight may also have some benefits under specific circumstances:

• In older women, some excess fat may produce extra estrogen that helps slow down bone loss, and insulates bones from fall-related injuries. It should be strongly noted, however, that when older overweight women lose weight they report less pain, improved vitality, and improved physical function. The same positive effect of excess weight does not appear to hold in older men.
• Conditioned athletes may have high BMIs because of very dense muscle tissue. Being fit in general may protect many overweight people.

Heart Disease and Stroke

Individuals with a BMI of at least 30 have a 10 - 50% increased rate of death from all causes, compared with individuals with a BMI of 20 - 25. Mortality rates from many causes are higher in obese people, but heart disease is the primary cause of death. People who are obese have almost three times the risk for heart disease as people with normal weights. Being physically unfit adds to the risk.

As mentioned above, weight concentrated around the abdomen and in the upper part of the body (apple shape) is particularly associated with insulin resistance and diabetes. It is also associated with increased risk of heart disease, high blood pressure, stroke, and unhealthy cholesterol levels. Fat that settles in a pear shape around the hips and lower body appears to have a lower association with these conditions.

Obesity poses many dangers to the heart and circulatory system.

Damage in the Blood Vessels. As people age, changes in body fat (particularly increasing abdominal fat) seem to cause stiffness in the aorta, the major blood vessel leading from the heart. Studies are finding higher levels of a factor called C-reactive protein (CRP) in people with obesity and abdominal fat. CRP is now considered to be a marker for inflammation and damage in the arteries. (Losing weight reduces CRP levels.)

High Blood Pressure. High blood pressure is the health problem most commonly associated with obesity, and the greater the weight, the greater the risk. High blood pressure carries serious risks of stroke, heart attack, and heart failure. The link between obesity and high blood pressure is complex, and may be a combination of genetic, population, and biological factors. Many studies have reported that modest weight loss is beneficial for reducing existing high blood pressure. [For more information, see In-Depth Report #14: High blood pressure.]

Heart Failure. Obesity is associated with both hypertension and type 2 diabetes, conditions that place people at risk for heart failure. Evidence strongly suggests that obesity itself is a major risk factor for heart failure, particularly in women. The biological mechanisms involved in obesity that lead specifically to heart failure are not clear. [For more information, see In-Depth Report #13: Heart failure.]

Unhealthy Cholesterol and Lipid Levels. The effect of obesity on cholesterol levels is complex. Although obesity does not appear to be strongly associated with overall cholesterol levels, triglyceride levels (the major form of fat storage in the body) are usually high in obese people, while HDL levels (the "good" cholesterol) tend to be low. Both conditions are risk factors for heart disease. [For more information, see In-Depth Report #23: Cholesterol.]

Stroke. Obesity is also associated with a higher risk for stroke. [For more information, see In-Depth Report #45: Stroke.]

Insulin Resistance, Type 2 Diabetes, and Metabolic Syndrome

Type 2 Diabetes and Insulin Resistance. Most people with type 2 diabetes are obese and, in fact, studies strongly suggest that weight loss may be the key in controlling the current epidemic of type 2 diabetes. The common factor appears to be insulin resistance. Insulin is a critical hormone in the use of sugar. In type 2 diabetes, different factors cause the body to become insulin resistant -- that is, the body can no longer respond properly to insulin. This has the effect of increasing sugar levels in the blood, the hallmark of diabetes. Both obesity and insulin resistance, at different phases, are marked by high levels of certain chemicals. It is not known yet if the higher levels are simply a product of obesity, or play some role in causing diabetes.

Insulin resistance is also associated with high blood pressure and abnormalities in blood clotting. Some research indicates that obesity, in fact, is the one common element linking insulin resistance, type 2 diabetes, and high blood pressure. [For more information, see In-Depth Report #60: Diabetes - type 2.]

Metabolic Syndrome. Metabolic syndrome (also called syndrome X) is a pre-diabetic condition that is significantly associated with heart disease and higher mortality rates from all causes. The syndrome consists of obesity marked by abdominal fat, unhealthy cholesterol levels, high blood pressure, and insulin resistance. A 2002 study estimated that nearly a quarter of the US population now has this condition. Even worse, according to a 2003 study, nearly a million American teenagers have this syndrome. A combination of weight loss and exercise is an effective treatment for this syndrome.

Cancer

The American Cancer Society (ACS) released new cancer prevention guidelines in September 2006. The guidelines stress the importance of keeping a healthy weight throughout life. The ACS indicates that healthy weight is even more important than eating specific healthy foods, when it comes to cancer prevention.

Obesity has been associated with a higher risk for cancer in general and specific cancers in particular. Studies have also suggested that restricting calories reduces the risk for cancer.

One way in which obesity may increase the risk for cancer is its association with high levels of hormones called growth factors, which can trigger rapid cell production, leading to cancer.

Uterine and Breast Cancers. The risk of uterine cancer in obese women appears to be two or three times higher than in thinner women. Increased levels of estrogen, which are present in overweight women, are thought to be the cause of this increase.

Studies are mixed on the association between obesity and breast cancer. A number of studies have linked obesity to breast cancer in postmenopausal women, particularly in women who began to gain weight after age 18.

Prostate Cancer. Studies report that obesity is associated with an increase in prostate cancer mortality in patients with more advanced prostate cancer, although not with the risk for less aggressive forms of prostate cancer.

Gallbladder Cancer. Obese women are at higher risk for gallbladder cancer.

Gastrointestinal Cancers. A number of cancers in the gastrointestinal (digestive) tract have been associated with obesity:

• Cancer of the esophagus may be due to a higher incidence of gastroesophageal reflux disorder (heartburn) in people who are overweight.
• Colon cancer has been linked to increased body mass in both men and women.
• Pancreatic cancer and obesity have been weakly linked, with one study reporting a lower risk in overweight people who are physically active.

Muscles and Bones

Obesity places stress on bones and muscles. Studies report that the incidence of osteoarthritis is significantly increased in people who are overweight. People who are obese are also at higher risk for carpal tunnel syndrome and other problems involving nerves in their wrists and hands.

Eyes and Mouth Disorders

Obesity increases the risk for the following mouth and eye disorders:

• Gum disease
• Cataracts
• Maculopathy, an eye disease related to aging

Reproductive and Hormonal Problems

Infertility. Abnormal amounts of body fat, either 10 - 15% too high or too low, can contribute to infertility in women. Obesity is especially related to certain infertility problems, such as uterine fibroids and menstrual irregularities. In men, obesity can contribute to reduced testosterone levels and erectile dysfunction.

Effect on Pregnancy. Obesity has many dangerous effects on pregnancy. These include high blood pressure, gestational diabetes (diabetes, usually temporary, that occurs during pregnancy), urinary tract infections, blood clots, prolonged labor, and higher fetal death rate in late stages of pregnancy. Obesity is also associated with increased rates of cesarean delivery. Infants of women who are obese are also at higher risk for neural tube birth defects, which affect the brain or spine. Folic acid supplements, ordinarily effective in preventing these conditions, may not be as protective in overweight women. Some evidence suggests an association between obesity and stillbirths, as well.

Effects on the Lungs

Obesity is thought to be a risk factor for symptoms of adult-onset asthma. Though there is evidence that obesity causes wheezing and shortness of breath, it does not appear to be strongly associated with the disease mechanisms in the lungs that cause true asthma.

Obesity also puts people at risk for hypoxia, a condition in which there is not enough oxygen to meet the body's needs. Obese people need to work harder to breathe. They tend to have breathing muscles and lungs that do not work as well as those in thinner people.

The Pickwickian syndrome, named for an overweight character in a Dickens novel, occurs in severe obesity when lack of oxygen produces intense and chronic sleepiness and, eventually, heart failure.

Effect on the Liver

Nonalcoholic Fatty Liver Disease. People with obesity, particularly if they also have type 2 diabetes, are at higher risk for a condition called nonalcoholic fatty liver disease, also called nonalcoholic steatohepatitis (NASH). This condition causes liver damage that is similar to liver injury seen in alcoholism. In some people, it can be very serious and require liver transplantation. NASH occurs in about half of people with diabetes, and 20 - 50% of obese people, depending on how severe their obesity is. NASH can also occur in overweight children.

Gallstones. The incidence of gallstones is significantly higher in obese women and men. The risk for stone formation is also high if a person loses weight too quickly. In people on ultra-low calorie diets, taking ursodeoxycholic acid (Actigall) may prevent gallstones.

Sleep Disorders

People who are obese and nap tend to fall asleep faster and sleep longer during the day. At night, however, it takes them longer to fall asleep, and they sleep less than people with normal weights. In an apparent vicious circle, studies have suggested that obesity not only interferes with sleep but that sleep problems may actually contribute to obesity.

Sleep Apnea. Obesity, particularly the apple shape, is strongly associated with sleep apnea, which occurs when the upper throat relaxes and collapses from time to time during sleep. This collapse temporarily blocks the passage of air. Sleep apnea is increasingly being viewed as a potentially serious health problem, which may lead to complications, such as heart disease and stroke. Some studies suggest that among overweight people, those who have sleep apnea have a greater risk of heart disease than those without it. In one study, the more obese a person with sleep apnea was, the higher the pressure on the airway, and therefore the greater the obstruction of the airway. Obstructive sleep apnea may also add to obesity, however, as sleepy people tend to be sedentary. Some studies indicate that treating sleep apnea may help people lose abdominal fat.

Emotional and Social Problems

Depression. A number of studies have reported an association between depression and obesity, particularly in obese women. There may be a number of factors to explain the link. In some cases of atypical depression, people overeat and may gain weight. Overweight people may also become depressed because of social problems and a poor self-image. In these cases, depression usually disappears when people lose weight.

There is evidence, however, that obesity itself may impair levels of tryptophan -- a chemical needed to make serotonin, a brain chemical associated with mental well-being. In one study, even after people lost weight, tryptophan levels were lower than normal.

There does not appear to be any association between depression and obesity in men.

Social Problems. One long-term study reported that overweight young women completed fewer years of school, were 20% less likely to be married, and had 10% higher rates of household poverty than their thinner peers. Obese young men were also less likely to be married, and their incomes were lower than their thinner peers. Nevertheless, studies consistently show that overweight males (both boys and men) are not as severely emotionally affected as females of any age. Women and girls tend to blame themselves for being heavy, while males tend to blame being overweight on outside factors.

Health Consequences of Childhood Overweight

Children and adolescents who are overweight have poorer health than other children. Studies are reporting unhealthy cholesterol levels and high blood pressure in overweight children and adolescents. Of great concern is the dramatic increase in type 2 diabetes in young people, which is largely due to the increase in overweight children.

Weight gain in children is also linked to asthma, gallbladder problems, sleep apnea, and liver abnormalities. Overweight girls are more likely to enter puberty early, according to a new study, and subsequently be at higher risk for breast cancer. It is not clear yet how many of these childhood problems persist in people who achieve normal weight as adults. Staying overweight into adulthood certainly carries health risks.