Health Risks of Obesity

Hypertension

High blood pressure is defined as mean systolic blood pressure 140 mm Hg, or mean diastolic blood pressure 90 mm Hg, or currently taking anti-hypertensive medication. The prevalence of high blood pressure in adults with BMI 30 is 38.4 percent for men and 32.2 percent for women, respectively, compared with 18.2 percent for men and 16.5 percent for women with BMI < 25, a relative risk of 2.1 and 1.9 for men and women, respectively. The direct and independent association between blood pressure and BMI or weight has been shown in numerous cross-sectional studies, including the large international study of salt (INTERSALT) carried out in more than 10,000 men and women.  INTERSALT reported that a 10 kg (22 lb) higher body weight is associated with 3.0 mm Hg higher systolic and 2.3 mm Hg higher diastolic blood pressure. These differences in blood pressure translate into an estimated 12 percent increased risk for CHD and 24 percent increased risk for stroke.

Type 2 Diabetes

The increased risk of diabetes as weight increases has been shown by prospective studies in Norway, the United States , Sweden, and Israel. More recently, the Nurses' Health Study, using data based on self-reported weights, found that the risk of developing type 2 diabetes increases as BMI increases from a BMI as low as 22. Since women in particular tend to under-report weight, the actual BMI values associated with these risks are likely to be higher than the Nurses' Health Study data would suggest. An association between type 2 diabetes and increasing relative weight is also observed in populations at high risk for obesity and diabetes, such as in American Indians.

In recent studies, the development of type 2 diabetes has been found to be associated with weight gain after age 18 in both men and women. The relative risk of diabetes increases by approximately 25 percent for each additional unit of BMI over 22 kg/m². Additionally, in a prospective study representative of the U.S. population, it was recently estimated that 27 percent of new cases of diabetes were attributable to weight gain in adulthood of 5 kg (11 lb) or more. Both cross-sectional and longitudinal studies show that abdominal obesity is a major risk factor for type 2 diabetes.

Coronary Heart Disease

Observational studies have shown that overweight, obesity, and excess abdominal fat are directly related to cardiovascular risk factors, including high levels of total cholesterol, LDL-cholesterol, triglycerides, blood pressure, fibrinogen, and insulin, and low levels of HDL-cholesterol.  Plasminogen activator inhibitor-1 causing impaired fibrinolytic activity is elevated in persons with abdominal obesity. Overweight, obesity, and abdominal fat are also associated with increased morbidity and mortality from CHD.

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Recent studies have shown that the risks of nonfatal myocardial infarction and CHD death increase with increasing levels of BMI. Risks are lowest in men and women with BMIs of 22 or less and increase with even modest elevations of BMI. In the Nurses' Health Study, which controlled for age, smoking, parental history of CHD, menopausal status, and hormone use, relative risks for CHD were twice as high at BMIs of 25 to 28.9, and more than three times as high at BMIs of 29 or greater, compared with BMIs of less than 21.  Weight gains of 5 to 8 kg (11 to 17.6 lb) increased CHD risk (nonfatal myocardial infarction and CHD death) by 25 percent, and weight gains of 20 kg (44 lb) or more increased risk more than 2.5 times in comparison with women whose weight was stable within a range of 5 kg (11 lb). In British men, CHD incidence increased at BMIs above 22 and an increase of 1 BMI unit was associated with a 10 percent increase in the rate of coronary events.

Congestive Heart Failure

Overweight and obesity have been identified as important and independent risk factors for congestive heart failure (CHF) in a number of studies, including the Framingham Heart Study. CHF is a frequent complication of severe obesity and a major cause of death; duration of the obesity is a strong predictor of CHF. Since hypertension and type 2 diabetes are positively associated with increasing weight, the coexistence of these conditions facilitates the development of CHF. Data from the Bogalusa Heart Study demonstrate that excess weight may lead to acquisition of left ventricular mass beyond that expected from normal growth.

Obesity can result in alterations in cardiac structure and function even in the absence of systemic hypertension or underlying heart disease. Ventricular dilatation and eccentric hypertrophy may result from elevated total blood volume and high cardiac output. Diastolic dysfunction from eccentric hypertrophy and systolic dysfunction from excessive wall stress result in so-called "obesity cardiomyopathy". The sleep apnea/obesity hyperventilation syndrome occurs in 5 percent of severely obese individuals, and is potentially life-threatening. Extreme hypoxemia induced by obstructive sleep apnea syndrome may result in heart failure in the absence of cardiac dysfunction.

Cerebrovascular Disease

The relationship of cerebrovascular disease to obesity and overweight has not been as well studied as the relationship to CHD. A report from the Framingham Heart Study suggested that overweight might contribute to the risk of stroke, independent of the known association of hypertension and diabetes with stroke.

More recently published reports are based on larger samples and delineate the importance of stroke subtypes in assessing these relationships. They also attempt to capture all stroke events, whether fatal or nonfatal. These studies suggest distinct risk factors for ischemic stroke as compared to hemorrhagic stroke, and found overweight to be associated with the former, but not the latter. This may explain why studies that use only fatal stroke outcomes (and thus overrepresent hemorrhagic strokes) show only weak relationships between overweight and stroke.

These recent prospective studies demonstrate that the risk of stroke shows a graded increase as BMI rises. For example, ischemic stroke risk is 75 percent higher in women with BMI > 27, and 137 percent higher in women with a BMI > 32, compared with women having a BMI < 21.

Gallstones

The risk of gallstones increases with adult weight. Risk of either gallstones or cholecystectomy is as high as 20 per 1,000 women per year when BMI is above 40, compared with 3 per 1,000 among women with BMI < 24.

According to NHANES III data, the prevalence of gallstone disease among women increased from 9.4 percent in the first quartile of BMI to 25.5 percent in the fourth quartile of BMI. Among men, the prevalence of gallstone disease increased from 4.6 percent in the first quartile of BMI to 10.8 percent in the fourth quartile of BMI.

Osteoarthritis

Individuals who are overweight or obese increase their risk for the development of osteoarthritis.  The association between increased weight and the risk for development of knee osteoarthritis is stronger in women than in men.  In a study of twin middle-aged women, it was estimated that for every kilogram increase of weight, the risk of developing osteoarthritis increases by 9 to 13 percent. The twins with knee osteoarthritis were generally 3 to 5 kg (6.6 to 11 lb) heavier than the co-twins with no disease.  An increase in weight is significantly associated with increased pain in weight-bearing joints. There is no evidence that the development of osteoarthritis leads to the subsequent onset of obesity.  A decrease in BMI of 2 units or more during a 10-year period decreased the odds for developing knee osteoarthritis by more than 50 percent; weight gain was associated with a slight increase in risk.

A randomized controlled trial of 6 months' duration examined the effect of weight loss on clinical improvement in patients with osteoarthritis. Patients taking phentermine had an average weight loss of 12.6 percent after 6 months while the control group had an average weight loss of 9.2 percent. There was improvement in pain-free range of motion and a decrease in analgesic use in association with weight loss; patients with knee disease showed a stronger association than those with hip disease. Similarly, improvement of joint pain was observed in individuals who had undergone gastric stapling, resulting in an average weight loss of 45 kg (99 lb).

Sleep Apnea

Obesity, particularly upper body obesity, is a risk factor for sleep apnea and has been shown to be related to its severity.  The major pathophysiologic consequences of severe sleep apnea include arterial hypoxemia, recurrent arousals from sleep, increased sympathetic tone, pulmonary and systemic hypertension, and cardiac arrhythmias. Most people with sleep apnea have a BMI > 30.  Large neck girth in both men and women who snore is highly predictive of sleep apnea. In general, men whose neck circumference is 17 inches or greater and women whose neck circumference is 16 inches or greater are at higher risk for sleep apnea.

 

Colon Cancer

Many studies have found a positive relation between obesity and colon cancer in men but a weaker association in women.  More recent data from the Nurses' Health Study suggest that the relationship between obesity and colon cancer in women may be similar to that seen in men. Twice as many women with a BMI of > 29 kg/m² had distal colon cancer as women with a BMI < 21 kg/m² . In men, the relationship between obesity and total colon cancer was weaker than that for distal colon cancer.

Other data from the Nurses' Health Study show a substantially stronger relationship between waist-to-hip ratio and the prevalence of colon polyps on sigmoidoscopy, than with BMI alone. Even among leaner women, a high waist-to-hip ratio is also associated with significantly increased risk of colon polyps.

 

Breast Cancer

Epidemiologic studies consistently show that obesity is directly related to mortality from breast cancer, predominantly in postmenopausal women⁶, but inversely related to the incidence of premenopausal breast cancer⁴⁵. Ten or more years after menopause, the premenopausal "benefit" of obesity has dissipated. Among postmenopausal women, peripheral fat is the primary source of estrogens, the major modifiable risk factor for postmenopausal breast cancer.

This crossover in the relationship of obesity with breast cancer, pre- and post-menopause, complicates prevention messages for this common female cancer. Recent data from the Nurses' Health Study, however, show that adult weight gain is positively related to risk of postmenopausal breast cancer. This relation is seen most clearly among women who do not use postmenopausal hormones. A gain of more than 20 lb from age 18 to midlife doubles a woman's risk of breast cancer. Even modest weight gains are positively related to risk of postmenopausal cancer.

 

Endometrial Cancer

Obesity increases the risk of endometrial cancer. The risk is three times higher among obese women (BMI 30 kg/m²) than among normal-weight women. However, the absolute risk of this condition is low when compared to breast cancer, heart disease, and diabetes.

 

 

 

 

 

Women's Reproductive Health

 

Menstrual Function and Fertility

Obesity in premenopausal women is associated with menstrual irregularity and amenorrhea.  As part of the Nurses' Health Study, a case control study suggested that the greater the BMI at age 18 years, even at levels lower than those considered obese, the greater the risk of subsequent ovulatory infertility. The most prominent condition associated with abdominal obesity is polycystic ovarian syndrome, a combination of infertility, menstrual disturbances, hirsutism, abdominal hyperandrogenism, and anovulation. This syndrome is strongly associated with hyperinsulinemia and insulin resistance.

 

Pregnancy

Pregnancy can result in excessive weight gain and retention. The 1988 National Maternal and Infant Survey observed that 41.6 percent of women reported retaining 9 lb of their gained weight during pregnancy, with 33.8 percent reporting 14 lb of retained weight gain.  The retained weight gain associated with pregnancy was corroborated by the study of Coronary Artery Risk Development in Young Adults (CARDIA). As a result of their first pregnancy, both black and white young women had a sustained weight gain of 2 to 3 kg (4.4 to 6.6 lb) of body weight.

Another study on a national cohort of women followed for 10 years reported that weight gain associated with childbearing ranged from 1.7 kg (3.7 lb) for those having one live birth during the study to 2.2 kg (4.9 lb) for those having three¹⁷⁸. In addition, higher prepregnancy weights have been shown to increase the risk of late fetal deaths.

Obesity during pregnancy is associated with increased morbidity for both the mother and the child. A tenfold increase in the prevalence of hypertension and a 10 percent incidence of gestational diabetes have been reported in obese pregnant women.  Obesity also is associated with difficulties in managing labor and delivery, leading to a higher rate of induction and primary Caesarean section.  Risks associated with anesthesia are higher in obese women, as there is greater tendency toward hypoxemia and greater technical difficulty in administering local or general anesthesia. Finally, obesity during pregnancy is associated with an increased risk of congenital malformations, particularly of neural tube defects.

A certain amount of weight gain during pregnancy is desirable. The fetus itself, expanded blood volume, uterine enlargement, breast tissue growth, and other products of conception generate an estimated 13 to 17 lb of extra weight. Weight gain beyond this, however, is predominantly maternal adipose tissue. It is this fat tissue that, in large measure, accounts for the postpartum retention of weight gained during pregnancy. In turn, this retention reflects a postpartum energy balance that does not lead to catabolism of the gained adipose tissue. In part, this may reflect reduced energy expenditure through decreased physical activity, even while caring for young children, but it may also reflect retention of the pattern of increased caloric intake acquired during pregnancy.

One difficulty in developing recommendations of optimal weight gain during pregnancy relates to the health of the infants. A balance must be achieved between high-birth-weight infants who may pose problems during delivery and who may face a higher rate of Caesarean sections and low-birth-weight infants who face a higher infant mortality rate.  However, data from the Pregnancy Nutrition Surveillance System from CDC showed that very overweight women would benefit from a reduced weight gain during pregnancy to help reduce the risk for high-birth-weight infants.

The Institute of Medicine report made recommendations concerning maternal weight gain. It recommended that each woman have her BMI measured and recorded at the time of entry into prenatal care. For women with a BMI of less than 20, the target weight gain should be 0.5 kg (1.1 lb) of weight gain per week during the second and third trimester. For a woman whose BMI is greater than 26, the weight gain target is 0.3 kg (0.7 lb) per week during the last two trimesters.