In the old spiritual, “Dem Bones,” each body part is linked to the next one in line: the thigh bone to the knee bone, the knee bone to the leg bone, and so on. But one body “part”-weight-is connected to virtually all of the others. A healthy weight sets the stage for bones, muscles, brain, heart, and others to play their parts smoothly and efficiently for many years.
Excess weight, especially obesity, diminishes almost every aspect of health, from reproductive and respiratory function to memory and mood. Obesity increases the risk of several debilitating, and deadly diseases, including diabetes, heart disease, and some cancers. It does this through a variety of pathways, some as straightforward as the mechanical stress of carrying extra pounds and some involving complex changes in hormones and metabolism. Obesity decreases the quality and length of life, and increases individual, national, and global healthcare costs. The good news, though, is that weight loss can curtail some obesity-related risks. Losing as little as 5 to 10 percent of body weight offers meaningful health benefits to people who are obese, even if they never achieve their “ideal” weight, and even if they only begin to lose weight later in life.
Entire books have been written detailing the effects of obesity on various measures of health. This article briefly summarizes associations between obesity and adult health.
Obesity and Diabetes
The condition most strongly influenced by body weight is type 2 diabetes. In the Nurses’ Health Study, which followed 114,000 middle-age women for 14 years, the risk of developing diabetes was 93 times higher among women who had a body mass index (BMI) of 35 or higher at the start of the study, compared with women with BMIs lower than 22. Weight gain during adulthood also increased diabetes risk, even among women with BMIs in the healthy range. The Health Professionals Follow-Up Study found a similar association in men.
More recently, investigators conducted a systematic review of 89 studies on weight-related diseases and then did a statistical summary, or meta-analysis, of the data. Of the 18 weight-related diseases they studied, diabetes was at the top of the risk list: Compared with men and women in the normal weight range (BMI lower than 25), men with BMIs of 30 or higher had a sevenfold higher risk of developing type 2 diabetes, and women with BMIs of 30 or higher had a 12-fold higher risk.
Fat cells, especially those stored around the waist,secrete hormones and other substances that fire inflammation. Although inflammation is an essential component of the immune system and part of the healing process, inappropriate inflammation causes a variety of health problems. Inflammation can make the body less responsive to insulin and change the way the body metabolizes fats and carbohydrates, leading to higher blood sugar levels and, eventually, to diabetes and its many complications. Several large trials have shown that moderate weight loss can prevent or delay the start of diabetes in people who are at high risk.
Obesity and Cardiovascular Disease
Body weight is directly associated with various cardiovascular risk factors. As BMI increases, so do blood pressure, low-density lipoprotein (LDL, or “bad”) cholesterol, triglycerides, blood sugar, and inflammation. These changes translate into increased risk for coronary heart disease, stroke, and cardiovascular death:
- Obesity and Coronary Artery Disease. Numerous studies have demonstrated a direct association between excess body weight and coronary artery disease (CAD). The BMI-CAD Collaboration Investigators conducted a meta-analysis of 21 long-term studies that followed more than 300,000 participants for an average of 16 years. Study participants who were overweight had a 32 percent higher risk of developing CAD, compared with participants who were at a normal weight; those who were obese had an 81 percent higher risk. Although adjustment for blood pressure and cholesterol levels slightly lowered the risk estimates, they remained highly significant for obesity. The investigators estimated that the effect of excess weight on blood pressure and blood cholesterol accounts for only about half of the obesity-related increased risk of coronary heart disease.
- Obesity and Stroke. Ischemic (clot-caused) stroke and coronary artery disease share many of the same disease processes and risk factors. A meta-analysis of 25 prospective cohort studies with 2.3 million participants demonstrated a direct, graded association between excess weight and stroke risk. Overweight increased the risk of ischemic stroke by 22 percent, and obesity increased it by 64 percent. There was no significant relationship between overweight or obesity and hemorrhagic (bleeding-caused) stroke, however. A repeat analysis that statistically accounted for blood pressure, cholesterol, and diabetes weakened the associations, suggesting that these factors mediate the effect of obesity on stroke.
- Obesity and Cardiovascular Death. In a meta-analysis of 26 observational studies that included 390,000 men and women, several racial and ethnic groups, and samples from the U.S. and other countries, obesity was significantly associated with death from CAD and cardiovascular disease. Women with BMIs of 30 or higher had a 62 percent greater risk of dying early from CAD and also had a 53 percent higher risk of dying early from any type of cardiovascular disease, compared with women who had BMIs in the normal range (18.5 to 24.9). Men with BMIs of 30 or higher had similarly elevated risks.
The good news is that weight loss of 5 to 10 percent of body weight can lower blood pressure, LDL cholesterol, and triglycerides, and improve other cardiovascular risk factors.
Obesity, Depression, and Quality of Life
The high rates of obesity and depression, and their individual links with cardiovascular disease, have prompted many investigators to explore the relationship between weight and mood. An analysis of 17 cross-sectional studies found that people who were obese were more likely to have depression than people with healthy weights. Since the studies included in the analysis assessed weight and mood only at one point in time, the investigators could not say whether obesity increases the risk of depression or depression increases the risk of obesity. New evidence confirms that the relationship between obesity and depression may be a two-way street: A meta-analysis of 15 long-term studies that followed 58,000 participants for up to 28 years found that people who were obese at the start of the study had a 55 percent higher risk of developing depression by the end of the follow-up period, and people who had depression at the start of the study had a 58 percent higher risk of becoming obese.
Although a biological link between obesity and depression has not yet been definitively identified, possible mechanisms include activation of inflammation, changes in the hypothalamic-pituitary-adrenal axis, insulin resistance, and social or cultural factors.
Obesity and Mortality
Given the adverse consequences of obesity on multiple aspects of health, it makes sense that the condition also shortens survival or increases premature mortality. However, pinning down the contribution of obesity to premature mortality has been fraught with methodological problems and controversy.
Two of the biggest problems that researchers must cope with are reverse causation-low body weight is often the result of chronic disease, rather than being a cause of it-and the effect of smoking. People with BMIs below 25 are a mix of healthy individuals and those who have lost weight due to cancer or some other disease that may or may not have been diagnosed. Smoking also confuses the issue because smokers tend to weigh less than their nonsmoking counterparts. When reverse causation and the adverse effects of smoking aren’t fully accounted for, death rates among lean individuals will be inflated and those among overweight and obese individuals will be diminished. That was a problem with a widely reported study based on data from NHANES, which estimated relatively low numbers of excess obesity-related deaths. A careful critique of using the NHANES data to estimate mortality demonstrated that correcting for statistical biases significantly increased the estimate of excess deaths attributable to obesity.
The Bottom Line
Obesity harms virtually every aspect of health, from shortening life and contributing to chronic conditions such as diabetes and cardiovascular disease to interfering with sexual function, breathing, mood, and social interactions. Obesity isn’t necessarily a permanent condition. Diet, exercise, medications and even surgery can lead to weight loss. Yet it is much much harder to lose weight than it is to gain it. Prevention of obesity, beginning at an early age and extending across a lifespan could vastly improve individual and public health, reduce suffering, and save billions of dollars each year in health care costs.
