Obesity and Hypertension: A Growing Problem

Introduction

We have heard a lot lately about the epidemic of obesity, and the numbers are certainly scary. The prevalence of obesity has increased markedly in the U.S. and other countries in the past 20 years: in 1978, one quarter of Americans were overweight, as defined by a body mass index (BMI) of 25-30 kg/m2, and in 1990, one third were overweight -- a 33% increase. The latest figures from the Centers for Disease Control and Prevention show that 60% are now overweight. Similarly, the prevalence of obesity (defined as a BMI of >30 kg/m2) has increased from 12% in 1991 to 19% in 1999.[1] It has been estimated that 300,000 deaths per year are attributable to obesity, and that it accounts for nearly 10% of national health costs.[1]

The consequences of this stealthy epidemic are only now being realized. The greatest publicity has been given to the alarming increase in juvenile diabetes, but it can be anticipated that hypertension will also be on the rise. In children and adolescents, the rate of increase in BMI, but not in height, is a strong predictor of the adult levels of blood pressure, insulin resistance, and lipids.[2] If the current trends continue, and there is not the slightest reason to think otherwise, we can anticipate an end to the decline in cardiovascular morbidity in the near future.

The reasons for the epidemic are not hard to discover. As we all know, body fat is determined by the balance between calorie intake and expenditure, both of which have strong cultural and behavioral components. The importance of physical activity as a determinant of body weight was well demonstrated in a 1970 publication of the Seven Countries Study conducted by Ancel Keys.[3] The group with the lowest calorie intake -- the Belgrade medical faculty -- were actually the most obese, while at the other end of the spectrum, Finnish lumbermen had the highest calorie intake and were the least obese. This paradox is explained by the close relationship between obesity and calorie expenditure, reflecting the predominance of physical activity over food consumption. There is evidence that American children are less physically active today than in the past, both at home and at school. Between 1984 and 1990, the percentage of high school students enrolled in physical education classes declined substantially.[3] Evidence that children today are less fit than previously comes from a comparison of the average time needed to complete a standard endurance run, which increased between 1980 and 1989.[4] There is also evidence that physical inactivity, as measured by television watching, correlates with several measures of obesity in children.[5] A lack of physical activity is also implicated by an analysis of adult Americans from the National Health and Nutrition Examination Survey (NHANES) database,[5] which confirmed the increase in BMI between 1978 and 1991, but found that total calorie and fat intake actually decreased over the same time span. The importance of the energy expenditure side of the equation was demonstrated by a study of Leibel et al.,[6] who measured changes in total energy expenditure in a group of sedentary, obese subjects before and after diet-induced weight gain and loss. The catch is that weight loss resulted in a reduction of resting energy expenditure and weight gain produced an increase, so that merely changing the diet will not result in the expected change in weight. This is why exercise is so important.

On the intake side, there is evidence that it is not just the amount that children eat that matters, but also what they eat. In the Coronary Artery Risk Development in Young Adults (CARDIA) study,[7] it was found that fiber consumption more strongly predicts weight gain, insulin levels, and other cardiovascular risk factors than does fat consumption. Another report from the same group[8] noted that sugar-sweetened drink consumption also predicted weight gain in children. The explanation is that the calories in sodas slip into the body unnoticed, and do not trigger the feeling of satiety that would normally occur with less energy-dense foods. The same mechanism might explain why low-fiber foods lead to obesity.

The long-term consequences of adolescent obesity are already clear. About 25% of obese children between the ages of 5 and 11 have elevated blood pressure,[9] and the Bogalusa Heart Study[10] showed that overweight adolescents are eight times more likely than lean adolescents to have hypertension as adults. Also, skinfold thickness in adolescence predicts high LDL, high triglycerides, and low HDL in adulthood.[11] Of course, not all overweight children grow up to be obese adults (the best estimate is that about one third will, however); on the other hand, only a small proportion (18%) of obese adults were obese as children.[9] The investigators of the Bogalusa Heart Study were able to obtain autopsy data on 93 of the subjects, who died from various causes (mostly trauma) between the ages of 2 and 39 (mean age, 20 years).[12] Among the cardiovascular risk factors, BMI, blood pressure, LDL and HDL cholesterol, and smoking were all related to the extent of plaques in the aorta and coronary arteries. The number of risk factors present in each subject correlated closely with the extent of the plaques. Finally, at least six published studies, reviewed by Must and Strauss have assessed the effects of childhood obesity on mortality. Despite varying definitions of obesity, the findings are quite consistent and indicate that childhood obesity results in a relative risk of about 1.5 for all-cause mortality and 2.0 for coronary heart disease mortality. The implication of these studies is that the incidence of new cases of cardiovascular disease is almost certainly going to increase in the next few years, as the plaques now incubating in our overweight adolescents finally mature. There are adverse social consequences of obesity as well. Men and women who were overweight in adolescence are less likely to be married by their late 20s, and for women there are additional penalties -- less education and lower income.[13]

A major issue is whether the increase in BMI in adolescents is accompanied by an increase in blood pressure, which one would certainly expect, although it has not so far been apparent in the NHANES surveys performed in adults. The best information we have on this comes from a survey of 8222 Minneapolis schoolchildren aged 10-14 years, which was taken in 1986 and repeated in another 10,241 children in 1996.[14] BMI increased in both boys (by 0.8 kg/m2) and girls (by 0.9 kg/m2), and systolic pressure also increased (1.5 mm Hg in boys, and 0.7 mm Hg in girls). Although numerically small, these changes were all highly statistically significant. The big surprise was that diastolic pressure, which was recorded using both the 4th and 5th phase of the Korotkoff sounds on both occasions, fell by 1-2 mm Hg in both sexes and with both methods. This finding is unlikely to be an artifact of measurement, since great care was taken in the training of the observers, but it has not been explained.

From a sociologic point of view, one of the more fascinating aspects of the obesity epidemic is the discrepancy between what is actually taking place and the public perception of what is the ideal body weight. One of the best examples of our obsession with thinness is the change in BMI of the winners of the Miss America Pageants, who showed a steady decrease in BMI between 1920 and 1990 (from 22 to 18 kg/m2), which places them below the World Health Organization criteria for malnutrition.[15] Another paradox is the observation that in the analysis of the NHANES data cited above, while the prevalence of overweight increased by 33% over a 20-year period, there was an even greater increase in the percentage of the population who consume low-calorie foods, up from 19% in 1978 to 76% in 1991.[5] Indeed, surveys have shown that more than two thirds of all Americans are trying to lose or maintain weight, but only 20% of those trying to lose weight are using the recommended combination of calorie restriction and exercise of more than 150 minutes/week.[16] A survey of high school students[17] indicated that 44% of girls and 15% of boys were trying to lose weight. An additional 26% and 15%, respectively, were trying to stop gaining weight.

There is no simple solution to the epidemic of obesity. Its origin lies in our culture and society, and is epitomized by the suburban mall, with its drive-through fast-food restaurants. Dealing with it will require action at several levels: policy, education (of both the public and heath care providers), and incorporation of a team approach to patient care that involves dietitians and health educators. We should encourage more vigorous physical activity in our schools, and place more emphasis on healthy diets. On the medical side, obesity has always been an orphan topic that no one wants to address, and physicians often feel that there is little they can do about it. In a recent survey of obese patients,[18] only 42% reported that their physicians had given them any advice to lose weight. However, it has been shown[19] that even brief counseling by a physician can influence patients' lifestyles, and can help them to lose weight. Obesity is too big a problem to ignore.

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