Introduction

We summarized the emerging views on syndrome X, also known as the metabolic syndrome, in a previous article ("Getting a Handle on Syndrome X" -- see link below). The recent new US guidelines on high blood cholesterol levels drew attention to the importance of this syndrome, and provided a working definition.¹ Now the Third National Health and Nutritional Examination Survey (NHANES III) has provided data that allows the estimation of the prevalence of the syndrome in a sample of the US population taken between 1988 and 1994. The findings of this study, recently reported in JAMA, are summarized here.

Method

The ATP III definition1 for the metabolic syndrome (syndrome X, or the insulin resistance syndrome) is three or more of the following:

• Waist circumference > 102 cm (40 inches) in men, >88 cm (35 inches) in women
• Hypertriglyceridemia: serum triglyceride >150 mg/dL (1.7 mmol/L)
• High density lipoprotein (HDL) cholesterol <40 mg/dL (1.04 mmol/L) in men, <50 mg/dL (1.29 mmol/L) in women
• Hypertension: >135/85 mm Hg
• Fasting blood glucose >110 mg/dL (6.1 mmol/L)

The investigators counted people reported as currently taking antihypertensives or antidiabetic medications as having one of the last two criteria, respectively. While obesity is associated with insulin resistance, abdominal obesity is more closely associated with the metabolic syndrome than the Body Mass Index (BMI).²

The prevalence of the metabolic syndrome was calculated for this USA population sample by age, sex, and race or ethnicity. Adjustments were made for age, so that the estimates were representative for the non-institutionalized US population.

Results

There were 8,814 people in the sample -- 4,265 men and 4,549 women. There were 3,599 white persons, 2,412 African-Americans, and 2,449 Mexican-Americans.

The overall unadjusted prevalence of the metabolic syndrome was 21.8%, and, after adjustment for age of the sample, it was 23.7%. There was a clear-cut age-related increase: the prevalence was 6.7% in 20 - 29 year-olds, increasing to 43.5% in 60 - 69 year-olds. Over 70, the prevalence leveled off at 42%. There was little difference between the prevalence in men and women.

Below is the age-adjusted prevalence of the syndrome for different ethnic groups:

	Women	Men	Sexes Combined
White	22.8%	24.8%	23.8%
African-American	25.7%	16.4%	21.6%
Mexican-American	35.6%	28.3%	31.9%

Women clearly had a higher prevalence among African-Americans and Mexican-Americans, while overall Mexican-Americans had the highest prevalence of the condition.

Conclusions

We make no excuse for bringing up the metabolic syndrome in these pages three times over the last 18 months. Even if prevalence rates remain unchanged since the NHANES III was done almost 10 years ago (which is unlikely, given the increasing rate of obesity in the USA), it seems likely that 50 million US residents have the syndrome.

Although the syndrome was clearly shown to have a genetic component in Scandinavia when it was first described, heredity is a less likely cause in the USA. Insulin resistance is an underlying feature (hence the synonym 'insulin resistance syndrome'); however, it is not always present. Hypertension, too, is an inconstant feature.

Can it be that the metabolic syndrome is not a single entity, but rather a convenient grouping of people with several recognized diabetic and cardiovascular risk factors? Time will tell, but the question is really academic -- there is no doubt that the presence of the metabolic syndrome is associated with increased morbidity and mortality. ³, ⁴ The syndrome nomination can be regarded as a scientific refinement of the coarse indicator of the rapidly growing health problem in USA -- obesity.

The authors of the present study are somewhat gloomy about the chances of tackling the problem represented by the metabolic syndrome. Proliferation of US-originated fast-food franchises in many other countries in the world suggests that it may well emerge everywhere there are no actual food shortages. It is to be hoped that the US experience will serve as a warning to others.