Reducing the Global Burden of Cardiovascular Disease: The Role of Risk Factors

James W. Levenson, MD, MPH, Patrick J. Skerrett, MS, J. Michael Gaziano, MD, MPH

Disclosures

Prev Cardiol. 2002;5(4) 

In This Article

Regional Trends in CVD Risk Factors

In large part, the global variation in CVD rates is related to temporal and regional variations in known risk behaviors and factors. Recent ecologic analyses of major CVD risk factors and mortality demonstrate high correlations between expected and observed mortality rates for three main risk factors -- smoking, serum cholesterol, and hypertension.[8] These analyses suggest that much of the regional variation is based on differences in conventional risk factors. Thus, identification and management of largely avoidable risk factors can substantially reduce age-adjusted death rates.

Smoking clearly represents an important avoidable cause of CVD and total mortality. In 1990, an estimated 6% of all deaths worldwide were attributable to tobacco. But as smoking rates rise in the developing economies, smoking will account for more than 12% of deaths by 2020,[9] killing more people than any single disease, including human immunodeficiency virus infection.[2]

Currently, more than 1 billion of the world's inhabitants smoke.[1] In the EstMEs and EmgMEs, smoking rates are beginning to decline, with the most substantial changes in the former (Figure 4). In the United States, for example, more than 40% of adults smoked in 1965, while only 23% smoked in 1997.[10] However, while cigarette smoking is declining overall, smoking is on the increase among young men and women in the United States.[11] In the EmgME countries, smoking rates are extremely high -- 59% of men and 26% of women smoked in 1995 -- but rates are stable or falling.[12]

Worldwide trends in per capita cigarette consumption, 1971-1991Reproduced with permission from World Health Organization. Making a Difference. 1999.[1]

In the DevE countries, tobacco represents an important cash crop and source of employment, two things that are often in short supply. On average, about 48% of adult men smoke, and smoking rates are increasing about 3.4% per year.[1] In some countries, included in the DevE group, smoking rates among men are staggeringly high, reaching 73% in Vietnam[13] and even higher in parts of Nepal.[14] Throughout the developing world, women have traditionally represented only a small proportion of the number of smokers. That is certain to change; as women's spending power increases, tobacco companies are targeting them as customers, even as women-specific health education and quitting programs are rare.[15]

In the EstME countries, cigarette smoking peaked late in the third phase of the transition. A unique feature of the DevE countries is the easy access to relatively low-cost cigarettes during early stages of the epidemiologic transition. In many DevE countries, which are in the first or second stage, smoking rates among men already exceed peak rates in the EstMEs and are expected to continue rising among both men and women. The impact of higher rates of smoking earlier in the epidemiologic transition means a more rapid increase in CVD rates as a country enters the third phase of the transition. In China, which is in the early stages of the epidemiologic transition, the 1996 National Prevalence Survey determined that 63% of men (but only 3.8% of women) were current smokers.[16] A massive retrospective study of 1 million deaths estimated that tobacco was responsible for 13% (600,000 deaths) of total mortality in China in 1990, and will account for 3 million deaths per year by 2025.[17]

Hypertension is clearly recognized as a risk factor for CHD and stroke. Elevated blood pressure, possibly associated with industrialization or urbanization, is an early hallmark of the transition out of the age of pestilence and famine. Among men and women living in rural communities in India, for example, the prevalence of hypertension is 14.0% and 10.8%, respectively, while among those living in urban areas it is 25.5% and 29.0%, respectively.[18] Although the relative increase in mortality associated with a given increase in blood pressure is similar in various regions of the world, the absolute risk at the same blood pressure level varies greatly.[19] In addition, the overall impact of hypertension may vary, depending on the proportion of individuals in a country with untreated hypertension.

In the EstME countries, hypertension remains a major cause of CVD morbidity and mortality despite high rates of detection and treatment. Given the direct relationship of blood pressure with age, the prevalence of hypertension is increasing in the mostly aging EstME countries. In this group, the proportion of the population with untreated hypertension is generally declining, though in the United States there has been a slight reversal of this trend.[20] In the EmgME countries, the prevalence of hypertension is at least as high as it is in the EstMEs, while rates of treatment are much lower. This may partly explain the higher stroke rates in these countries in relation to CHD rates.

Across the DevE countries, hypertension rates are quite variable. In northern Asian countries, such as China and South Korea, for example, hypertension is rapidly increasing,[21,22] with higher rates in urban areas than rural areas. One major concern in these countries is the high rate of undetected, and therefore untreated, hypertension.

While dietary habits clearly vary from country to country, intake of dietary fat, particularly animal fat, generally increases with annual per capita income, and thus tends to be low in many DevE countries and high in many EstME countries (Figure 5). For example, fat contributes less than 20% of calories in rural China and India,[23] compared with approximately 35% in the United States. Caloric contributions from fat appear to be falling in the EstME. In the United States, for example, the percent of calories from fat has steadily declined over the last 30 years, from 45% of calories in 1965 to 34% in 1994, although the total amount of fat in the diet has increased slightly since 1989.[24] Even in the DevE countries, which are broadly characterized by low fat intake, fat intake varies greatly, tending to increase with industrialization and urbanization. Compare, for example, the less than 20% contribution of calories from fat in rural China and India with the 36% contribution in Taiwan.[25]

Association between income and dietary intake, based on country-level sources of energyReproduced with permission of International Life Sciences Institute from Drewnowski A, Popkin BM. The nutrition transition: new trends in the global diet. Nutr Rev. 1997;55:31-43.

The causal association between plasma cholesterol levels and risk of CVD is indisputable. Low levels of HDL and elevated triglycerides are also clearly associated with excess risk of CVD, and this association holds across racial and ethnic divisions. The lipid profile appears to have a greater impact on CHD than on stroke.

As countries move through the epidemiologic transition, mean population levels of plasma cholesterol tend to rise. This shift is largely driven by greater consumption of dietary fats -- primarily from animal products and processed vegetable oils -- and decreased physical activity. Cross-cultural differences in mean cholesterol levels reflect this pattern. In rural Nigeria, which is in an early phase of the epidemiologic transition, mean cholesterol levels are 120 mg/dL (3.1 mmol/L),[26] while in the heavily industrialized United States and northern Europe, which are in the fourth phase, mean cholesterol levels are 200 mg/dL (5.2 mmol/L) and 240 mg/dL (6.2 mmol/L),[27] respectively. In the EstME countries, mean population cholesterol levels are generally falling, while in the EmgME countries, they tend to be high but stable or rising.

In the countries constituting the DevE group, mean population cholesterol levels tend to be increasing rapidly, although there is wide variation. A population survey in Bogota, Columbia, for example, found that 46% of men had serum cholesterol levels greater than 250 mg/dL (6.5 mmol/L).[28] In sub-Saharan Africa, mean cholesterol levels in rural areas are similar to the low levels seen in China, while levels are considerably higher in urban centers.[29] Given the high level of global capacity to produce Western-style food products at low cost, developing countries can now afford to adopt a Western dietary lifestyle earlier in the economic transition than was possible in the past.

As is true for hypertension, rates of hypercholesterolemia are increasing far faster than the resources needed for widespread detection and treatment. Thus, the impact of both of these on atherosclerosis may be far greater than they have been in developed economies.

One byproduct of the increased mechanization that accompanies the economic transition is decreased physical activity. In the United States, approximately 25% of the population does not participate in any leisure-time physical activity and only 22% report engaging in sustained physical activity for at least 30 minutes on 5 or more days per week (the current recommendation).[30] The shift from physically demanding, agricultural-based work to largely sedentary industrial- and office-based work is occurring throughout the developing world. This is also accompanied by a switch from physically demanding transportation to mechanized transportation.

Diabetes mellitus (DM) and impaired glucose tolerance represent strong risk factors for vascular disease. As a consequence of, or in addition to, increasing body mass index and decreasing levels of physical activity, worldwide rates of DM -- predominantly type 2 (non-insulin-dependent) DM -- are on the rise. According to World Health Organization models, the number of individuals with DM will swell from 135 million people in 1995 to 300 million in 2025, a 35% increase in worldwide prevalence (from 4.0% to 5.4%).[31] The largest increases will be in China (up 68% between 1995 and 2025) and India (up 59%), followed by Latin America and the Caribbean (41%), other Asian countries and islands (41%), and the Middle Eastern Crescent (30%). The EstME and EmgME groups will experience increases between 26% and 28%.

While much of the increase in DM can be traced to changes in diet and physical activity, the clear genetic susceptibilities of various racial and ethnic groups also play a role. For example, Pima Indians living in the southwestern United States are eight times more likely to develop DM as the general US population,[32] and Hispanic Americans have higher rates than white Americans.[33] Migration studies suggest that South Asians and Indians also tend to be at higher risk than those of European extraction.[34]

Obesity is clearly associated with increased risk of CHD. However, much of this risk may be mediated by other CVD risk factors, including hypertension, DM, and lipid profile imbalances. Obesity tends to increase later in the epidemiologic transition, unlike smoking and hypertension, which tend to increase earlier.

In the mid-1980s, the World Health Organization's Multinational Monitoring of Trends and Determinants in Cardiovascular Disease (MONICA) Project sampled 48 populations for cardiovascular risk factors. In all but one male population (China), and in most of the female populations, between 50% and 75% of adults aged 35-64 were overweight or obese.[35] A later follow-up study showed that the prevalence of obesity has continued to increase.[36] In many EstME countries, mean body mass index is rising at an alarming rate even as mean plasma cholesterol levels are falling and age-adjusted hypertension levels remain fairly stable. In the United States, increases in body mass index are occurring in all sectors of the population, and are increasing fastest among minorities and women.[37] Overweight and obesity are not limited to established or emerging market economies. In many of the DevE countries, obesity appears to coexist with undernutrition and malnutrition and, while it is not as common as in the EstME and EmgME countries, it is on the rise there as well. In Mauritius, for example, rapid increases were documented in obesity between 1987 and 1992.[38] Among African women living in the Cape Peninsula, an estimated 44% were obese.[39]

Further increases in the prevalence of overweight and obesity are to be expected if the alarming data on childhood and adolescent obesity hold true,[40] with sharp increases in developed countries, such as the United States[41] and the United Kingdom,[42] but also in China[43] and other developing countries.[44] Early obesity not only increases the likelihood of adult obesity, but it also increases the prevalence of weight-related disorders, including CVD.

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