(Nutrition Foundation IN: Obesity And Physical Activity

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Obesity And Physical Activity

P.S. Shetty

Obesity is becoming an increasingly serious public health problem. It is now estimated that about 100 million people worldwide are obese. Obesity is associated with an increased likelihood of non-insulin dependent diabetes mellitus (NIDDM), hypertension, hyperlipidaemia and cardiovascular diseases. It is also associated with increased rates of breast, colo-rectal and uterine cancers. Obesity is thus an important factor in the increasing morbidity and mortality due to chronic non-communicable diseases.

The practical and clinical definition of obesity is based on the Body Mass Index (BMI; weight (kg)/height (m2)]1. It is generally agreed that a BMI of greater than 30 is indicative of clinical obesity, while a BMI of 25.0-29.9 is suggestive of overweight in an individual. The recommended cut-offs are appropriate for the identification of the extent of overweight or obesity in individuals and population groups. The WHO Expert Committee concluded in its Report 1 that weight gain and overweight or obesity is associated with increased morbidity and mortality and that weight cycling may also be associated with increased morbidity and mortality. The Committee was of the opinion that weight loss in obesity is difficult to sustain and is of uncertain benefit to health in the long-term and may lead to weight cycling. Hence, the primary prevention of obesity must be our main concern.

Secular Trends In Obesity

The prevalence of obesity is increasing both in developed and developing countries. Industrialised, developed countries are showing increasing trends in prevalence of obesity over the last two or more decades. Developing countries are also showing a rise in overweight and obesity among their populations along with economic development and urbanisation. Two critical factors that have influenced this explosion in the prevalence of obesity are changes in dietary intake and levels of physical activity. Obesity is the result of energy intake being chronically in excess of energy expenditure, resulting in a positive energy balance and weight gain.

In developed countries, despite the steady decline in per capita energy intake over the last three to four decades, the levels of physical activity have also declined due to sedentary lifestyles. Occupational activity levels have declined, and in spite of an increased participation in leisure time activities at most ages, energy expenditure levels still continue to decline. The growing concern of industrialised affluent societies about secular trends in the prevalence of obesity, particularly among children2, is also mirrored in industrialising, developing economies in rapid transition. Repeat surveys in developing countries have shown an association between a dramatic increase in mean BMI of the population and acculturation indices3 or modernity scores4,5.

Changes in dietary intake, food consumption patterns and physical activity levels have also contributed to the problem of increasing obesity in developing countries.

Prevalence Of Obesity

Europe and USA: Prevalence of obesity is high in USA and Europe, particularly in eastern Europe and the Mediterranean countries. In USA, the prevalence of obesity is as high as 32 and 33.5 per cent in males and females respectively6, and this is despite the fact that surveys have used a BMI cut-off of 27.8 for males and 27.3 for females to define obesity. The trend is even higher among American blacks as compared to whites, and also among the indigenous population of American Indians and Hispanic Americans7.

Obesity is relatively common in southern and eastern Europe, especially among women8. Apart from the Netherlands, where prevalence of obesity has remained stable between 1974 and 19869, in most other countries of Europe the trends are indicative of an increase in obesity among the adult populations. For instance, the prevalence doubled from 6 and 8 per cent in 1980 to 13 and 15 per cent in 1991 in England10. Increases in the prevalence of obesity are also seen in most countries of Europe over the last decade. This prevalence has increased also in Scandinavian countries, and in Australia, Canada and Israel.

Africa and Asia: The prevalence of obesity is much lower in African and Asian countries. However, obesity is increasing in several countries of the developing world, particularly those in economic transition, and in some of them high rates of obesity are already evident in children as well as adults. The present prevalence of obesity is highly variable between developing countries and within the population of a country. In general, the prevalence of obesity is higher in women as compared to men. Table 111 shows the problem of adult obesity in some countries of Africa, Asia and Latin America. In Brazil, between 1974 and 1989, the prevalence of overweight increased from 3.1 to 5.9 per cent in males and from 8.2 to 13.3 per cent in females12.

In the Asian region, recent data from the China Health and Nutrition Surveys show that the percentage of overweight in the study population increased from 5.3 to 6.5 per cent (severely overweight from 2.3 to 2.9 per cent), in rural areas from 8.2 to 9.8 per cent (severely overweight from 4.9 to 5.4 per cent), in urban residents between the survey years 1989 and 199113.

Over this short period of time, an increase of 2.7 and 2.4 per cent was noticed in the numbers of the overweight alone, in the middle and upper tertiles of household incomes, respectively.

Dietary Changes

Energy intake and food consumption patterns have changed in the developed world. In the UK, for instance, there has been a steady decline in per capita energy intake from the 1950s to the present. Over the same 50 years, there has been an increase in the proportion of fat and a consequent decrease in the carbohydrate content in the diet. The energy density and the fat content of the diet appear to have an important effect on the overall intake of food energy. Reducing the energy density of the diet by reducing fat has repeatedly been shown to lower energy intakes14, while increasing fat energy of a diet has been shown to increase food intake, at the same time interacting quite strongly with the level of physical activity15.

In developing countries, the rapid changes in dietary intakes have been indicative of an increase in per capita availability of food. Food balance data from FAO (1993) shows that in Asian countries such as China, Indonesia, Malaysia and Thailand, the change in energy intake has been small while large changes have occurred in the consumption of animal products, sugars and fats. National food consumption data for China for the period 1978 to 1987, collected by the State Statistical Bureau, shows that per capita cereal and vegetable consumption had increased and stabilised by 1984, while the consumption of meat, edible oils, sugar, eggs and fish increased throughout this period. The net effect has been a marked shift in diet with energy from fat (both animal and vegetable) increasing year by year. Countries such as China have thus not only altered overall dietary adequacy but have also seen a marked change in dietary composition, with increasing proportions of the population consuming more than 30 per cent of energy from fat. Data from Delhi also showed that higher income groups, consumed a diet with 32 per cent energy from fat while in the lower income groups only 17 per cent of the energy came from fat. More recent dietary surveys in Delhi also confirm that the higher income groups in urban India consume higher levels of energy and fat as compared to the urban poor or rural populations18.

Secular Trends

In the industrialised world, physical activity has declined as a result of the increasing mechanisation of life19. The time dedicated to employment or paid work in a single day or week has declined in several industrialised countries since the early 1960s and is the result of shorter work shifts, shorter weeks and longer vacations. This is compounded by the fact that the decline in time dedicated to productive work has been accompanied by a reduction in energy spent at work due to the increasing mechanisation of occupational work. Concurrent with this decrease in the energy expenditure due to occupational activities, increased urbanisation, universal use of motor cars, and increased mechanisation of most manual jobs outside occupational leisure time, has aggravated this trend. The increased leisure time is most often dedicated to television viewing, thus altering the structure of leisure time and encroaching on time normally allocated to other activities including weekday sleep20,21. In the US, television viewing has increased, leading to a steady decline in regular physical activity and an increase in undesirable levels of sedentariness22. In the UK, the increasing hours of television viewing have been shown to have a strong effect on the social classes, with the lower social classes showing higher hours of television viewing per week. This trend is associated with a higher prevalence of obesity23.

Computer games and videos in children, together with television watching, have been identified as one of the most important determinants of childhood and adolescent obesity24.

In the process of economic development, communities often evolve from rural societies where physical activity is needed for agricultural production, into urbanised and industrialised communities where the demand for physical labour and activity declines. There is little, if any, information on secular trends in patterns of physical activity in developing countries. The exception is China where the changes in the diet and the prevalence of obesity has been shown to be associated with marked changes in physical activity patterns. Data from China shows that urban residents in all income groups demonstrate a trend towards increased levels of sedentary activities in 1991 as compared to 1989. In contrast, rural Chinese show a significant increase from low and moderate activity patterns to high activity patterns (Table 2)25. Corresponding data on activity patterns of urban or rural residents of developing countries are not currently available and need to be collected.

Table 2: Physical Activity Patterns of Chinese Aged 20-45 by Residence and Income Levels

Physiological Responses To Physical Exercise

The human body’s physiological responses to episodic physical exercise (both aerobic and resistance) are seen in the cardio-respiratory, musculo-skeletal, endocrine and immune systems.

Cardio-respiratory systems respond to exercise in order to meet the metabolic demand for oxygen and nutrients, and for the disposal of metabolic waste products. Heart rate and cardiac output increase with an increase in exercise. Arterial blood pressure increases (largely seen as a rise in systolic blood pressure) and the pattern of blood flow changes dramatically in favour of increasing blood supply to the active skeletal muscles and to the skin for disposal of heat as body temperature rises with exercise. The coronary blood flow increases as a result of increase in coronary perfusion pressure and from coronary vasodilation. The arterio-venous oxygen difference increases with increasing work due to exercise and the increased oxygen extraction from arterial blood as it passes through the exercising muscle. The respiratory system also responds by increasing pulmonary ventilation both by increasing the rate and depth of respiratory movements.

Endocrine responses are aimed at integrating the various physiological changes and are essential to maintain homoeostasis during exercise. Table 3 summarizes some of the important endocrine changes that accompany physical exercise.

Immune response to exercise depends upon the intensity and duration of the exercise. Moderate exercise bolsters the function of certain components of the immune system which include natural killer cells, circulating T and B lymphocytes as well as the cells of the monocyte-macrophage system, and may thus help avert infections. However, high intensity exercise and exercise of long duration may have adverse effects on the body’s immune function and may be related to the fall in plasma glutamine levels that accompany such exercise.

Exercise training over a long period may result in long-term adaptations in skeletal muscle, bone, as well as in the cardio-respiratory systems.

Physical Activity And Health

The effects of physical activity on health and disease include the following:

Overall mortality: Higher levels of regular activity are associated with lower mortality rates among adults and even moderate activity on a regular basis results in lower mortality rates as compared with the rates for those who are least active.

Cardio-vascular diseases: Regular physical activity decreases the risk of cardio-vascular disease mortality, particularly the risk posed by coronary heart disease (CHD). The level of reduced risk of CHD attributable to regular physical activity is similar to that from not smoking. Exercise reduces blood pressure in hypertensives, while regular physical activity prevents or delays the development of high blood pressure.

Non-insulin dependent diabetes mellitus(NIDDM): The risk of developing NIDDM is lowered with regular physical activity.

Cancer: Regular physical activity is associated with decreased risk of colon cancer, while the association between physical activity and other cancers such as ovarian, endometrial, testicular and breast or prostate are largely unproved.

Osteoporosis: Although weight-bearing physical activity is essential for normal skeletal development as well as for achieving and maintaining peak bone mass during childhood and adolescence, it is unclear whether physical activity can reduce the accelerated bone mass loss that occurs in post-menopausal women.

Obesity: Low levels of physical activity contribute to the development of obesity while physical activity may favourably affect body composition and body fat distribution.

Mental health: Physical activity appears to relieve the symptoms of depression and anxiety and is a mood enhancer. It may also reduce the risk of developing depression.

Health-related quality of life: Physical activity appears to improve the health-related quality of life by enhancing psychological well-being and by improving physical functioning in persons affected by poor health, injuries and the development of osteoarthritis in later life.

Adverse effects: Physical activity of competitive nature may have adverse effects which include sports-related and musculo-skeletal injuries and the development of osteoarthritis in later life.

Chronic Diseases

Obesity is a major risk factor for several non-communicable diseases. There is a large body of epidemiological evidence that links obesity to increased mortality and the risk of chronic diseases such as cardio-vascular diseases, diabetes, hypertension, certain cancers, gall bladder diseases, osteoarthritis and osteoporosis.

The link between obesity and CHD28 is strong, although much of this relationship is ascribed to increases in blood pressure and cholesterol levels and the independent effect of obesity is either very small or non-existent. Nevertheless, it is important to recognise that weight loss in these individuals has a beneficial effect on both blood pressure and cholesterol levels (with HDL cholesterol levels rising), hence from a policy point of view it is useful to include overweight and obesity as a risk factor for heart diseases. Obesity is a major risk factor for NIDDM and the risk appears to be related both to the duration and degree of obesity.

Pubic Health Issues

It is important that developing countries, especially those in rapid transition, make immediate efforts to collate data on the prevalence of obesity in their populations and then set goals or targets to reduce its prevalence. The following two sub-groups within populations can be defined:

(i) Those who are already obese and who need help to reduce weight successfully to maintain it

(ii) Those who are at an increased risk of becoming obese and who need help to avoid it.

In addition, a strategy has to be developed to prevent the population from becoming obese and the median BMI of the country rising as an upward shifting mean would imply an increase in deviant numbers. While those who are obese need to be treated, public health measures which are preventive in nature are addressed to those who are at an increased risk and also the overall population. The two major arms of any preventive public health strategy to reduce obesity in the population would be to reduce the food energy from fat and to increase the levels of physical activity30.

Reducing food energy from fat can be achieved by:

Improving the nutrition knowledge of the public through education and information;
Increasing availability and access to foods with lower fat content;
Enhancing healthier food choices especially outside the home; and
Ensuring that health professionals/systems promote dietary changes.
Increasing the levels of physical activity can be promoted by:

Disseminating the positive health benefits of physical fitness;

  • Encouraging physical activity in educational institutions, for example, schools and colleges;
  • Providing opportunities for physical activity in work places and industrial setups;
  • Increasing public facilities for physical activity.

Each national group has to devise strategies which are culturally and socially relevant to its population with the broad aims of reducing dietary excess, decreasing fat proportion of energy and increasing levels of energy expenditure by promoting physical activity and discouraging sedentariness. The benefits are not only confined to reducing the problem of obesity but will also help reduce many other chronic non-communicable diseases and improve the health and well-being of the populace.


Developing nations have to learn from the experience of industrialized and affluent countries to tackle the emerging crisis of chronic diseases, which include the burgeoning problem of obesity among their populations. They have had to deal with the problems of under nutrition and malnutrition and are now providing sufficient outlays of resources to tackle these. The emerging health burden of chronic diseases, affecting mainly the economically productive adult population, will consume even more of their scarce resources. However, it is important to realize that poor countries will be hurt even further in the long run if strategies are not developed to address these emerging health problems on an urgent basis.

The traditional focus on under nutrition has to be widened to encompass all aspects of malnutrition, which includes over nutrition. Rather than separate issues of deficit and excess, we need to create health and nutrition messages that broadly address the concerns of unbalanced nutrition in the community -- both under and over nutrition. It is important to emphasize the need for primary prevention of diet and lifestyle-determined health problems such as obesity in countries where resources are scarce and limited. The need for a nutrition-driven policy that encompasses the cooperation of all sectors that influence dietary and food practices and lifestyles related to physical activity is essential to generate changes in health-related behaviour that will address both the problems of dietary deficiency and dietary excess.

P.S. Shetty is Professor and Head, Human Nutrition Unit, Department of Public Health and Policy, London School of Hygiene and Tropical Medicine.

The present excerpt is based on his presentation at the ANF symposium. The complete text will be published in the proceedings of the symposium.