Title: Obesity – causes and considerations
Key words: obese, mortality, fat, body weight, body mass index, BMI, physical activity, exercise, macronutrients, children, adolescents, muscle mass, genetic factors, brown fat, carbohydrates, wheat and sugar, omega 3
Date: Jan 2002
Category: Weight management
Author: Dr M Draper
Obesity – causes and considerations
A recent Daily Mail article (1) revealed the alarming statistic that 1 in 5 people in England is now classified as obese and leads with a title 'A deadly warning as heavyweights triple in 20 years ' (see Table 1 and 2 for change in incidence and mortality statistics). Obesity in the Western or Developed world is a rapidly increasing nutritional disorder that has short and long term health implications and threatens to become the foremost cause of chronic diseases (2). It is normal and desirable to have some reserves of fat in the body. In lean men about 16 per cent of body weight is fat at age 25, increasing to 24 per cent by age 65. In lean woman, about 30 per cent of body weight is fat at age 25, increasing to about 36 per cent by age 65. (3).
Definitions of overweight and obesity
Overweight is defined when a individual's weight is 10 to 20% greater than the acceptable range
Obesity is the term used when it is greater than 20% .
The World Health Organisation defines overweight using body mass index (BMI , equals body weight (in kg) divided by height squared (in metres). Acceptable range for BMI is 18.5-24.9 kg/m2. Values of below 20 are associated with undernutrition, while BMI between 25-29.9 indicates overweight and > 30 are classified as obese. Waist circumference below for men 94cm /37inches and woman 88cm/35 inches reflects low coronary risk and greater than 102cm/40inches and 88cm/35inches indicates the need for weight management and increased coronary risk.
Key factors underlying the increasing levels of obesity
The causes of obesity is often simplified into the following three components that govern energy balance: diet, physical activity and genetic factors which determine individual predisposition by effects on appetite and metabolism. Obesity results from an imbalance between energy intake and energy expenditure (4) and the interaction of these three factors. The rapidly changing pattern of excess body fat in UK (see Table1) indicates that the main factors are diet and lack of physical exercise because the genetic makeup is unlikely to have changed dramatically between 1980 and 1998 when the number of people classified as obese has trebled .
Does changing the dietary macronutrients influence obesity?
Populations with food provision in excess of requirements , with >30 % dietary energy from fat and low levels of physical activity are those with the highest prevalence of overweight people however there is no conclusive evidence that under isoenergetic conditions dietary fat intake promotes the development of obesity more so than other macronutrients(4). US dietary surveys indicate that the percentage of total nutrient energy consumed as fat has substantially decreased over the past 20 years and this corresponded with a massive increase in obesity (5). There appears little doubt that to to prevent age-related weight gain there must be a reduced intake and that weight gain cannot be prevented simply by substituting carbohydrates for fat(2). In the UK around 20-25% less calories per head per day is eaten than in the mid 1970's (there may be bias in this data) (6). The proportion of carbohydate has declined and those of fat has increased, protein unchanged (6). Research indicates that energy density of the diet is important especially in sedentary individuals because as the percentage of fat increases from 20 to 60% they develop more of a positive energy balance compared with active individuals (7). For sedentary individuals lowering dietary fat intake may help to prevent weight gain.
What are the effects of physical activity on obesity?
The trend of reduced physical activity through car ownership and mechanisation parrallels the increases in the hours spent watching television are associated with an increased risk of weight gain (6,8) and this often starts in childhood with increases in TV watching, playing computer games, transport to school and social activities and reduction in outdoor pursuits and games at school. (2, 9). The alarming rise in obesity children and adolescents (9) can partly be explained by these changes in society and is especially concerning because obese children tend to become obese adults. The resting metabolic rate (RMR) declines with age and this appears to be linked with an age related reduction of muscle mass (2). Maintaining muscle mass through physical activity should be a high priority in health education programs for children and aging adults. The effect of socioeconomic factors ie the increase in obesity with lower social class (6) may in part be explained by the above average rates of physical activity and below average hours watching TV comparing social class 1&2 with 4&5 (9).
What influence do genetic factors have on obesity?
Some overweight people strongly believe that they consume the same amount or less food than do their thin counterparts and recent research (10) shows this to be true. There maybe genetic and nutritional factors that influence a persons basal metabolic rate (BMR) and this could partly explain why some people gain weight more easily than others. One theory to explain this maybe variations in the amount of brown adipose tissue. There are two types of body fat , the ordinary subcutaneous fat which is mainly an energy store when excess calories are consumed and a different type of fat which is more metabolically active and is found between the shoulder blades and around the kidneys. The brown fat appears to have the ability to burn upexcess energy and appears to be linked to thermogenesis.
Obese people may have less brown fat or have brown fat which is less active (11,12). Some researchers suspect this may be one the causes of familial obesity. It is possible that a number of nutritional factors including the balance of vitamins, minerals and essential fatty acids may influence the activity and metabolism of brown fat.
How can we reduce obesity in the UK?
Obesity often starts in childhood and von Kries et al (13) in Bavaria found that the risk of obesity at age 5 or 6 years could be reduced by 35% if children had been exclusively breast fed for 3 to 5 months. The risk can be reduced even further if children are breast fed for longer than 5 months. Throughout the UK the percentage of woman breast feeding remains less than 44% at 6 weeks and 28% at 4 months (14). Women often stop breast feeding to return to work (perhaps explaining the lower rates among lower social class (14)) and health education programs to change the incidence of breast feeding have been unsuccessful between 1980 and 1995 (14). It does seem that in the vast majority of cases obesity is caused by consuming more calories than are needed to maintain normal weight. A theory put forward by Groves (15) for the increase in obesity seen in civilised populations is the increase in our diets of carbohydrates especially those derived from wheat and sugar. His arguments are based on archeological and anthropological data. He believes that during our evolution Homo erectus lived some 500,000 years ago was mainly meat eating and only when meat was in short supply did our ancestors eat nuts, berries and fruits. He uses anthropological data from modern tribes to show that when Eskimo in Greenland increase their intake of preserved food , dried potatoes, canned foods and cereals they develope the diseases of civilisation .(3) A similiar trend occurs when Masai in Africa eat less meat and replace it with maize and beans. He says the health of Kikuyu improved when they stopped being wholly vegetarian and started eating meat. The health trend we have all been encouraging is to reduce animal fats, increase carbohydrates and maintain protein, perhaps we need to look in a more complex way and remind ourselves that our present eating habits were influenced by the demands placed on agriculture by the industrial revolution and the need to produce food that could feed urban populations with food that did not perish. The present low levels of Omega 3 essential oils may be partly due to the fact that these oils do not keep and the popularity of white flour is because even the weevils will not eat it ! The eating of wholefoods (meats, nuts, seeds, vegetables and fruit) that have not been processed and modified may be a move in the right direction because the ratios of carbohydrate, protein, fats and oils tends to more closely paralleled to that which is considered healthy. We can apparently in the civilised world overproduce poor quality food and a return to organic methods, lower pesticide residues and increase mineral and vitamin intake may help to reduce the overall calorie intake, increase satiety and reduce body weight as some observational studies are suggesting (unpublished data).
1. Daily Mail Thursday 15 February 2001 p17.
2. GRUNDY, S, Multifactorial causation of obesity: implications for prevention, Am J Clin Nutr 1998 :67(suppl):563S-572S
3. BENDER, D.A.(1997) Introduction to Nutrition and Metabolism p179-196 Taylor & Francis.
4. SEIDELL,J, Dietary fat and Obesity: an epidemologic perspective, Am J Clin Nutr 1998;67(suppl):546S-550S.
5. WILLETT, W, Is dietary fat a major determinant of body fat? Am J Clin Nutr 1998;67(suppl.):556S-562S
6. WISEMAN,M, MSc lecture Surrey University January 2001 Principles of Applied Nutritional Science
7. STUBBS, R.J. , Macronutrient effect on appetite. Int J Obes Relat Metab Disord, 1995 Nov , 19 Suppl. 5: S11-19
8. JEBB, S.A.& MOORE, M.S, Contribution of a sedentary lifestyle and inactivity to the etiology of overweight and obesity: current evidence and research issues. Med Sci Sports Exerc, 1999 Nov.31:11 Suppl, S534-541.
9. REILLY, J.J., DOROSTY, A.R., EMMETT, P.M., Prevalence of overweight and obesity in British Children: cohort study - British Medical Journal 1999; 319:1039-1039
10.ROSE, G.A, WILLIAMS, R.T., Metabolic studies on large eaters and small eaters. Br J Nutr.1961;15:188.
11.JUNG, R.T, et al. Reduced thermogenesis in obesity. Nature 1979; 279:323.
12.HIMMS-HAGEN, J. ,Obesity may be due to a malfunctioning of brown fat. Can.Med.Assn J. 1979; 121: 1361.
13.VON KRIES, R. et al. , Breast feeding seems to reduce the risk of obesity in children - British Medical Journal 1999; 319:147-150. 14.Office for National Statistics. Breastfeeding in the United kingdom in 1995.
15. WAHLQVIST, M. L., KOURIS-BLAZOS, A., WATTANAPENPAIBOON, N. (1999) The Significance of Eating Patterns: An Elderly Greek Case Study - Appetite, v 32 n 1 p.23-32 Academic Press.
15. GROVES, B. (1999) The Calorie Fallacy p. 20-34 - Bookmarque 16. WILLIAMS, C.L. et al (1998) Healthy Start: A Comprehensive Health Education Program for Preschool Children - Preventive Medicine, v 27 n 2 p216-223. Academic Press.