Title: Obesity - why the increase?

Key words: chronic disease, genetic factors, energy, dietary fat intake, energy density, resting metabolic rate, stress, weight gain, micronutrient intake

Date: March 2001

Category: Weight management

Type: Article

Author: Kate Neil (NS3)


Obesity - why the increase?

Obesity threatens to become the foremost cause of chronic disease in the world and is increasing in incidence1. Reasons for the increase include: urbanisation of the population, increased availability of food supplies, and reduction of physical activity1.

Moderate obesity is undoubtedly multi-factorial in origin. Acquired influences probably exceed genetic factors in its causation1. Appetite drive in humans is variable and could be under genetic control, as could be the regulation of energy expenditure, which could encourage obesity in sensitive individuals1.

Obesity results from an imbalance between energy intake and energy expenditure2. There is no conclusive evidence from epidemiological studies that under isoenergetic conditions dietary fat intake promotes the development of obesity more so than other macronutrients2.

US dietary surveys indicate that the percentage of total nutrient energy consumed as fat has substantially decreased over the past two decades and this has corresponded with a massive increase in obesity3. There appears little doubt that a reduction in nutrient energy intake with aging is required to prevent age-related weight gain, but that weight gain cannot be prevented simply by substituting carbohydrates for fat1.

Around 20-25% less calories per head per day is eaten than in the mid 1970ís (there may be biases in the data)4. The proportion of carbohydrate calories has declined and those of fat has increased, protein unchanged4. Research suggests that itís the energy density of the diet that is important in determining interaction through physical activity4. Fat proportion, because it affects energy density, interacts with the level of physical activity to determine energy balance4.

Several reports note that the resting metabolic rate (RMR) declines with age, likely as a result of age-related reduction in muscle mass1. If energy intake does not fall in parallel with loss of muscle mass, body fat will increase1. Maintaining muscle mass through physical activity should receive a high public health priority as adults age1 .

Reduced physical activity often starts in childhood1 e.g.

The alarming increase in obesity in children and adolescents can be partly explained by these changes in society and culture, which is usually carried into adulthood1.

Although much of the weight gain with aging is probably related to declining energy expenditure, food intake may actually increase after age 201. Once in a settled relationship, meals often become: more regular, frequent or generous in size, as eaten in restaurants or social gatherings1.

A common pattern of response to increasing stress levels in adult life is to fall back on pleasurable escapes like eating, drinking and smoking. A common pattern in overweight, middle-aged adults includes periods of weight gain while under stress1.

Personal and parental educational attainment and socio-economic status have been found to be significant predictors of higher levels of adult activity5. Women from social class 4 and 5 are also more likely to be obese4.

Research has suggested that a major controller of our daily food intake is the ability of our body to sense the presence of nutrients in the blood6. Perhaps, considering changes in micronutrient intakes alongside changes in macronutrient intake would be of merit?



  1. Grundy S, Multifactorial causation of obesity: implications for prevention, Am J Clin Nutr 1998;67(suppl):563S-72S
  2. Seidell, J, Dietary fat and obesity: an epidemologic perspective, Am J Clin Nutr 1998:67(suppl):546S-50S
  3. Willett, W, Is dietary fat a major determinant of body fat?, Am J Clin Nutr 1998;67(suppl):556S-62S
  4. Wiseman M, MSc lecture Surrey University 19/1/00, Principles of Applied Nutritional Science
  5. Trends in Patterns of Disease and Diet
  6. Koopmans et al (1991), Brain Res Bull 27:429-34