What is obesity?

Obesity is defined as: “A state of adiposity in which body fatness is above the ideal; a body mass index of 30-39.”

Morbid obesity is defined as: “A state of adiposity in which body weight is 100% above the ideal body weight; a body mass index of 45 or greater.”

(Mahan & Escott-Stump, 2000)

Obesity and overweight can be defined by using a measure called the Body Mass Index (BMI). This is calculated as follows:
BMI = mass (kg)/ height (m)2

A person is considered obese when their BMI is greater than or equal to 30 kg/m2, while morbidly obese patients have a BMI of 45 or more.

Overweight is regarded as a BMI between 25 and 29 kg/m2.

The BMI is regarded as a relatively good measure of body fat in most people, except someone who is very heavily muscled. In this case, the BMI could be high, but the percentage of body fat low. Body builders and people with highly developed muscles should have their body fat percentage measured to determine if they are overweight or not.

Other measures are often used in overweight or obese people to gauge the level of risk associated with their degree of overweight. The percentage body fat and the distribution of this fat are important and the waist-to-hip ratio is a simple method for determining a person’s distribution of body fat. This is determined by dividing the waist circumference by hip circumference. Waist circumference is defined as the smallest circumference between the rib cage and belly button. Hip circumference is defined as the largest circumference of the hip-buttocks region.

The table below relates BMI and waist-to-hip ratio with disease risk:



Disease risk



22.00 to 24.99

Very low

Less than 0.85

Less than 0.80

25.00 to 29.99


0.85 to 0.89

0.80 to 0.84

30.00 to 34.99


0.90 to 0.99

0.85 to 0.95

35.00 to 39.99


1.00 to 1.10

0.96 to 1.05

40.00 and above

Very high

Greater than 1.10

Greater than 1.05

Since the development of obesity is deeply rooted in the enlargement of fat cells, a person’s percentage body fat is important. This is accurately measured using a series of skin-fold estimates from different parts of the body. Other techniques such as electrical impedance are far less accurate.

The table below shows body-fat norms based on the percentage of mass consisting of fat:

Percentage fat




Essential fat

13.0 - 14.0

3.0 - 5.0


12.0 - 22.0

5.0 - 13.0


16.0 - 25.0

12.0 - 18.0

Potential risk

26.0 - 31.0

19.0 - 24.0


32.0 and higher

25.0 and higher

What causes obesity?

Overweight and obesity usually result from an energy imbalance, where more energy is ingested via food eaten than is used up for body processes and physical activity. There are three components to energy expenditure:

  • “Resting” energy expenditure or RMR (the amount of energy that the body requires to keep its functions such as respiration, digestion, blood circulation, etc, going - even when the person is sleeping or at rest)
  • Physical activity (work and exercise)
  • The thermogenic effect of food (the amount of energy available to the body from food, which is influenced primarily by the total energy content of meals, rather than their composition)

(Kinabo & Durnin, 1990; Bissoli et al, 1999)

Resting energy expenditure is most strongly associated with fat-free body mass and is influenced by body size, gender and hormones, and the percentage of lean muscle tissue in the body. Consequently men generally, but not always, tend to have a higher RMR than women and are less likely to gain weight. Restriction of energy intake (e.g. low-energy intake when trying to lose weight), lowers the RMR by up to 15% within 2 weeks, thus slowing down weight loss. This is one of the prime reasons why weight loss on slimming diets with very low energy contents (e.g. semi-starvation diets), tends to result in less than satisfactory weight loss. (Mahan & Escott-Stump, 2000)

The macronutrient composition of the diet also influences weight gain. Proteins, carbohydrates (starches and sugars) and fat are the three major macronutrients. The energy contents of 1 gram portions of carbohydrate, protein and fat are 16 kJ, 17 kJ and 37 kJ, respectively. A high fat intake (37 kJ/gram), therefore, usually produces an increase in weight, but excessive kilojoule consumption (overeating) of all 3 macronutrients will also cause weight gain.

In addition, the body is less efficient when storing carbohydrates and protein. Only the fat stores can readily expand to accommodate increasing levels of food intake above those required for daily energy needs.

The amount of energy expended in physical activity is directly related to body weight. In most people physical activity declines as they age and the vast majority find it difficult to maintain a regular exercise programme, particularly as they get older.

The thermic effect of food is the third component of energy expenditure. After eating there is a rise in energy expenditure equivalent to about 10% of the day’s total energy expenditure. However, in very fat or obese people this effect is not as marked because of problems with insulin resistance, which will be discussed later.

Obesity is a reflection of increased fat stores, both under the skin and in the so-called ‘fat depots’ in the rest of the body. Fat depots are gender-determined: in men fat is usually stored in the abdomen and back of the neck (buffalo hump), while women store fat in their hips, thighs and breasts. Deposition of abdominal fat is regarded as particularly harmful and is associated with an increased risk of type 2 diabetes, heart disease and other diseases of lifestyle.

Obese people have enlarged fat cells. The many chemicals made and secreted by these fat cells play an important role in the clinical consequences of obesity. Insulin, a hormone produced by the pancreas, also plays a significant role in the deposition and mobilisation of fat.

Research arising from the Human Genome Project indicates that between 45 and 60% of eating behaviour which can lead to obesity, is due to genetic factors. The interaction of a genetic disposition and lifestyle factors such as a high-fat or high-energy diet and lack of physical activity, is regarded as a prime cause of the current obesity epidemic.

(Joffe, 2010)

The detailed physiology and biochemistry of the development of obesity is now much better understood. This greater understanding is starting to contribute to the way in which obesity is treated.

Who is obese and what are the risk factors for developing obesity?

People can become overweight at any age, but it is more common at certain times of life.

Several surveys show that one third of overweight adults become overweight before the age of 20 and two thirds after that. This means that between 75% and 80% of adults will become overweight at some time in their lives.

Between 20 and 25% of the population will become overweight before the age of 20 and 50% after the age of 20.

Some overweight individuals will develop clinically significant problems such as diabetes, hypertension, gallbladder disease or the metabolic syndrome. The latter is a syndrome (collection of symptoms and signs), which is characterised by resistance to insulin (the body cannot use insulin effectively), problems with glucose tolerance, abnormal levels of lipids (fats) in the body (dyslipidaemia), central obesity (fat around the midriff), an increased risk of thrombosis and raised uric acid levels. It is easy to see how the metabolic syndrome can lead to other clinical problems such as diabetes, hypertension, gout and heart disease.

Obesity is on the increase in the Western world. In South Africa, more than 56% of women, 29% of men, and 10% of children suffer from overweight or obesity. Because obesity can lead to so many diseases of lifestyle, it is important that this condition should be prevented and controlled.

(Labadarios et al, 2008; Bartels, 2010)

Risk factors associated with obesity

  • Overweight parents (genetic factors and learned eating habits)
  • Lower socioeconomic status
  • Giving up smoking
  • Low level of physical activity
  • Low metabolic rate
  • Overweight as a child
  • Being a heavy as a baby
  • Lack of maternal knowledge of a child’s sweet and cool drink intake and snacking habits
  • Recent marriage
  • Pregnancy and particularly multiple births
  • Eating an unbalanced diet rich in fat or energy

Other factors that also impact on obesity:

Body weight during adolescence is a good predictor of adult weight status. Overweight adolescents have a five to twenty-fold increased chance of being overweight as adults.

Most women gain weight after puberty, which may be precipitated by pregnancy, oral contraceptives and the menopause.

Pregnancy itself may leave a legacy of increased weight with some women never regaining their pre-pregnancy weight.ghuio

Can obesity be prevented?

A population can be divided into four subgroups: never overweight, pre-overweight, pre-clinical-overweight and clinically overweight.

The first - never overweight – are people who will never become overweight. The second group includes all people who have a BMI below 25 kg/m2.

When someone becomes overweight for reasons that are not medically significant,  they are pre-clinically overweight. As time goes on, these same people may develop the medical complications associated with obesity such as diabetes, hypertension, and dyslipidaemia, and once these are present they are classified as clinically overweight.

Prevention strategies focus around these groupings. Pre-overweight and pre-clinically- overweight people are assessed in terms of the risk factors outlined above. Those normal- weight people who appear likely to become overweight should start measures such as a sensible, low-fat, moderately energy-reduced diet and a regular exercise programme to ensure that they do not gain weight.

People who are already overweight, but who do not yet have any associated medical problems should be encouraged to lose weight through a sensible, low-fat eating plan and a regular exercise programme.

If these interventions are made early enough, and people stick to them, then obesity can be prevented.

How is obesity treated?

The basic idea behind treating obesity is simple: reduce energy intake and increase energy expenditure. However, to be successful, this involves completely changing one’s lifestyle, something that is never easy.

It is important that the overweight or obese person understands the potential medical complications which can result from remaining overweight. This can provide motivation for weight loss.

People should be encouraged to change their eating patterns in such a way that they can maintain these changes for life. Fad diets, slimming supplements and pseudo-scientific diet programmes are seldom successful in the long-term and can be positively harmful.

A diet that is high in complex carbohydrates, contains medium amounts of protein and is low in fats, particularly saturated fats, will aid weight loss.

Exercise has a major role to play in encouraging and maintaining weight loss. Very high intensity exercise, generally sport-specific training, will increase the rate of weight loss with an appropriate diet. However, this is not possible for most people since they are not prepared to go to such lengths. But there is plenty of evidence to show that moderate to low intensity exercise will also aid weight loss. What is more important is that regular exercise helps to maintain weight loss over time. All adults should be encouraged to exercise at moderate intensity for about 30 minutes on most days. This can be walking, swimming, jogging, cycling, modern dancing, or any other activity that is enjoyable and uses up energy.

Changing from a generally sedentary lifestyle also helps. Parking the car further from the shops and using stairs instead of lifts are simple measures which can increase your daily energy expenditure.

Slimming drugs are a last resort and should only be used for people whose BMI is more than 30 kg/m2 and who have failed to lose weight through diet and exercise. Most of the slimming drugs currently available to treat obesity are appetite suppressants, which act on the central nervous system and are only approved for short-term use and under the supervision of a medical doctor.

Pharmacological slimming medications containing sibutramine can help patients achieve weight loss of 5 to 10% of their body weight. Its side effects include dry mouth, insomnia and constipation. However, recent evidence from post-marketing research indicates that sibutramine should not be used by anyone with blood pressure, heart or circulatory problems, as it can cause an increase in heart attacks and strokes in susceptible patients (Richwine, 2010). It is, therefore, essential that patients should be carefully screened by a medical doctor before sibutramine is prescribed and that individuals who suffer from heart or circulatory disease, should not take these medications. Even if obese patients have been declared fit to use sibutramine products for slimming purposes, they should be carefully monitored by their doctor for signs and symptoms of high blood pressure or irregular heartbeat.

Orlistat is a drug which blocks the absorption of fat in the intestine. In clinical trials lasting up to two years, this drug was associated with an average weight loss of 10% at the end of one year in patients who were eating a low-fat diet.

Effective use of either of these medications requires changes in lifestyle as well. There is always diet modification involved in weight loss, as well as an increase in physical activity if possible.

Bariatric surgery is reserved for those who suffer serious medical consequences of gross obesity and who cannot lose weight in any other way.

With any treatment it is important to know what your goals are and obesity is no different. For most people, weight rises with age, slowly and inexorably. The main preventative strategy would be to stop further weight gain, which in itself would be beneficial in terms of averting the medical complications of obesity and decreasing the incidence of this condition in the population. Weight loss of less than five percent is generally considered inadequate and weight loss of up to 12% is required to prevent the onset of type 2 diabetes in susceptible people.

An ideal outcome of treatment would be a return of body weight to the normal range with no further weight gain later. However, this is unrealistic for most people. A more realistic goal is usually a loss of between five and 15% of body weight and good maintenance of the achieved target weight.

Weight loss of more than 15% of initial weight is regarded as an excellent outcome. In most patients, however, this is usually difficult to achieve in practice.

An improved quality of life is also an important outcome which can take many forms. Any reduction in the medical complications of obesity, such as type 2 diabetes and hypertension, will improve quality of life, not least because of the reduction in the cost of treating these conditions. Weight loss also reduces the wear and tear on osteoarthritic joints and in some cases slows or reverses the development of osteoarthritis altogether.

A loss of five percent or more of initial weight almost always translates into improved mobility, increased ability to exercise and greater self-esteem and confidence.

What is the outcome of obesity?

The major consequences of obesity are the increased rates of death and disease associated with the condition.

Most of the medical complications of obesity result from increased production of fatty acids due to the enlargement of fat cells. Other consequences are caused by the increased mass of fat in the body.

It has been estimated that in 2000, obesity was responsible for nearly 36 500 deaths in South Africa (7% of mortality). These excess deaths are usually due to heart disease, diabetes, hypertension and cancer.

(Joubert et al, 2007)

Diseases which seem to be associated with enlarged fat cells and their metabolic products include type 2 diabetes, heart disease, hypertension, gallbladder disease and cancer.

Even small increases in BMI and the size of fat cells are associated with a significant increase in the risk of developing type 2 diabetes. It appears that the metabolic products of the fat cells place an increased demand on the pancreas for insulin. Weight loss of 12% or more which is then maintained, may in some cases be able to restore the normal function of the pancreas, underlining the importance of the prevention and early treatment of obesity.

Increasing deaths from heart disease occur with increasing obesity in both men and women. This increase is most pronounced in people with BMIs greater than 27 to 29 kg/m2. This increased risk can be attributed to a number of things. Levels of high-density lipoprotein (HDL) cholesterol ("good" cholesterol) decrease in the presence of insulin resistance and obesity. Increased blood pressure associated with obesity also plays a role in increased risk of heart disease. All these factors can revert towards normal ranges if the patient can achieve weight loss.

Blood pressure rises with increasing BMI, as does the risk of developing gallstone disease.

Cancer of the womb (endometrium), breast, colon and gallbladder is more common among obese women. Cancer of the prostate and colon are more common among obese men.

Sleep apnoea is a serious problem among overweight people and is more common in men than women. In this condition the airways are intermittently obstructed at night, leading to fitful sleep and lack of oxygen. This leads to increased sleepiness during the day. The increased mass of fat in the pharyngeal (throat) area partly explains this complication of obesity.

Osteoarthritis eventually occurs in most older people, but is far more marked among overweight and obese individuals. In women in particular, damage to the knee and hip joints becomes severe in the overweight and obese. This is a potentially costly complication of obesity in terms of the discomfort caused by the damaged joints and the actual costs involved in replacing them if this should become necessary.

One of the most damaging consequences of obesity is social disapproval and stigmatization. Extra weight is obvious to everyone and fat people are generally erroneously characterised as being lazy and lacking self-control. Fat men and women suffer social stigmatization which detracts from their quality of life. Social stigmatization is particularly severe in women, where social disapproval is one of the main pressures for weight loss. Some studies have even shown that fat people have more trouble finding employment, and when they are employed, they are paid less than their leaner counterparts.

When to see your doctor/dietician

If you are gaining weight and/or are overweight already, then see your doctor or dietician to discuss ways of preventing further weight gain and losing weight if necessary.

Updated by Dr Ingrid van Heerden, D.Sc., July 2010.


Bartels K (2010). The grave truth about obesity. Press Release, 26/04/2010.

 Bissoli L et al (1999). Resting metabolic rate and thermogenic effect of food in vegetarian diets compared with Mediterranean diets. Ann Nutr Metab,43(3):140-4.

 Joffe Y (2010). The role of genetics in weight regulation. ADSA Workshop ‘The genetics of obesity and weight loss’. Pretoria, 26 July 2010.

 Joubert J et al (2007). Estimating the burden of disease attributable to excess body weight in South Africa in 2000. S Afr Med J, 97(8):683-690.

 Kinabo JL, Durnin JV (1990). Thermic effect of food in man: effect of meal composition, and energy content. Br J Nutr, 64(1):37-44.

 Labadarios D et al (2008). Executive Summary, National Food Consumption Survey- Fortification Baseline (NFCS-FB-II), South Africa, 2005. S Afr J Clin Nutr, 21(3)(Suppl 2):245-300.

 Mahan LK, Escott-Stump S (2000). Krause’s Food, Nutrition & Diet Therapy, 10th Ed., WB Saunders Co, Philadelphia, USA.

 Richwine L (2010). EU agency ban on diet drug. Reuters Health. 2010-01-21.

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