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Obesity: an increasing problem across the globe

Michael Rennie
Symers Professor of Physiology
Division of Molecular Physiology
School of Life Sciences
University of Dundee

Normal body mass index (BMI: weight [kg]/height2 [m(2)]) is 18-24 for young to middle-aged men, and 20-25 for women. Values up to 28 for men and 30 for women are regarded as signifying overweight. Values above this signify obesity. In a normal healthy male, approximately 17% of the body weight is fat; for women, approximately 23%. In the obese the lower limits are almost double those figures. BMI increases naturally with age, but even in 60-70- year-olds it should not increase by more than 2 or 3 units above normal.
So why all the worry? Because obesity is linked to acute functional deficits (poor mobility, difficulties in breathing, poor temperature regulation in the heat) and increases the risk of chronic disease (coronary artery disease and hypertension, type 2 diabetes, cancer of the colon, fungal overgrowth of deep skinfolds, osteoarthritis, stress incontinence and sleep apnoea).(1)
In addition, obese people suffer social prejudice. They have difficulty buying fashionable clothes and fitting into cinema or airline seats. They often have a poor self-image and may suffer from chronic anxiety and depression.

The incidence of obesity has markedly increased in all developed societies over the last 50 years, but the increases are most pronounced in the USA, where almost 55% of the population are now defined as overweight or obese.(1) Other countries with increasingly Americanised cultures, including the UK, are rapidly catching up. The changes are slower in Europe, particularly the Nordic countries and Holland, where the incidence of obesity is only 25% of that in the USA. This large and rapid rise in the prevalence provides a strong argument against a simple genetic cause (see below).

There is also an increase in the rate of childhood obesity, again more obvious in the USA. Obesity is a disease of general affluence and leisure, and even in developing countries (such as China and India) it affects the children of the affluent elite. Nevertheless, it is the lower social classes, in whom unhealthy eating and sedentary leisure patterns are more prevalent, who are most affected.
Excess fat is stored when energy expended (chiefly through physical activity) is less than energy intake (as food). This is effectively a restatement of the first law of thermodynamics. It was proven by the extensive studies carried out at the Dunn Nutrition Institute in Cambridge by Prentice and coworkers in the 1990s.(2) There are no metabolic inefficiencies, hormonal balances or inherited characteristics that can adequately explain the current incidence of obesity, and individuals who claim a metabolic peculiarity are, in the majority of cases, wrong. This is consonant with the observation that obesity has increased markedly over a period when the genetic and other metabolic characteristics of individuals in populations have remained constant.

Nevertheless, studies of twins reared apart and investigations into the susceptibility to obesity of genetically isolated populations suggest that some people may have genetic predisposition to a number of metabolic problems that cause, for example, development of insulin resistance in skeletal muscle (but not fat) in response to overeating. This leads to chronic hypoglycaemia and thus an increased rate of fat deposition. Being chronically overweight may set a metabolic catch, possibly through insulin resistance, which becomes difficult to undo, making weight loss difficult. Analogous neurochemical defects may adversely affect satiety in a small number of people.


Diet and exercise
The cheapest and most successful treatment is better education and the imbuing of healthy eating and activity patterns. Obesity is becoming recognised as a major public health issue, and there should be local resources available to help obese patients come to terms with their problem and to deal with it. Obesity is a life-diminishing and shortening condition, and those who suffer from it deserve respect and help. From a purely utilitarian point of view, the cost of associated chronic healthcare is enormous and unnecessary.
The only realistic long-term treatment for most patients is a hypoenergetic diet, limiting food (and ­alcohol, which has 75% of the energy density of fat), combined with an exercise programme. However, experience suggests that this is difficult for most overweight people to achieve, especially since they want fast results. Even total starvation with the maintenance of a normal physical energy expenditure of about 4-5MJ/day would cause a loss of only about 100g of fat/day. Since semistarvation causes a fall in the basal metabolic rate (due to decreased thyroid hormone secretion and conversion to the more active tri- iodothyronine), it is not an ­efficient way for obese people to lose weight.

Lean tissue protein sparing
An increase in physical activity has the twin benefits of increasing the energy expenditure and preserving muscle mass. However, the idea that replacing fat with lean tissue will allow the basal metabolic rate to rise and permit dietary intake at the accustomed rate before weight loss is wrong. All weight loss is accompanied by loss of lean tissue (muscle) mass. The aim is to make sure that this is of the order of 33% and not 50% of the weight loss. This is best accomplished by a gradual reduction of weight - say 1-2kg per week.

Fad diets
Modern fad diets are rarely subjected to clinical trials, so evidence of efficacy that is more than anecdotal is ­difficult to obtain. Nevertheless, it appears that high-protein, low-energy diets, despite claims by their ­proponents, are no more effective than a general low-energy/exercise regime. Removing carbohydrate from the diet and concentrating on high-protein foods induces satiety quickly, but since most high-protein foods also contain fat (often "invisible"), the energy intake may not be reduced by much. Also, such diets may cause increased ketosis (due to oxidation of excess protein and lack of carbohydrate for complete fat oxidation). It may also be that continued consumption of an inevitably increased proportion of dietary saturated fat as a result of cutting out carbohydrate will cause harm by promoting heart disease and bowel cancer, and lack of carbohydrate is associated with slowing of gastrointestinal motility.
Rate of weight loss and keeping it off
The maximum rate of weight loss should be moderate (about 1-2kg per week with diet alone, or double this with exercise). This will allow the patient to learn to adjust their eating behaviour. Maintaining weight loss is difficult, and previously obese patients may need substantial support to accommodate a lifelong change of lifestyle. Cognitive behavioural therapy may be of major benefit in countering poor self-image and defeatist attitudes to weight loss.
Pharmacological approaches
Standard practice in the USA is to reserve pharmacological treatment for patients with a BMI over 30 and with no additional risk factors, or patients with a BMI over 27 with sleep apnoea, hypertension, dyslipidaemia, coronary heart disease (CHD) or type 2 diabetes.(1) The National Institute for Clinical Excellence (NICE) has recommended a slighter tougher set of recommendations, with the cutoffs at 35 or 30 with comorbidities.(3,4)

Orlistat inhibits pancreatic lipase activity in the small intestine, so that the components of fatty food (which are normally extremely efficiently digested, absorbed and stored) become largely unavailable to the body. In clinical trials, orlistat appears to be effective in controlling energy intake, but, as pointed out above, without an exercise programme to increase energy expenditure the rate of weight loss is low. Furthermore, since the fat that is not absorbed is passed to the colon, where the local fauna have little difficulty in metabolising it, there are a number of side-effects, including flatulence, occasional bloating and the production of noxious fatty stools that can result in what the manufacturers coyly call "spotting" of underwear. Long-term use may necessitate fat-soluble vitamin supplementation (particularly vitamins A and D).

A second approach has been the use of anorectic agents. There is a long history of pharmacological suppression of appetite: amphetamines were readily prescribed by doctors for rich clients 50 years ago. The associated unwanted effects on cognitive ­function, and other side-effects such as anxiety and dependency, have weakened the popularity of this approach. Even drugs that have few of these side-effects were shown to cause cardiovascular problems (eg, dexfenfluramine).
More modern drugs (such as sibutramine) target the hypothalamic appetite centres more directly and thus are more effective. The efficacy of these drugs in common practice remains uncertain, although they may have a useful place in an integrated approach. Side-effects include increases in blood pressure and heart rate. The drug should not be used in patients with a history of hypertension, CHD, congestive heart failure or arrhythmia. The benefit is likely to be small unless physical activity increases. It may be useful to maintain weight loss with sibutramine in patients with comorbidity.

Surgical intervention (eg, gastroplasty) may be an option for carefully selected patients with clinically severe obesity (BMI of more than 35-40) when other methods have failed and the patient is at high risk of other diseases or death. Weight loss does occur reasonably successfully, but lifelong surveillance is necessary.

Integrated treatment plan
Diminution of food intake alone is unlikely to result in noticeable and sustainable weight loss for most patients. It is likely that an integrated treatment plan will have the best effect. This must include:

  • A clear-sighted acceptance by the patient of the nature of the problem and the likely time course of progress.
  • Acceptance of the need to change lifestyle, ­including eating behaviour (meals rather than snacks, more fruit, more carbohydrate, less fat of all kinds) and physical activity patterns.
  • An individualised approach depending on ­circumstances - not all patients are the same.
  • A timetable of at least one year to achieve an acceptable slow rate of weight loss that can be ­sustained.
  • Recognition that maintenance of weight loss requires a change for life or the likelihood is weight regain within a year.

For those who are morbidly obese, appetite suppressants and lipolyis inhibitors may help them achieve sufficient rapid and noticeable results, but these approaches should be discontinued as soon as practicable in favour of diet and exercise. Patient self-help groups may play important motivational roles in demonstrating what others have done and in reinforcing motivation through peer praise. Cognitive psychotherapy may be a useful adjunct to regular monitoring of weight and other support strategies by, for example, community or ­practice nurses.
The treatment of childhood obesity should be straightforward inasmuch as parents and school teachers have a large measure of control over food choice and partitioning a child's day into work and leisure. In practice this is as difficult as most other attempts at changing childhood behaviour.
Prevention is better than cure. Breastfed infants are less likely to be obese; mothers who encourage good dietary practices and activity patterns in their children will save themselves much later anguish. It is not considered appropriate to use pharmacological approaches in children.


  1. The Practical Guide: identification, evaluation, and treatment of ­overweight and obesity in adults. Available from URL: guidelines/obesity/practgde.htm
  2. Prentice AM, Jebb SA. Obesity in Britain: gluttony or sloth? BMJ 995;311:437-9.
  3. NICE. Orlistat for treatment of obesity in adults - full guidance. Available from URL: http://
  4. NICE. Guidance on the use of ­sibutramine for the treatment of obesity in adults. Available from URL:

Prentice AM. Overeating: the health risks. Obes Res 2001;9(Suppl 4):234S-8
Clinical guidelines on the ­identification, evaluation and treatment of ­overweight and obesity in adults. US NIH
publication. Available from URL:
A whole range of obesity-related topics can be found on the OMNI website