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The relative importance of lack of fitness versus fatness |
An important complication of being overweight or obese is the increased chance of having the metabolic syndrome (MS) or syndrome X.
The MS has reached centre stage as an important cluster of risk factors that will influence morbidity and mortality from cardiovascular disease (CVD). The whole concept of risk for CVD has changed as a result of modern lifestyles in developed and developing countries. In addition to the more traditional risk factors of smoking, high cholesterol and hypertension, we now also need to focus on this increasingly prevalent cluster of MS risk factors first described as recently as 1988 by Gerald Reaven, Professor Emeritus at Stanford University.The National Cholesterol Education Program definition for MS (ATP 111) is shown below.
Countries like Australia now find that about one quarter of the childhood and adolescent population is overweight or obese and the MS is now being found in children and young people with worrying future health implications. Whilst the traditional risk factors are just as important as they always were, GPs should also be aware of the MS, which tends not to have high fasting LDL cholesterol as a feature so often, but rather a low HDL, high triglyceride dyslipidaemia, which also carries significant CVD risk. The fundamental defect for the MS is insulin resistance in adipose and muscle tissue leading to hyperinsulinaemia.
Insulin resistance can easily be deduced by performing a fasting BSL, which if greater than 5.5mmol/l, a full GTT should be ordered to determine if there is impaired fasting glucose, or impaired glucose tolerance, or in some instances type 2 diabetes. Serum insulin levels are difficult to measure and interpret and are not currently indicated in clinical general practice.
The big question is what to do about it? We are well aware that bringing about a therapeutic lifestyle change is the first line of approach to try to prevent the progression from impaired fasting glucose or impaired glucose tolerance to overt type 2 diabetes. Medications are sometimes needed particularly where therapeutic lifestyle change is not sufficiently controlling metabolic abnormalities.
But what is the relative importance of losing weight versus becoming more physically active? Whilst both increasing physical activity and improving the diet is a common strategy for many, others will increase exercise without losing significant weight and some will lose weight without increasing exercise by dietary measures alone.
There is some controversy as to which is the most important - lack of fitness or obesity as a predictor of mortality.
A well known researcher in this field is Steven Blair from the Cooper Institute, Dallas, Texas, USA. He now has a prospective study of more than 30,000 individuals followed for an average of 10 years.(1) Some of the findings suggest that obese individuals who are fit have a much lower risk of mortality compared with lean individuals who are unfit, and that low cardiorespiratory fitness in overweight or obese men is as hazardous as having diabetes, smoking, or having high levels of cholesterol or blood pressure. His data suggested that low fitness is more important than obesity as a predictor of mortality. He believes that public health messages should include greater emphasis on physical activity than currently.
Others are totally opposed to this emphasis and believe that this message may give some people a convenient excuse not to try to lose weight because of their self perceived high levels of physical activity. Arne Astrup from the Department of Nutrition, RVA University, Copenhagen, speaking at the Physical Activity and Weight Management Satellite Symposium (in connection with the 12th European Congress on Obesity) acknowledges that physical activity and fitness exert some diabetes protective effect, but that its importance should not be exaggerated.
Most of the research this is based on are prospective observational studies using BMI as a measure for fatness. It is important to consider where fat lies in people and that muscular subjects can have BMIs in the overweight or obese range and yet not have high levels of intra-abdominal fat, which is strongly associated with insulin resistance predisposing to type 2 diabetes. To fully control for associations between fitness and type 2 diabetes for fatness requires adjustments for total body fat, intra-abdominal fat and peripheral fat deposit stores.
He goes on to report that intervention studies such as the Swedish Obesity Subject (SOS) study clearly demonstrate that a major weight loss using gastric surgery, where the weight loss is around 28 kgm without exercise training, can almost eliminate type 2 diabetes in obese subjects with good sustained effects over five years, even when there is a small amount of weight regain.
Even smaller weight losses of 4-7 kgm, such as seen in the Finnish and American Diabetes Prevention Studies have shown to be sufficient to reduce the incidence of type 2 diabetes by 56% and further analyses support that it is the weight loss per se and not the minor increase in daily physical activity that is responsible for the diabetes protective effect.
This is further supported by the Swedish Xendos study that showed that the addition of the weight loss drug Orlistat to an intensive lifestyle intervention reduced body weight by 2.8 kgm and the incidence of type 2 diabetes by 37%.(2)
Dr Astrup concluded that the maintenance of a normal body weight is the most important strategy to prevent type 2 diabetes and a slight weight loss in obese high risk patients substantially reduces the risk of the development of type 2 diabetes. The addition of 45-60 minutes of daily physical activity may contribute to maintenance of a normal body weight and add some effect on glucose tolerance giving further diabetes protective effect. Weight loss achieved by diet and surgery is far more important in prevention and treatment of impaired glucose tolerance and type 2 diabetes, but reducing physical inactivity may be essential for prevention of weight gain and regain.
Another way of looking at this he has put forward is that an obese person may be consuming 2,500 calories per day. On a 1000 calorie a day diet the deficit would be 1500 calories per day. To try to match that with exercise would have one exercising all day, which is obviously unrealistic.(3)
So in general practice we should clearly be encouraging our overweight and obese patients to lose weight as a primary aim. Dietary intervention is essential, but physical activity will also be helpful to prevent weight gain in the first place, prevent regain in those that have managed to lose some weight, and improve insulin sensitivity that will assist with diabetes prevention and management.
References:
(1) Wei M et al. Relationship between low cardiorespiratory fitness and mortality in normal weight, overweight and obese men. JAMA 1999; 282:1547-53
(2) Sjostrom L et al. Xendos - a landmark study - Poster presentation at ICO, Sao Paulo, 2002
(3) Astrup A - 38th EASD Annual Meeting Budapest, Hungary 1st-5th Sept 2002 - A debate for independent thinkers - topic 3 - Is there conclusive evidence that exercise alone reduces glucose intolerance? - http://conferences.bjdvd.com/event1/debate/index.asp - (Ed note: this is a highly recommended website seminar with sound and Powerpoint presentation.)
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