VOL: 99, ISSUE: 43, PAGE NO: 20
Ian Rollo, BSc, is personal trainer/health consultant, BirminghamIan Rollo, BSc, is personal trainer/health consultant, Birmingham
The increase in the severity and prevalence of obesity represents a major public health issue in the UK, with most adults trying to lose or maintain weight (Offer, 2001). The approach that restricts carbohydrates and increases consumption of protein has stemmed predominantly from Atkins' book, Dr Atkins' Diet Revolution.
The cost of obesity
The ketogenic diet
The principle of high-fat, low-carbohydrate diets is that a large carbohydrate intake increases insulin production, which makes the body store fat. Removing carbohydrate from the diet would cause insulin levels to drop and the body would burn fat instead of carbohydrate. The mobilisation of fat for energy generates excess plasma ketone bodies that allegedly suppresses appetite.
The Atkins' diet eliminates carbohydrates from food without restricting protein and fat intake. Deprived of carbohydrates, the body uses fat for fuel. Subsequently a small part of the metabolised fat is eliminated in the urine as ketone bodies. Ketones lost in urine represent unused energy, the elimination of which is supposed to further weight loss and is the reason such diets are called 'ketogenic'.
The Atkins' diet
The Atkins' diet has become popular because of the documented significant weight loss. Even though very low-carbohydrate diets are popular, little is known about their effects on blood lipids and other risk factors for cardiovascular disease (Volek et al, 2003). These diets supply large quantities of saturated fat and cholesterol, which are traditionally associated with cardiovascular disease and obesity.
There is still a great deal that is unknown about the Atkins' diet. However, there are now a growing number of research papers that have investigated the diet allowing a systematic review of the literature to be made to provide evidence on the key questions:
- Does the Atkins' diet cause significant weight loss?
- Are there any detrimental effects arising from the diet in the short and long-term?
- Can people stick to the diet?
- Is the Atkins' diet more effective than conventional low-fat high-carbohydrate diets?
- Which diet is best for weight loss and health benefits?
Diets that are low in carbohydrate and high in fat and protein have been associated with both increased weight loss and improved risk of cardiovascular disease in several randomised controlled trials in people with obesity (Foster, 2003) and lean populations (Volek et al, 2002).
Weight loss was found to be significantly greater over the duration of three to six months in individuals who are obese when following a ketogenic diet compared with a high-carbohydrate diet (Foster, 2003; Sondike, 2003; Volek et al, 2002). However, no significant difference in weight loss was observed after 12 months (Foster, 2003).
A common finding in the literature was people's increased adherence to the low-carbohydrate diet compared with the low-fat diet (Samaha, 2002).
In trials of people with obesity a 20 per cent reduction in serum triglyceride was observed in low-carbohydrate diets (Foster, 2003). This was accompanied by about a 20 per cent increase in high-density lipoprotein cholesterol (HDL-C) and no change in total or low-density lipoprotein cholesterol (LDL-C) (Foster, 2003).
In women of normal weight who were also normolipidaemic, a short-term very low-carbohydrate diet modestly increased LDL-C. However, there were also favourable effects on their risk status for cardiovascular disease by a relatively larger increase in HDL-C (Volek et al, 2003). A low-carbohydrate diet could also be an effective method for short-term weight loss in overweight adolescents, as significant weight loss occurred with no harm to blood lipid profiles (Sondike, 2003).
When 12 men of normal weight were switched from a regular diet to a very low-carbohydrate ketogenic diet, they lost approximately five pounds by the sixth week. Body fat levels in the arms, legs and trunk were reduced from an average of 20.5 per cent to 16.9 per cent.
The men also had a significant decrease in fasting blood insulin levels on the ketogenic diet. Uncontrolled insulin levels have been linked to such health conditions as obesity, diabetes and heart disease. Therefore, regulating insulin levels by regulating carbohydrate intake may cut the risk for these conditions (Volek et al, 2002).
The importance of exercise
Weight loss and fat-lowering interventions are more successful when they address both energy intake and energy expenditure.
Modifications have been made to incorporate exercise into the Atkins' programme. However, when people restrict carbohydrates in their diet, they experience more fatigue, more psychological negative effect, and less positive effect in response to exercise (Butki, 2003). A low-carbohydrate diet rapidly compromises energy reserve for vigorous physical activity or regular training.
Regular exercise is vital to gain the health-related benefits of decreased cardiovascular disease risk and obesity. Carbohydrate is digested and absorbed rapidly and provides energy faster than fat or protein. During sub-maximal prolonged exercise there is a progressively greater use of fat, which can contribute up to 80 per cent of the total caloric expenditure. Endurance training results in a shift of the metabolism towards a greater use of fat during the same absolute and relative exercise loads.
Low concentrations of HDL-C are a recognised risk factor for atherosclerotic cardiovascular disease. Endurance exercise training in sedentary middle-aged men and older men and women has been shown to increase in HDL-C subfractions.
Physiological thresholds have been established from cross-sectional and longitudinal exercise training studies. These indicate that 15 to 20 miles per week of brisk walking or jogging, eliciting 1,200-2,200kcals of energy expenditure per week, is associated with LDL-C reductions and HDL-C increases (Durstine et al, 2002).
However, exercise will almost never alter total cholesterol and LDL-C levels unless accompanied by a reduction in dietary fat intake and loss of body weight, associated with the exercise-training programme (Durstine et al, 2002). A 24 per cent reduction in cardiovascular events was observed in participants reducing their dietary fat intake to within current UK guidelines after a period of two years.
On average, total fat was reduced to 35 per cent of total energy intake and saturated fats were cut to 8.5 per cent of total energy intake (Hooper, 2001).
Almost all reducing diets are nutritionally inadequate. The rate of anticipated weight loss will vary according to the age, sex, weight, basal energy requirement and activity level of an individual. Reducing diets do not work, but changes in lifestyle and eating habits do. Therefore, when advising patients about a change in nutrition, it should be done so to complement a regular exercise regime.
A low-fat, high-carbohydrate diet (Box 1) combined with an effective exercise programme (Box 2) is recommended for long-term weight control and overall heath. Low total fat intake should reduce the risk of cardiovascular event by 24 per cent after a period of two years (Hooper, 2001).
A balanced diet and adequate carbohydrate intake will allow exercise to be performed, increasing energy expenditure while having a positive effect on cardiovascular risk (Hooper, 2001).