| Carbohydrates and health—the FAO/WHO consultation
Thomas Wolever
Abstract Carbohydrates are the single most important source of food energy in the world and have significant additional health influences. In April 1997, the Food and Agriculture Organization of the United Nations and the World Health Organization convened an expert consultation on carbohydrates in human nutrition in Rome that involved scientists from 13 countries. The consultation considered all aspects of carbohydrate nutrition and interpreted the current science on the health impact of dietary carbohydrates, especially where controversy existed. The ultimate aim was to promote a nutritious and safe food supply globally. Progress in carbohydrate chemistry and our growing understanding of the diverse physiological roles of carbohydrates in recent decades has led to new dietary approaches in which carbohydrates play a key role. Diet is one of the major risk factors in many modern diseases and dietary carbohydrates can beneficially influence obesity, type 2 diabetes, coronary heart disease and some cancers. The expert consultation released 22 recommendations to be passed to member countries to assist them to develop their own dietary guidelines. This paper focuses on the findings and recommendations related to the role of carbohydrates in the maintenance of health, including recommendations on the minimum content of carbohydrate in the diet, the most appropriate sources, the role of carbohydrates in body weight and endurance exercise, the practical applications of the glycaemic index and implications of carbohydrates for diabetes, cancer and cardiovascular disease. (Aust J Nutr Diet 2001;58 Suppl 1:S3–S8)
Dietary strategies for weight management—the importance of carbohydrates
Arne Astrup
Abstract The prevalence of obesity is increasing rapidly in all age groups globally and is one of the fastest growing epidemics, now affecting 4 to 8% of children and 10 to 20% of adults. Obesity is followed by serious co-morbidities such as type 2 diabetes, cardiovascular disease, certain cancers, and reduced life expectancy, and these complications may account for five to ten per cent of all health costs. There is robust evidence to support the view that a diet which is low in carbohydrates and high in fat is energy dense and, together with physical inactivity, is an independent risk factor for weight gain and obesity. Furthermore, interactions between dietary fat and physical fitness determine fat balance, so that the obesity promoting effect of a high fat diet is enhanced in susceptible subjects, particularly in sedentary individuals with a genetic predisposition to obesity. Skipping breakfast may further increase the risk of obesity. A diet with a higher fat content seems to be better tolerated without weight gain by physically active individuals than by sedentary people. Ad libitum consumption of diets low in fat (20–30% of energy) and high in protein (15–20%) and carbohydrates, contributes to the prevention of weight gain in normal weight subjects. It also causes a spontaneous weight loss of 3 to 4 kg in overweight subjects, and has beneficial effects on risk factors for diabetes and cardiovascular disease. The source of carbohydrate is less important than the fat content. The main effect of the low fat, high carbohydrate diet composition on energy balance is exerted through enhanced satiety, increased faecal energy loss and slightly increased energy expenditure. The addition of daily physical activity to the diet doubles the weight loss in overweight subjects. Implementation of dietary change and increased physical activity may reduce the mean body weight of the population and decrease the prevalence of obesity and diabetes substantially. In conclusion, important interactions exist between genetic make-up, dietary fat and carbohydrate, meal pattern and physical fitness. Increasing daily physical activity and reducing dietary fat content may be more effective in combination than separately in preventing weight gain and obesity. Energy balance seems only to be achieved with less energy-dense diets and fat intakes of 20 to 25% of energy in sedentary subjects, but 25 to 30% in highly physically active subjects. (Aust J Nutr Diet 2001;58 Suppl 1:S9–S12)
Carbohydrates and appetite control
John Blundell and Joanna le Noury
Abstract Many studies have shown that consumption of high carbohydrate foods can give rise to a clear modulation of the expression of human appetite. The potency and time course of the effects of various carbohydrates on satiety vary with the amount consumed and the chemical structure. There is evidence that this biological effect can modulate the temporal profile of hunger and the eating pattern of meals and snacks. One important issue is the action of carbohydrate foods on satiation (within meals) and satiety (after meals). These effects can be contrasted with the relatively weaker effects of high fat foods. The physiological mechanisms through which carbohydrates exert an action on appetite include plasma glucose levels, glucoreceptors, hepatic glucose metabolism and glycogen stores. Experimental evidence indicates that the encouragement to eat high carbohydrate (low fat) snacks or high carbohydrate breakfasts can significantly reduce daily fat intake, limit energy intake, prevent weight gain and even induce weight loss. It is therefore possible to design high carbohydrate diets that provide good nutrition with adequate control over appetite and a beneficial effect on body weight. (Aust J Nutr Diet 2001;58 Suppl 1:S13–S18)
The fuels for exercise
John Hawley
Abstract Compared with the body’s limited carbohydrate (CHO) stores, the reserves of fat in humans are plentiful: if fat was the sole source of energy, it could sustain skeletal muscle contraction for approximately 120 hours of continuous, moderate intensity (65% of maximal oxygen uptake [VO2 max]) exercise. On the other hand, if CHO was the only fuel oxidised, it could provide energy for approximately 90 minutes of intense (85% of VO2 max) activity. As the men’s world record for the marathon (42.2 km) is approximately 125 minutes, this highlights the importance of fuel integration during exercise. In contrast to fatty acid metabolism, the rate of CHO oxidation during exercise is regulated tightly with glucose availability closely matching the requirements of the working muscles. Both the absolute (power output or speed) and relative (percentage of individual VO2 max or maximal heart rate) intensity of exercise have important roles in the regulation of substrate metabolism: the absolute work rate or energy flux determines the total quantity of fuel required, while the relative intensity determines the fuel mix (i.e. the proportion of fat and CHO oxidised by the working muscles). During moderate exercise more than 50% of total energy is derived from fatty acid oxidation. However, during intense exercise, CHO fuels predominate with muscle glycogen and glucose utilisation scaling exponentially to the relative workrate. Although controversy exists with regard to the mechanism(s) that regulate substrate choice for oxidation by skeletal muscle during exercise, there is a growing body of evidence to suggest that CHO metabolism in general and muscle glycolytic flux in particular, regulate fatty acid oxidation by direct inhibition of long-chain fatty acid oxidation. CHO stores are limited and often substantially less than the requirements of the training and competition sessions undertaken by many athletes. Furthermore, muscle and liver glycogen depletion often coincide with fatigue during both endurance events and many team sports. Therefore, any nutritional strategy that promotes fatty acid oxidation and conserves endogenous CHO stores could, potentially, improve exercise capacity. (Aust J Nutr Diet 2001;58 Suppl 1:S19–S22)
Fat adaptation and glycogen restoration for prolonged cycling—recent studies from the Australian Institute of Sport
Louise Burke
Abstract Current sports nutrition strategies are based on the theory that body carbohydrate (CHO) stores are limited, and that extra CHO consumed before and during a workout will continue to make this important fuel available. These strategies have been shown to be effective in delaying the onset of fatigue and enhancing the performance of endurance sports (> 90 minutes duration). However, CHO consumed during prolonged exercise provides an additional fuel source for the muscle, rather than altering the rate of depletion of muscle glycogen stores. An alternative angle to enhance performance is to find a fuel source for exercise that could replace glycogen and slow its rate of use. The classical carbohydrate-loading studies of the 1960s found that short-term exposure to a high fat, low CHO diet caused dramatic reductions in muscle glycogen stores and decreased exercise capacity. By contrast, studies that lengthened the duration of this diet showed, with time, that the body adapts to CHO deprivation by increasing the utilisation of fat during exercise, and using the precious glycogen stores at a slower rate. These adaptations may occur after as little as five days of a high fat, low CHO diet. A recent Australian Institute of Sport study investigated whether fat-loading strategies provide additional benefits to the performance of endurance athletes. We exposed well-trained athletes to five days of a high fat, low CHO diet, followed by one day of high CHO intake to refuel glycogen stores before a performance ride on day seven. The athletes also consumed a high CHO breakfast and drank sports drink throughout the performance test according to current sports nutrition guidelines. These strategies ensured that CHO availability was optimal. The fat adaptation diet caused major changes in fuel utilisation during sub-maximal exercise, with at least some of the adaptations persisting on day seven, even in the face of a plentiful CHO supply. As dramatic as these metabolic changes were, they failed to improve the performance of the cyclists’ time trial. Together with other research, this study fails to find evidence that fat adaptation strategies offer any benefits for the endurance athlete. The only remaining question is whether there are any advantages for ultra-endurance athletes who compete in events undertaken at a lower intensity and for longer periods (e.g. four hours or more). For these athletes, fat is the predominant fuel source. (Aust J Nutr Diet 2001;58 Suppl 1:S23–S27)
Obesity in children—the importance of physical activity
Kate Steinbeck
Abstract The focus of this paper is the role of energy output, i.e. physical activity, in the prevention and management of childhood obesity. However, this does not mean that the other side of the energy balance equation, food intake, is not important. An excess fat gain is the result of an imbalance between energy intake (food) and energy output (physical activity). There is evidence that physical activity is declining for the whole population, including children, and that this decline is a major factor in the increasing prevalence of obesity. Many childhood leisure activities, including viewing television, increase sedentariness. Active time may be limited by safety concerns, lack of suitable environments and lack of family time. There is little scientific evidence on the role of increasing physical activity in the management of childhood obesity. The intuitive and practical advantages of increasing physical activity are a counterbalance to food-related issues. Healthy eating (as outlined in the national dietary guidelines) remains important but does not become the only focus for family change. Additionally, increasing physical activity allows children to grow into their weight and an improvement in the metabolic (lipid, insulin) status of obese children. Physical activity in children has genetic, environmental and behavioural components. Genes influence spontaneous physical activity and also muscle fibre type and nutrient partitioning (how the body selects which fuel to burn—fat or carbohydrate). Space, access and appropriate types of play are vital. Children model behaviours on parental behaviours and the family activity philosophy is an important one, just as are the family’s food beliefs and eating. Thus any proposed change in physical activity must take these three components into account. Maintaining a healthy level of physical activity in childhood has a number of potential advantages, including the initiation of a lifelong habit and improvement in physical skills and achievement, which in turn support the activity habit. Evidence for a decline in physical activity in populations, in an environment that is becoming more and more supportive of inactivity, is presented. This inactivity increases the risk of obesity. Lifestyle activity or the activity of day-to-day living is very important for overall energy expenditure in children. Families influence childhood physical activity patterns in a number of interconnecting ways. (Aust J Nutr Diet 2001;58 Suppl 1:S28–S32) |
Australian Journal of Nutrition and Dietetics, Volume 58, Supplement 1, June 2001