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Chapter 2 - The role of carbohydrates in maintenance of health

Carbohydrates in the diet
Energy balance
Physical activity
Carbohydrate and behaviour
Carbohydrate through the life cycle

Carbohydrates in the diet

While the amount of carbohydrate required to avoid ketosis is very small (about 50 g/day), carbohydrate provides the majority of energy in the diets of most people. There are many reasons why this is desirable. In addition to providing easily available energy for oxidative metabolism, carbohydrate-containing foods are vehicles for important micronutrients and phytochemicals. Dietary carbohydrate is important to maintain glycemic homeostasis and for gastrointestinal integrity and function. Unlike fat and protein, high levels of dietary carbohydrate, provided it is obtained from a variety of sources, is not associated with adverse health effects. Finally, diets high in carbohydrate as compared to those high in fat, reduce the likelihood of developing obesity and its co-morbid conditions. An optimum diet should consist of at least 55% of total energy coming from carbohydrate obtained from a variety of food sources.

The consultation agreed that when carbohydrate consumption levels are at or above 75% of total energy there could be significant adverse effects on nutritional status by the exclusion of adequate quantities of protein, fat and other essential nutrients. In arriving at its recommendation of a minimum of 55% of total energy from carbohydrate, the consultation realised that a significant percentage of total energy needs to be provided by protein and fat, but that their contribution to total energy intakes will vary from one country to another on the basis of food consumption patterns and food availability.

Energy balance

In adults, it is important that the amount of energy ingested be matched to the amount of energy expended. Maintenance of energy balance is important in order to avoid obesity and its associated co-morbidities such as diabetes and cardiovascular disease. Positive energy balance and obesity occur when total energy intake exceeds total energy expenditure, regardless of composition of the excess energy. However, the composition of the diet can affect whether and to what extent positive energy balance occurs.

The composition of the diet can also affect the ability to maintain energy balance. In particular, diets containing at least 55% of energy from a variety of carbohydrate sources, as compared to high fat diets, reduce the likelihood that body fat accumulation will occur. Substantial data suggest that diets high in fat content tend to promote consumption of more total energy than diets high in carbohydrates (58,70). This effect may be due to the low energy density of high carbohydrate diets, since total volume of food consumed appears to provide an important satiety cue (71). There are no data to suggest that different types of carbohydrates differentially affect total energy intake.

In addition to affecting the chance of having excess energy available, the composition of the diet also affects the proportion of excess energy that will be stored as body fat. The body has a large fat storage capacity and excess dietary fat is stored very efficiently in adipose tissue. Alternatively, the body's capacity to store carbohydrate is limited and excess carbohydrate is not efficiently stored as body fat (72). Instead, excess carbohydrate tends to be oxidized, leading to indirect fat accumulation via reductions in fat oxidation (73).

Excess fat and carbohydrate were previously thought to be equally fattening. This was due to the assumption that de novo lipogenesis was a commonly used pathway for disposal of excess carbohydrate. The available data suggest, however, that this process occurs rarely in human subjects and only in situations of appreciable carbohydrate overfeeding (74). In most usual circumstances, accumulation of body fat via de novo lipogenesis is quantitatively very low.

While noting the low overall contribution of de novo lipogenesis to body fat accumulation, it should be noted that de novo lipogenesis is increased with insulin resistance and with extremely high consumption of sucrose or fructose (74).

Physical activity

Maintenance of energy balance is dependent both on energy intake and energy expenditure. Maintaining regular physical activity greatly reduces the likelihood of creating positive energy balance, regardless of the composition of the diet. There is agreement that the combination of a high carbohydrate diet and regular physical activity is the optimal arrangement to avoid positive energy balance and obesity.

The increased energy needs of physical activity can be supplied by carbohydrate or fat. The importance of carbohydrate in the diet becomes more critical as the amount and intensity of physical activity increases.

In many developing countries, the major challenge is to meet daily energy needs created by high levels of daily physical labour. In such cases, any combination of carbohydrate and fat which provides sufficient energy is to be encouraged.

Many countries recommend increasing leisure time physical activity. While increased physical activity would clearly increase energy needs, these do not create needs for specific macronutrients. Rather, the optimum diet identified above is considered sufficient to provide for such physical activity.

There is substantial evidence that supplemental carbohydrate can improve performance for the elite endurance-trained athlete. A high carbohydrate diet during a few days preceding an endurance event, carbohydrate loading, a high carbohydrate pre-event meal and carbohydrate supplementation in the form of carbohydrate-containing beverages have all been shown to enhance performance during long-distance cycling and running. There is, however, no evidence that such carbohydrate supplementation would improve performance for the majority of people who engage in recreational physical activity of lower intensity and duration. On the other hand, carbohydrate intake following exercise can help to quickly replenish depleted glycogen stores (75).

Carbohydrate and behaviour

It has been suggested that food intake could have important effects on behaviour. While providing breakfast to children who do not typically eat breakfast can increase cognitive performance (76), it is less clear that the overall composition of the diet can affect behaviour. It has been suggested that sugar consumption leads to hyperactivity in children. However, an extensive review of the literature in this area (77) concluded that there is no evidence to support the claim that refined sugar intake has any significant influence on either behaviour or" cognitive performance in children.

Because glucose is an essential fuel for the central nervous system, carbohydrate has also been suggested to play a role in memory and cognitive function. While there appears to be a relationship between glucose levels and memory processing, the clinical significance of this relationship remains unclear.

Carbohydrate through the life cycle

Energy and nutrient needs are increased in pregnancy and lactation, and the primary challenge for pregnant women is to meet these increased energy needs in order to ensure healthy offspring. It has been observed that where variety in the food supply is low and carbohydrate intake is high, a low birth weight is more common. This raises concerns about the adequacy of high carbohydrate diets to meet the energy and nutrient needs of pregnancy when food variety is limited. Energy and nutrient needs should be met by consumption of a wide variety of carbohydrate foods. There is also some concern about excessive fat intake in pregnancy since it may be associated with risk of obesity in the mother.

In many countries, infants receive 45-55% of energy from fat through breastmilk or formulas and 35-45% of energy from carbohydrate. While specific reductions in fat intake are not recommended below the age of two years, infants in many countries consume lower fat diets. This does not present a problem as long as energy requirements are fulfilled. From the age of two and on, the optimum diet (at least 55% of total energy from a variety of carbohydrate sources) should be gradually introduced.

During the first four to six months of life, exclusive breast feeding is recommended as this tailors the concentration of lactose to the maturing neonatal and infant gut, particularly while colonic microflora and pancreatic amylase production are developing. For infants fed on formula, the carbohydrate and other nutrient components should usually mimic breast milk to the extent possible and in accordance with standards of the Codex Alimentarius (78).

Carbohydrate digestion in the neonate and young infant is significantly influenced by both gastrointestinal maturation and the chemical nature of the carbohydrate ingested. The establishment of colonic microflora is responsible for colonic carbohydrate scavenging, converting any carbohydrate entering the colon into short chain fatty acids. Any disturbances or inappropriate development of this microflora (incorrect infant formula, antibiotics, infection) leads to colonic carbohydrate overloading and diarrhoea.

Lactose from dairy products can be a major source of carbohydrate for young children. In addition, milk represents an excellent source of high quality protein, calcium, and riboflavin. In most populations, even those with low lactase activity, milk can be ingested in small amounts, especially after meals with dilution by co-ingestion. Fermented dairy products can be valuable items in the diet of most people irrespective of intestinal lactase status.

Often the transition from childhood to adulthood is associated with changes in dietary pattern. In developing countries, children frequently consume very high carbohydrate intakes from a single or a small number of sources, while adults have greater variety. In such cases, the adult diet is preferred. In developed countries, on the other hand, surveys indicate that children have higher intakes of carbohydrate from more sources than adults. In those countries, the diet consumed by children would seem to be more beneficial. In both situations, at least 55% of carbohydrate energy from a variety of sources is the optimum.

Individualization of carbohydrate intake is necessary for elderly populations. Elderly individuals in many countries are at risk as regards both malnutrition and obesity. Food intake patterns can be altered by changes in taste perception, chronic disease and medication use. While a high carbohydrate diet is recommended for prevention of weight gain and obesity, it should be recognized that some individuals may need diets higher in energy density (e.g. fats) in order to prevent malnutrition. Optimizing intake of carbohydrate to minimize glucose intolerance in later life is a consideration in countries where such intolerance is a problem.

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