Carbohydrates and Sugar
Carbohydrates are biological molecules consisting of carbon, hydrogen and oxygen atoms, which serve as a source of energy. The simplest form of carbohydrates are the monosaccharides – also known as simple sugars – which include glucose, fructose and galactose. Disaccharides are composed of two monosaccharides and include sucrose (glucose and fructose), lactose (glucose and galactose) and maltose (two molecules of glucose).
Complex carbohydrates are linear chains of glucose molecules called polysaccharides and are commonly referred to as starch. The term complex carbohydrate was first introduced to distinguish sugars from other carbohydrates which were thought to be nutritionally superior.
The relative impact of carbohydrates on blood glucose levels can assessed by glycemic index (GI). A standardised amount of ingested carbohydrate is ingested followed by measurement of blood glucose levels usually two hours later. The increase in blood glucose level is then compared relative to pure glucose, arbitrarily given a score of 100. A GI less than 55 is considered low, while 70 or above is high. Glycemic load (GL) is an extension of GI that considers the potential impact of carbohydrates on blood glucose levels, taking into account the total amount of absorbable glucose contained within a carbohydrate source.
The current Dietary Guidelines for Americans and Australian Dietary Guidelines recommends that carbohydrates should constitute 45-65% of total energy intake. The UK dietary reference values also indicate that carbohydrates should constitute about 50% of energy intake(10). These recommendations for high carbohydrate diets largely stem from a perceived need to limit dietary fat while ensuring adequate energy intake.
The need for glucose by red blood cells is often misinterpreted as meaning that humans have an obligate need for dietary carbohydrate. However, this need for glucose can be met from gluconeogenesis, where glucose is formed from non-carbohydrate precursors, including glycerol, lactate and some amino acids.
Optimal functioning of the central nervous system is commonly thought to be dependent on dietary carbohydrates. It is now well understood however, that the brain can utilise ketones as a source of energy in the absence of glucose. In a prolonged fasted state, ketones from fat oxidation can provide most of the energy needs of the brain, with gluconeogenesis able to provide the balance of required energy.
Carbohydrates are therefore not an essential nutrient. In other words, the theoretical minimal level of carbohydrate intake consistent with good health is zero. This has been demonstrated over many years in those on strict ketogenic diets for the management of epilepsy.
Despite evidence that carbohydrates are not an essential nutrient, current Dietary Guidelines for Americans recommend a minimum carbohydrate intake of at least 130g per day to support neurological functioning. The recommendation to consume at least 130g of carbohydrates per day arose from an Institute of Medicine report estimating the brain to have a capacity to metabolise 100g of glucose per day in the absence of other energy substrate. The recommended intake of 130g was thought to provide a margin of error over the 100g usage identified. Despite compelling evidence to the contrary, this recommendation has persisted through revisions of the Dietary Guidelines for Americans.
In contrast, the Australian Dietary Guidelines and UK dietary reference values reflect the current understanding that carbohydrate is not an essential nutrient for adults. Neither guidelines set values for recommended daily intake (RDI), estimated average requirement (EAR) or adequate intake (AI). The Australian Dietary Guidelines recommend a minimum carbohydrate intake for infants due to uncertainty as to whether gluconeogenesis can provide all the glucose needs of this population.
Recent research has shed light upon the efficacy of high and reduced carbohydrate diets.
The PURE (Prospective Urban Rural Epidemiology) study, which examined the dietary intake of over 135 000 individuals from 18 countries using validated food frequency questionnaires, found that over a median follow-up period of 7.4 years, high carbohydrate diets were associated with a higher risk of total mortality. In contrast, higher total fat intake, including saturated fat, was related to lower total mortality. Comparison of the highest and lowest quintiles of carbohydrate intake found a total mortality hazard ratio of 1.28, reflecting a 28% increased chance of dying in those on the highest carbohydrate diets.
Despite its prospective nature, there were methodological limitations in this study which have been pointed out by critics. These include grouping of carbohydrate sources, the possibility that poverty may be associated with high carbohydrate diets and the use of food frequency questionnaires. The authors of this study stand by their findings however, having extensively controlled for covariates with the potential to confound results, and conclude that ‘global dietary guidelines should be reconsidered’.
Monosaccharides, or single sugars, are the simplest form of carbohydrate, and include glucose, fructose and galactose. Disaccharides are composed of two monosaccharides and include sucrose (glucose and fructose), lactose (glucose and galactose) and maltose (two molecules of glucose). Sucrose is arguably the most significant sugar in the Australian diet, with a fructose content of 50% which is comparable to high fructose corn syrup (typically 42-55% fructose) – a sweetener made from corn starch that has been processed to convert its glucose into fructose.
Fructose is found in fruits, vegetables, and honey. It has been strongly implicated in the development and progression of non-alcoholic fatty liver disease (NAFLD), in both animal and human studies. Following digestion, fructose enters the portal circulation and is almost completely extracted at first pass and converted to lactate and glucose. When glycogen stores are full, these products stimulate de novo lipogenesis, contributing to fatty liver. This is consistent with large cohort studies demonstrating significantly increased risk of NAFLD with sugar-sweetened beverage intake. NAFLD has been shown to be causally associated with insulin resistance, playing a significant role in the pathophysiology of diabetes, and fructose has been shown to be significantly more potent than glucose in contributing to hepatic steatosis and insulin resistance.
In 2015, the World Health Organization (WHO) released a recommendation that no more than 10% of total daily energy intake should come from added sugars. This recommendation is largely based on the relationship between added sugars and excess weight gain and dental caries. Additionally, high sugar diets have been shown to increase the risk of both type 2 diabetes and cardiovascular mortality. For an average adult intake of 8700 kJ (2080 kcal), 10% of daily energy intake is equivalent to 52 grams or 13 teaspoons of added sugar. Although this may seem like a considerable amount, more than half of Australians consume added sugar exceeding this level.
Fifty per cent of Australian children aged 12 years and older have evidence of tooth decay of their adult teeth. The WHO conditionally recommends reducing energy intake from added sugar to less than 5% for improved dental health. Further benefits in reducing added sugar intake to less than 5% of energy intake include improved intake of essential nutrients(41)and reduction in obesity and its biopsychosocial implications.