Carbohydrates were put under the microscope in Sydney last week in a symposium organised by the International Life Sciences Institute (ILSI). The risks and benefits of carbohydrate intake were widely discussed. Here is a taste of what we heard, some observations and a few key references.
Recommended carbohydrate intakes
A couple of speakers discussed the recommended range for carbohydrate intakes, which is 45-65% of daily calories in both Australia and the United States. In the United States, the major factor that determined the upper boundary of intake was adverse effects of carbohydrate on levels of triglycerides and HDL-cholesterol in the blood. At the lower end of the scale it was argued that fibre requirements are unlikely to be met at intakes of carbohydrate below 45% of energy (in the low fibre US context). It’s interesting that dietary fibre is considered as a carbohydrate-amount issue in the United States. To my mind it is very much a carbohydrate quality issue.
Mean adult intake of carbohydrate in Australia is about 46% of daily calories, right at the lower end of recommended intake range. This was perceived as low by some speakers and a reason for focussing on glycaemic index as the preferred means of lowering the total glycaemic load of the Australian diet. Others saw the current intake as ‘moderate’, pointing out that the beneficial diets in the Diogenes study contained just 43% of calories from carbohydrate – below the lower boundary of recommended carbohydrate intake.
I introduced my talk by arguing that both the upper and lower boundaries of carbohydrate intake were too high. They were framed at a time when it was thought that a higher percentage of dietary energy from fat was detrimental to health. However, the preferred model for healthy eating is now one with more unsaturated fats and less of both saturated fat and carbohydrate – a Mediterranean-type diet.
No speaker advocated very low carbohydrate diets for the general population. Whether the adverse effect on blood vessel function of very low carbohydrate diets is due to low carbohydrate intake per se or due to the typically high saturated fat content of such diets is unclear and is being researched.
The benefits of carbohydrate intake
Professor Manny Noakes and Dr Alan Barclay gave two excellent talks addressing the benefits and risks of carbohydrate intakes. Provision of micronutrients and dietary fibre were the obvious benefits of carbohydrate-rich foods. Dr Barclay summarised the findings of a recent review showing that a high dietary fibre intake is associated with lower risk for obesity, type 2 diabetes, cardiovascular disease and colorectal cancer at varying levels of evidence. As much dietary fibre comes from wholegrains it’s difficult to clearly attribute the benefit to one or the other. Professor Noakes mentioned ‘intriguing’ data suggesting that wholegrains may affect body composition but also that CSIRO studies were unable to find any effect of wholegrains on other risk markers or risk factors for coronary heart disease.
Dietary carbohydrate also lowers LDL-cholesterol in the blood when it replaces saturated fat, which should provide benefit. However, this exchange also raises serum triglycerides and lowers HDL-cholesterol so whether there is net benefit on blood lipid-related risk for heart disease is an interesting point for debate.
Professor Noakes acknowledged US data showing that those who successfully maintain weight loss tend to be on low carbohydrate diets. However, she said prescriptions for low carbohydrate diets often threw ‘the baby out with the bathwater’, a reference to recommendations to not only restrict intake of nutrient-poor carbohydrate foods such as soft drinks, confectionery, pastries, biscuits and pies but also to restrict nutrient-rich carbohydrate foods such as fruit, bread and breakfast cereals. The benefits of carbohydrate-rich foods in providing essential nutrients and fibre need to be balanced against any risks associated with carbohydrate intake.
Risks: sugar or GI?
Discussions on the risks associated with carbohydrate intake revolved around sugar and glycaemic index/glycaemic load, reflecting the current international debate. Dr Barclay discussed the latest review on dietary sugars and body weight conducted for the World Health Organization (WHO) by a team from Otago University in Dunedin. They found that intake of free sugars or sugar-sweetened beverages is a determinant of body weight. Like the review for the Dietary Guidelines for Americans, this review found that the best evidence on sugars and body weight is from studies on sugar-sweetened drinks and perhaps the authors of the WHO review should have limited their findings to beverages. Importantly, their other key finding was that exchanging sugars for other carbohydrates was not associated with weight change. That is, sugar and starch have the same effect!
While you are considering the WHO review take a look at the editorial that accompanied it, written by Walter Willett and David Ludwig. Although generally supportive of the WHO review, these authors take a subtle shot at the WHO for previously disregarding evidence suggesting that starchy products have metabolic effects comparable to those of sugar. They state: Efforts to reduce sugar intake are appropriate, but they should form part of a broader effort to improve the quality of carbohydrates …
This is the nub of the current debate. Carbohydrate quality is not a simple concept that can be reduced down to ‘limit sugar’. It’s complicated and has several dimensions, including nutrient density, dietary fibre/wholegrains and glycaemic index.
Dr Barclay argued that glycaemic index was more important that sugar. He cited the Institute of Medicine’s report on sugar which recommends an upper limit on added sugar intake of 25% of daily calories. But he drew our attention to the rationale for this rather high upper limit, which is not the prevention of disease – it’s to limit nutrient dilution. In contrast, Dr Barclay argued, glycaemic index and glycaemic load are predictors of disease, including obesity, type 2 diabetes and coronary heart disease. His key message was to focus on the body’s response to carbohydrate foods, rather than on whether the carbohydrate was in the form of sugars or starch.
• Very low carbohydrate diets are not recommended for the general public
• Moderate intakes of nutrient-rich carbohydrate foods are recommended
• Lower GI and high fibre/wholegrain foods are protective