Carbohydrate quality: a new model

The long-held view that carbohydrate-rich foods are protective against heart disease, or at least benign, turns out to be wrong. Carbohydrate recommendations are too high but which carbohydrate-rich foods should be recommended in a healthy diet and which foods should be culled?

One of the most important findings from nutrition research in recent years was that carbohydrate confers similar risk for coronary heart disease to saturated fat. This turned 20 years of dietary advice on its head as it meant that the long-recommended low fat diet was not protective against heart disease. It followed that carbohydrate recommendations were too high and if lower intakes of carbohydrate were to be recommended, it would make sense to preference ‘good’ carbohydrate.

But what is good carbohydrate? Ask this question to a room full of nutritionists and the argument will go on for hours. Thirty years ago starch was considered good and sugars were bad, based on the assumption that starch was digested more slowly and raised blood glucose levels more gradually than sugars. However, studies of glycaemic index showed this assumption to be wrong. So should nutritionists forget about sugar and start recommending low GI foods? There are also arguments over whether ‘wholegrain’ or dietary fibre is the better measure of a good cereal food. In the United States, nutrition authorities have adopted nutrient density as an over-arching principle of their latest dietary guidelines. It would certainly make sense to favour nutrient-rich carbohydrate foods if total carbohydrate intake (or calories) was to be limited. So there are lots of options and opinions but no consensus on what represents good and bad carbohydrate.

Against this swirling background Professor Manny Noakes from CSIRO and I recently published a new model for assessing the nutritional quality of carbohydrate-rich foods.

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Should we worry about trans fats?

Trans fats are bad news for heart health but what should we do about them? Should we give them top billing in the new Dietary Guidelines, demand trans fats be declared in nutrition information panels on food labels, or regulate them in some other way? Hang on … haven’t we already dealt with trans fats?

All fats in our diet are made of substances called fatty acids. Although there are hundreds of them, fatty acids can be grouped into three broad classes – saturated, monounsaturated and polyunsaturated. By far the majority of mono- and polyunsaturated fatty acids have a characteristic ‘bent’ shape, which the scientists call cis. However, a small proportion of unsaturated fatty acids are ‘straight’ and this trans shape gives them different physical properties, more like saturated fats. Fats with a high proportion of trans fats (or saturated fats) tend to be solid, not liquid like oils rich in cis fatty acids.

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Fruits and vegetables don’t prevent cancer

Evidence that fruits and vegetables prevent cancer has fallen away over the last decade but our national nutrition authorities continue to perpetuate the myth. Why don’t they just review the science and give us the facts?

In 1981 the eminent epidemiologists Richard Doll and Richard Peto wrote a famous paper in which they argued that many cases of cancer were due to environmental factors and were therefore potentially preventable. It was suggested that poor diet was second only to smoking as a cause of cancer and could account for 35% of cases. Based on the evidence available at the time, Doll and Peto suggested that increased intakes of fruits and vegetables may prevent cancer. A generation of nutritionists and dietitians embraced the idea with a passion and started communicating the good news.

Better evidence now available

The early evidence suggesting protective effects of fruits and vegetables against cancer came primarily from case-control studies, with all their well-documented bias. Over the last two decades much better evidence in relation to fruits, vegetables and cancer has become available as large prospective studies and in some cases pooled analyses of these studies have been conducted. Most of the results are negative.

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Nutrition authorities ignoring vitamin D deficiency

On 12 June a symposium titled ‘Should Australia and New Zealand allow more vitamin D into the food supply?’ was conducted in Melbourne under the auspices of the International Life Sciences Institute (ILSI). Excellent speakers outlined the current vitamin D status of Australians, the consequences of vitamin D deficiency, current dietary recommendations and intakes, and options for public health action. But are our national nutrition authorities listening?

Vitamin D recommendations

Dietary vitamin D recommendations are relatively new in Australia. After long assuming that a sunny country provided its people with adequate vitamin D, our health authorities introduced Nutrient Reference Values in 2006 in response to increasing evidence that a significant number of Australians and New Zealanders may have less than optimal (vitamin) D status. Adequate Intakes of vitamin D were considered to be 5 μg/day for children and young adults, 10 μg/day for adults aged 51-70 years and 15 μg/day for adults over 70 years of age. These recommendations mirrored those in the United States at the time.

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