As the focus of public health nutrition switches from fats to carbohydrates there is uncertainty about how carbohydrates actually affect the risk for chronic diseases, such as type 2 diabetes and coronary heart disease. Early dietary advice on fats was based on effects on blood cholesterol but what mechanisms are driving the risk associated with carbohydrate?
Glycaemic response or fructose?
One view is that the key driver of carbohydrate-related risk is glycaemic response i.e. the degree to which carbohydrates raise blood glucose and insulin levels. If glycaemic response is the critical factor then the dietary measure of interest would be glycaemic load, which takes into consideration both the amount of carbohydrate consumed and its potential to raise blood glucose.
An alternative view is that fructose drives the chronic disease risk associated with dietary carbohydrate. If so, we should expect fructose or perhaps sucrose (the major dietary source of fructose) to be associated with risk for chronic disease in large population studies. And, importantly, glycaemic load would not be linked with risk.
Let’s look at the epidemiology and see if it supports one argument or the other.
Glycaemic load and coronary heart disease risk
Four meta-analyses of prospective cohort studies of glycaemic load and risk for coronary heart disease have been published in the last couple of years (Dong 2012, Fan 2012, Ma 2012, Mirrahimi 2012). The findings are essentially the same – glycaemic load is associated with heart disease in women but not men. In two of the meta-analyses the links between glycaemic load and heart disease were stronger in overweight subjects. Fan (2012) also found an association with stroke incidence, as did Sieri (2013) in a recent cohort study. In all four meta-analyses the relative risks for heart disease in men were positive – they just failed to reach statistical significance, perhaps due to smaller numbers.
A study in a Chinese cohort published last year found glycaemic load was associated with coronary heart disease risk in both genders (Yu 2013). Compared to studies in western populations carbohydrate intake was high (68% of calories) but sugar intake was relatively low. Most of the carbohydrate was starch, 87% of which came from white rice and refined wheat products.
Fructose or sucrose and coronary heart disease risk
There are virtually no prospective cohort studies that find a significant association between fructose intake and risk for coronary disease. The few studies that have looked at total sugars and heart disease risk generally find no association (Beulens 2007, Sieri 2013). A recent exception found an association between ‘added sugars’ and cardiovascular disease in a study in the United States (Yang 2014).
There are few data about the consumption of sugar-sweetened beverages and risk for coronary heart disease. And the findings are mixed – Fung (2009) found a positive association but this was not confirmed by Eshak (2012). Eshak (2012) and Larsson (2014) found links with stroke, albeit inconsistent between genders.
Glycaemic load and risk for type 2 diabetes
In the last three years at least four meta-analyses of cohort studies have considered whether glycaemic load affects the risk for type 2 diabetes, the latest published this year by the Harvard team (Bhupathiraju 2014). All four meta-analyses found that high glycaemic load increases risk for type 2 diabetes.
A recent meta-analysis found that consumption of white rice, a high GI food, was associated with increased risk for type 2 diabetes in Asian populations where it is by far the major contributor to glycaemic load (Hu 2012). The association was also positive in western populations but failed to reach significance.
Fructose or sucrose and risk for type 2 diabetes
No meta-analysis of studies into whether fructose or sucrose affects risk for type 2 diabetes has been published and there are just a few cohort studies to go on. The findings are mixed. In a small cohort Montonen (2007) found intake of glucose and fructose combined, but not sucrose, was associated with increased risk of type 2 diabetes whereas in the large EPIC cohort sucrose, glucose and fructose were not associated with diabetes risk in men or women.
A couple of months ago a meta-analysis of studies into sugar-sweetened beverages and risk for type 2 diabetes and found a positive association (Greenwood 2014). Interestingly, the same study also found a positive association between non-sugar beverage consumption and diabetes. This raises questions about whether these links are causal or alternatively whether soft drink consumption is a marker for other lifestyle behaviours that are associated with risk.
Glycaemic load or fructose?
Given the public debate about fructose and sucrose the paucity of data linking these sugars with risk for type 2 diabetes and coronary heart disease risk comes as a surprise. It is clear that glycaemic load is a more consistent predictor of risk for these conditions than fructose or any individual sugar or total sugars or added sugars. This doesn’t let sugars off the hook but rather suggests that the risk for chronic disease posed by sugars should be seen in the context of their contribution to glycaemic load.
The significant contribution of sugar-sweetened beverages to glycaemic load in western diets has an interesting parallel in Asian diets – white rice, a high GI, starch-rich food. Both are also nutrient-poor. Some dietary recommendations in Australia still encourage the replacement of sugars with starch but the rationale for this is lacking if limiting glycaemic load is the most effective means of lowering carbohydrate-related risk.
Poor quality carbohydrate is not a narrow concept. It’s broad and includes some traditional foods currently recommended for healthy eating.