EXPERT REACTION: Bread or butter? High carb diets may be worse than high fat

This is an observational study which only shows associations and cannot prove causation. It is of relatively short duration for a study of this design, with only a median follow-up of 7.5 years.

Food intake was only assessed at base-line using a Food Frequency Questionnaire (FFQ) that was validated using unstated methods and no results of the validation studies are presented in the paper. We therefore do not know how well the FFQ assessed people's carbohydrate, fat (saturated, mono and polyunsaturated) and protein intakes in each country. Finally, the Hazard Ratios are modest for the extreme quintiles  (Q1 and Q5).

From Figure 1, we can see that total mortality was lowest for carbohydrate intakes between approximately 45 per cent of energy and 60 per cent of energy. With respect to saturated fats, risk of mortality was highest at very low intakes and started to increase when more than 6 per cent of energy was consumed from this nutrient.

Overall, the conclusions of the paper are overstated – a major overhaul of existing dietary guidelines is not warranted based on this additional evidence.

It is important to put the results of the Dehghan, et al. paper in to an Australian context: based on our most recent national health survey, Australian’s consumed on average just over 43 per cent of energy from carbohydrate and 11.5 per cent of energy from saturated fat. On face value, this means that Australians should be consuming more carbohydrate and less saturated fat – consistent with our current dietary guidelines.

Australians are ahead of the curve. We recognised that carbohydrates were not created equal over 3 decades ago – some were harmful because they increased fluctuations in blood glucose (ie they had a high GI). Since then we have reduced our intake of added sugars and as well as high GI starches.  Australians and the Australian food industry should be congratulated.

The Prospective Urban Rural Epidemiology (PURE) study is a very large prospective observational study that assesses association of intake of dietary components (estimated by food frequency questionnaire) with health outcomes, in more than 135,000 people from 18 countries. This paper reports initial associations, mainly in low and middle-income countries, between macronutrients (with a focus on carbohydrate, total and different types of fat) and cardiovascular disease and mortality.
 
The findings that higher intakes of fats including saturated fatty acids, monounsaturated fatty acids, and total polyunsaturated fatty acids, and also animal protein, were associated with lower mortality, whereas carbohydrate intake was associated with increased mortality make an important contribution to nutrition.
 
However, the types and food sources of carbohydrate are not reported in this paper. Yet the PURE study has shown previously an association between higher intakes of fruit, legumes and vegetables (that provide carbohydrates) and increased mortality, suggesting that it is mainly carbohydrate from added sugars and refined grains that may be problematic. Further, the food sources of fats have also not been reported or controlled for in this paper.

An explanation of the findings could be that, in the early stages of the nutrition-transition to more western diets, animal products may help increase life expectancy in low and middle income countries, as they are a rich source of micronutrients that can be lacking in many of the countries in the PURE study.
 
In addition, the results may not be generalisable to Australia, as the upper levels of intakes of carbohydrate reported in the study are much higher and the lower intakes of fats are very much lower than consumed here.
 
It would be useful to see greater control of cultural, social, economic and other confounding variables, and more detailed analysis, including adoption of a food-based approach, in future reports of the PURE study.

This is a good study, across a range of countries with high, mid and low average income populations. What it shows is that fats - saturated, mono-unsaturated, polyunsaturated - are not the no-no we have all been brought up to believe in the context of high carbohydrate diets when the source of the latter is refined sources such as sugar, rice etc.

Highly significantly, lower levels of various cardiovascular morbidity and mortality were seen between diets with up to 35 per cent of calories from fat over those low in fat and high in carbohydrate. Complex carbohydrates - as in fruit and vegetables - are probably another thing, and not similarly a problem.

So go for dairy, olive oil and even the occasional wagyu beef burger, have lots of grains, fruit and vegetables, and lay off the sweet stuff - especially the empty calories in the 16 teaspoonfuls of trouble in sugar-sweetened soft drinks. Sounds a bit like the Mediterranean Diet, with wagyu rather then meat sauce.

This study provides further evidence that high carbohydrate diets are not the preferred model for healthy diets. Although this evidence has been accruing for years it was largely ignored by the National Health and Medical Research Council during the development of the latest Australian Dietary Guidelines. 

The NHMRC chose not to review the Nutrient Reference Value for total carbohydrate intake, or the evidence for glycaemic load and chronic disease risk, or the evidence on the associations between dietary saturated fat, carbohydrate and coronary heart disease, all of which would have shed light on the issue.

During the development of the Guidelines several health agencies wrote to the NHMRC arguing that advice to replace saturated fat with carbohydrate to lower coronary disease risk was no longer evidence-based, but this was ignored. 

Consequently, the latest Australian Dietary Guidelines were 10 years out-of-date when they were published.

The controversy that this study inevitably accentuates will not be resolved by such macronutrient-based studies.

Food pattern, food habit and socio-ecological studies already provide us with the advice we need.That is to support biodiversity and access to public open space so that we can increase our prospects of plant-based minimally processed, biodiverse, affordable diets and be physically active so that we can eat enough without being overfat.

There is a sea-change in establishment thinking around the diet-heart hypothesis, and what constitutes a healthy diet. The long awaited PURE study results, follow a favourable review of best-selling book “The Big Fat Surprise” also in the The Lancet(last week, and 2 years after publication) summarising the evidence for where we got it wrong in demonising fat.

PURE found that lower intakes of fat and saturated fat (SFA) were associated with higher mortality, and had no benefit in terms of cardiovascular mortality. Importantly, this study refutes the claim often made that SFA variations in epidemiology just aren’t low enough to see the benefit of lower SFA.

In the PURE study, there are low intakes of SFA, and plainly we not only see no evidence of benefit, we see harm in increased all-cause mortality.

If there were such harmful effects of saturated fat, you’d expect to see some evidence in studies of these sorts.

PURE tells us, along with mounting other evidence, that it is finally time to move on from banishing dietary fat, including saturated fat.

I’d concentrate on getting people to eat less highly processed and refined foods, especially sugar and highly refined carbohydrates. The totality of the evidence says to 'eat foods low in human interference'. If our food was clearly recently alive in nature, that’s a good start to a healthy diet.

This study presented at the European Society of Cardiology Congress and published in The Lancet suggests that a high carbohydrate intake is associated with a higher risk of total mortality, whereas total fat and individual types of fat were related to lower total mortality.  

This observation together with the findings that total fat and types of fat were not associated with cardiovascular disease and saturated fat was inversely associated with stroke lead the authors to suggest that 'removing current restrictions on fat intake but limiting carbohydrate intake (when high) might improve health'. They conclude that global dietary guidelines should be reconsidered in the light of their findings.  

Given that their findings are based on observations made on over 135,000 people in 18 countries one might assume that such conclusions are definitive. There are however major limitations to these conclusions some of which are acknowledged by the authors. The limitations apply to countries like New Zealand as well as to countries which traditionally have a high carbohydrate intake such as China, which the authors suggest may particularly benefit from their recommendations.

It is important to consider this study in the context of a large body of evidence regarding nutrition and health, and not consider the results of this single study in isolation. Importantly the strengths and limitations of each study must be considered. Important strengths of this study are the large sample size, and inclusion of populations from a wide variety of countries and regions throughout the world. The pooling together of such diverse populations with diverse patterns of lifestyle and dietary patterns poses some challenges however with respect to interpretation of the results.

A key limitation is that there is no distinction between carbohydrates which have been repeatedly shown to be detrimental to health (e.g. free sugars such as table sugar, refined grains) and those which have been clearly shown to have health benefits (e.g. fibre-rich wholegrains, legumes, vegetables and fruits).  

Other very large cohort studies have shown that there are health benefits when saturated fat is replaced either by polyunsaturated fat or wholegrains but not when replaced by sugars or refined grains.

A major difficulty in interpretation results from the fact that the most striking effects were seen when comparing extreme levels of intake. For example, the risk of death was 28% higher among those with diets high in carbohydrate than in those with the lowest intakes. However, those with the highest intake were obtaining 77% of energy from carbohydrate and those with lowest intake 46% energy from carbohydrate. 

In New Zealand, current intakes are not appreciably different from those in the low intake category. The 'benefits' of fat are similarly mainly apparent when comparing extreme levels of intake.

Carbohydrate intakes are highest amongst the predominantly rice-eating countries including China and countries in South Asia (carbohydrates providing 65 to 68% total calories) and it is these countries which the authors suggest might be particularly at risk from their high carbohydrate intakes.  

However, in the largest cities in China fat intake has increased appreciably at the expense of carbohydrate consumption and rates of obesity, diabetes and cardiovascular disease have increased. In China, this new dietary trend which appears to be compatible with the recommendations of the authors is also associated with increasing cholesterol levels. There would seem to be considerable risk associated with offering recommendations which are in conflict with traditional dietary patterns.  

Japan was not represented in this study but it is noteworthy that in that country where rice is a staple food life expectancy is the greatest in the world.

National and international dietary guidelines are increasingly emphasising diet quality and that a wide range of macronutrient intakes can contribute to a diet associated with positive health benefits. Recommending the optimal sources of carbohydrate and fat is more important than precise amounts.  

Current guidelines which we endorse recommend that people continue to eat a diet that is rich in vegetables and fruit, legumes, pulses, nuts, wholegrains, and vegetable oils. Importantly, people should limit the amount of free sugars, salt and highly processed food.  A range of dietary patterns, including Mediterranean, Asian style and other traditional dietary patterns can be consistent with this approach.