An important new dietary trial has produced challenging results. And there is more to come.
A decade ago two similar studies were undertaken in Finland and the United States to test whether type 2 diabetes could be prevented by diet and lifestyle. In both studies, overweight subjects with pre-diabetes were placed on low fat, weight reduction diets and exercise programs. Amazingly, the results of the two studies were exactly the same: the interventions lowered the incidence of diabetes by 58% over three years. The key drivers of the benefit were considered to be weight loss and increased physical activity.
One issue that these trials did not explore in any depth was the role of diet quality in diabetes prevention i.e. was the ‘low fat’ aspect of the diets relevant? The recent results of the PREDIMED trial provide some tantalising insights.
The PREDIMED study
The PREDIMED study is a multi-centre, randomised trial designed to test whether the Mediterranean diet is better than a low fat diet for the prevention of cardiovascular disease. One centre involved in the trial specifically looked at the effect of these dietary options on diabetes risk after four years of intervention and the results were impressive. Despite there being no differences in body weight or physical activity, the incidence of diabetes among those eating the Mediterranean diet was half that of those eating the low fat diet. This benefit was due to diet quality.
Mediterranean diet recommendations
Let’s have a close look at the Mediterranean diet that was recommended:
• abundant use of olive oil for cooking and dressing
• increased consumption of fruits, vegetables, legumes and fish
• reduction in total meat intake; white meat preferred to red or processed meats
• preparation of homemade sauce with tomato, garlic, onion and spices with olive oil to dress vegetables, pasta, rice and other dishes
• avoidance of butter, cream, fast food, sweets, pastries and sugar-sweetened beverages
• moderate consumption of red wine (for those who drank alcohol)
In addition, there were two approaches to the Mediterranean diet built into the PREDIMED study – one with extra virgin olive oil and the other with mixed nuts. Participants were given free olive oil (1 litre/week) or mixed nuts (30 g/day) to encourage subjects to stick to the dietary advice.
Teasing apart ‘diet quality’
So what aspects of the Mediterranean diet conferred the protection against the development of diabetes? There was certainly a lot going on. Firstly, the fat quality in the Mediterranean diets was good – saturated fats were restricted through limiting butter, cream, fast food, pastries and meats, and replaced with monounsaturated fats and some polyunsaturated fats from olive oil, nuts and fish.
Carbohydrate quality was also good – plenty of nutrient-rich, fibre-rich fruits, vegetables and legumes and less sugar-sweetened beverages and sweets. Dietary fibre appears to have a role to play in the prevention of diabetes, cereal fibre being more protective than fibre from fruits and vegetables.
Finally, the total amount of carbohydrate in the Mediterranean diet was lower. This, together with the improvement in carbohydrate quality, would have lowered the overall gylcaemic load. The role of glycaemic index and glycaemic load in diabetes risk has been explored in recent reviews by Livesey (2008) and Thomas (2009).
But what was the telling factor?
The interesting aspect of this study was the diet chosen for the control group. It was not ‘normal diet’ or ‘normal diet minus some fat’. It was the National Cholesterol Education Program’s diet from a decade ago – a healthy diet, but with an emphasis on lowering total fat as well as saturated fat. So a major difference between the intervention and the control in the PREDIMED study appears to be the amount of carbohydrate and monounsaturated fat they contained.
When a diet is enriched in monounsaturated fat at the expense of carbohydrate and proves to be beneficial it is impossible to say whether the presence of monounsaturated fats or the absence of carbohydrate (lower glycaemic load) produced the benefit. We just know that this exchange of macronutrients is beneficial.
However, as a consequence of this exchange, total fat intake was higher in the Mediterranean diets, about 40% of daily calories. Although not high by Mediterranean standards, 40% of calories from fat is well outside the recommended range of fat consumption in Australia of 20-35% of calories, yet body weights were unaffected and the risk for diabetes was lower. This begs the question of whether both boundaries of this recommended range for fat intakes are just too low, a legacy of outdated views on the virtues of low fat diets.
Big test still to come
Although the apparent protection of a monounsaturated fat-rich Mediterranean diet against diabetes is an intriguing result, the real excitement from the PREDIMED study may come with the publication of findings relating to the prevention of cardiovascular disease. These are expected in the next month or two. If the mono-rich diet again proves to be protective relative to the low fat diet, the policy makers will have some serious thinking to do about the amount of fat that should be recommended in healthy diets. Provided fat quality is good, more may be better.
Watch this space.