Rank Nutrition lecture: the jigsaw puzzle of dietary evidence
Wednesday 8th March
The opening named lecture of the conference – the Rank Nutrition lecture – was presented this year by Nita Forouhi, from the University of Cambridge, who gave an overview of the often contradictory evidence linking diet to diabetes risk.
Forouhi leads the Nutritional Epidemiology Group of the Medical Research Council’s Epidemiology Unit, and multiple times in her talk she stressed the importance of looking at the totality of the evidence, pointing out that the results of single studies can be misleading.
Modeling studies show that, of all risk factors for diabetes, diet is the one with the greatest preventative potential if modified. But it is extremely complex, difficult to measure, and prone to multiple biases. On top of that, there has been an explosion of publications within the field, of highly varying quality, many of which are industry sponsored, creating an increasingly confused situation.
Nevertheless, Forouhi called for researchers to tackle the situation “with renewed fervor and bring our best minds to the field, and take on those challenges and turn those challenges into opportunities.”
Her talk focused on three such challenges: which diet is best for weight control overall, and the quality and quantity of nutrients, specifically carbohydrates and fats.
The weight of evidence to date shows that low-fat diets are, on average, not effective for weight control over 1 year. And although low-carbohydrate diets seem significantly more effective than low-fat diets, the average difference in weight loss amounts to only around 1 kg. Research into various “named diets” showed them to all be effective in the short term, but most participants fail to stick to them.
“I think the research gap is to address how to get to grips with helping people to maintain diets; this is the biggest challenge,” said Forouhi. “And in the simplest terms, the diet that works for you is the best diet, and experiment with different diets until you find the one that works for you.”
Moving onto carbohydrates, Forouhi outlined research showing that low-carbohydrate diets may result in increased high-density lipoprotein cholesterol levels and reduced triglycerides, but may increase low-density lipoprotein cholesterol, with a neutral effect on glycemic control. However, conclusions from studies of low-carbohydrate diets are limited by factors including variable definitions, variations in overall calorie intake, and failure to account for carbohydrate quality.
And she stressed that carbohydrate quality matters, with solid evidence supporting the benefits of wholegrain and fiber. She advises people to check food labels for the ratio of total carbohydrate to fiber, with a ratio below 5:1 “fantastic,” below 10:1 acceptable, and above 10:1 to be avoided.
“So that’s a good rule of thumb to take away,” she said.
Forouhi several times commented on the influence of the food and drink manufacturers, in particular highlighting the storm of criticism from the soft drinks industry in response to research linking consumption of sugar-sweetened drinks to diabetes risk, and suggesting that swapping these for artificially sweetened drinks and fruit juice is a less than ideal solution.
Saturated fats, on the face of it, have less of a role in diabetes, with meta-analysis findings not supporting a connection to diabetes risk. However, the more recent research that Forouhi outlined has revealed a complex underlying situation, with higher plasma levels of even-chain saturated fatty acids potentially increasing diabetes risk, but higher levels of odd-chain saturated fatty acids associated with a reduced diabetes risk. Odd-chain saturated fatty acids are derived mainly from dairy products, and further research has refined this further, to fermented dairy products.
But while underlining the importance of the source of dietary fat, Forouhi also cautioned that such a focus on specific dietary nutrients is “a recipe for manipulation by the food industry and poor food choices by consumers.”
Finally, she outlined pressing research needs, including in ethnic minority groups, on gene–diet interactions (for which she said the evidence is currently weak), and on the gut microbiome. Forouhi believes more progress will come with greater appreciation of the existence of both obesity-dependent and -independent pathways to type 2 diabetes, highlighting the complex effects of diet quality. She also insisted that policy-makers must be more ready to accept evidence “below” that of randomized trials, given the closely matched results obtainable from observational studies, and the near impossibility of conducting long-term randomized controlled dietary trials.