Dietary guidelines: controversies and missing evidence
Friday 9th June
medwireNews: In a lively session on Friday afternoon, presenters looked at the evidence (or lack thereof) underlying dietary guidelines for patients with diabetes.
Hope Warshaw (Hope Warshaw Associates, LLC, Alexandria, Virginia, USA) led off with a look at the current US guidelines and how they can help patients with glycemic control and cardiovascular risk reduction. She noted that the guidelines have become more practical in recent years, and that the grading system has changed to reflect that. Likewise, Pam Dyson (University of Oxford, UK), who gave the second presentation in the session, said that the UK guidelines have moved towards being “food-based.”
While outlining the US guidelines, Warshaw drew attention to the complex, hard to interpret evidence for the benefits of low glycemic index or glycemic load, and she noted that although the guidelines support plant stanols or sterols for reducing lipid levels, such foods are very difficult to find in the USA. By contrast, Dyson said they are widely available in the UK, but less recommended because of their low cost-effectiveness.
Warshaw surmised that the guidelines do not support any specific macronutrient intake for diabetes patients, but here she gave a “dose of reality,” noting that even if such a thing existed, patients would find it extremely hard to adhere to major dietary changes in the long-term. Instead, she noted that a number of diets, such as the Mediterranean diet, the DASH diet, and plant-based vegetarian/vegan diets, can all help patients to control their weight and blood sugar, saying that it is “about what the individual is willing and able to do.”
She stressed that although dietary change works when effectively implemented, this is rarely the case, with support for patients not usually provided as recommended. However, the authors of the joint ADA/American Association of Diabetes Educators/Academy of Nutrition and Dietetics position statement on self-management education and support have released a toolkit, available here, to help practitioners support their patients in achieving the best outcomes.
The two presenters had been asked to highlight controversies, among which Warshaw listed: which is the best diet for weight loss; which types of fat are more or less healthy; and what percentage of food should be formed of carbohydrates. Dyson also raised the subject of carbohydrates, quoting Edwin Gale (University of Bristol, UK), who said that “passion in science is an infallible marker of lack of evidence,” and noting that, until recently, it “probably applied to low-carbohydrate diets.”
The cumulative evidence does now appear to support such diets, she said, but cautioned that they are no better than any other diet and the long-term effects remain to be determined.
She also picked up on the theme of which types of fat patients should be advised to eat and avoid, reminding the audience of the media headlines generated by claims that saturated fat might not after all be a cause of cardiovascular disease. The research apparently refuting the effect did not examine high-fat diets, and there is some evidence for reduced cardiovascular disease if people reduce saturated fat intake, noted Dyson.
However, the controversy over saturated fats leads to another thing Dyson classed as a controversy: conflicting guidelines. The UK’s National Obesity Forum recently released guidelines that advocated aggressive sugar reduction, but gave patients license to eat as much saturated fat as they wish, contradicting other UK guideline-makers.
Dyson, while clearly not a fan of the National Obesity Forum recommendations, apportions part of the blame for conflicting guidelines on the poor underlying evidence. Problems include short-term, often epidemiologic, studies with surrogate endpoints, frequent lack of control groups, and variable means of measurement, all producing uncertain findings that are then poorly represented by the mainstream media.
Nevertheless, Dyson concluded that there is some consistency among the UK guidelines, with all presenting weight management as the key strategy, along with carbohydrate control.
Gestational diabetes: Mind the evidence gap
The evidence underlying dietary guidelines may be confusing and of variable quality, but the two presenters tasked with discussing dietary guidance for women with gestational diabetes had a different problem: a near absence of evidence, resulting in a near absence of guidelines.
The first presenter, Robyn Ann Barnes (Bankstown Diabetes Centre, Sydney, New South Wales) said that in Australia guidelines were so completely absent that it prompted a dietician to undertake a survey of current practice, as part of her Ph.D. This, unsurprisingly, revealed large variations in the recommendations of dieticians to their patients. For example, although almost all dieticians surveyed discussed carbohydrates with their patients, the percentage intake they recommended ranged from 20% to 75%.
UK guidelines are also lacking in specific dietary advice for women with gestational diabetes. Barnes noted that fairly consistent advice is given in the UK’s National Health Service, of 30–40% carbohydrate and a maximum of 120 g/day, but these recommendations have evolved within the healthcare system without actually being evidence-based.
Across Europe, there is wide practice variability, but the International Life Sciences Institute has recently set up a Gestational Diabetes and Diet Taskforce, to examine the evidence and produce recommendations.
In the last presentation of the session, Teri Hernandez outlined current US recommendations, and also raised several issues with producing dietary advice for pregnant women. One such problem is that diet trials usually demonstrate success over 6 months, whereas women with gestational diabetes need results in a much shorter time. Hernandez also noted that there are ethnic differences in how women present with gestational diabetes, suggesting that one diet may not be suitable for all women. Another problem, summed up by the “law of unintended consequences,” is the long-term effect that a change in maternal diet might have on the growing fetus, with such “programming” thought to contribute to later obesity in children of overweight mothers.
The little evidence that does exist to guide nutrition in women with gestational diabetes has major issues with study quality, and Hernandez outlined how she would like future researchers to approach their trials. Her wish-list is:
- To have consensus on measures used for maternal and infant outcomes.
- To find means of better controlling study variables, eg, by providing food.
- For trials to be prospectively planned.
- For researchers to consistently report body mass index and to report weight gain during the intervention (not just across the whole pregnancy).
- For studies to focus on homogeneous samples of women, ie, from the same racial background, to avoid major genetic confounders and build a stronger evidence base.
Despite the sparse evidence, Hernandez and team identified 10 randomized controlled trials looking at carbohydrate intake in women with gestational diabetes. The overall findings seem to suggest that women can benefit from improving the quality and complexity of the carbohydrates they eat, rather than worry about reducing them.
Hernandez stressed that such an approach is ideal for pregnant women, who are often anxious or fearful upon receiving a diagnosis of gestational diabetes, and may feel better able to manage a shift in their carbohydrates than to attempt a reduction.
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