Prevention and management of type 2 diabetes: which diet is best?
The prevalence of type 2 diabetes is reaching epidemic proportions worldwide. Despite the availability of dietary guidelines, pharmacological interventions and advances in insulin therapy, many people with diabetes fail to control their symptoms. Type 2 diabetes is also increasingly affecting young people, with a reported prevalence of up to 5.3% in children and adolescents worldwide.1 It is unanimously agreed that dietary interventions play an important role in the development and management of type 2 diabetes.2 Clinicians should be aware of expected outcomes from possible interventions so that they can advise people with diabetes on how to achieve their metabolic goals. However, faced with patient questions about fashionable diets and sensationalist media headlines, how does the clinician know which dietary interventions are of value in diabetes prevention and management?
A large body of evidence supports the traditional low-fat, high-carbohydrate diet in the prevention and management of diabetes.3-5 However, low carbohydrate diets, which typically aim for less than 50 g carbohydrate per day, have become a popular approach to weight loss. A recent review of adult studies presented 12 points of evidence supporting the use of low-carbohydrate diets as the first approach to treating type 2 diabetes.6 In a 2014 systematic review of studies involving children and adolescents, three of the larger and better quality studies found that very low carbohydrate diets improved insulin levels and/or insulin resistance.7 In addition, a 2005 literature review concluded that a low carbohydrate regimen improved metabolic syndrome compared with low fat diets even in the absence of weight loss, and that the lower the carbohydrate content, the greater the improvement.8
Another fashionable diet involves high protein intake. A 2012 systematic review of 24 trials concluded that compared with standard low fat diets, high protein, low fat diets performed slightly better in terms of weight loss but showed no differences in terms of fasting plasma glucose and fasting insulin.9 In children and adolescents, six studies analysed in the systematic review discussed above investigated high protein diets but no differences were seen compared with standard diets in terms of fasting glucose, fasting insulin and insulin sensitivity.7
A 2004 retrospective study of data from 69,554 women found that meat consumption was associated with increased diabetes risk.10 Plant-based diets are associated with numerous health advantages and have been shown to improve glycaemic control in individuals with type 2 diabetes, but evidence is largely based on small studies,11 and there is insufficient evidence to recommend a vegetarian or vegan diet.
The glycaemic index (GI) of carbohydrates is important in choosing an appropriate diet for the prevention and management of diabetes. A 2003 meta-analysis of 14 studies concluded that replacing high-GI foods with low-GI foods has a small but clinically meaningful effect on glycaemic control in patients with diabetes.12 However, the American Diabetes Association did not consider this evidence strong enough tp recommend a low-GI diet as a primary nutrition therapy strategy.13
Very low energy diets, comprising less than 800 calories, are used as short-term interventions for people trying to achieve rapid weight loss. A small study of 30 participants created huge media interest earlier this year after finding that a very low calorie diet for 8 weeks, followed by a stepped return to a normal healthy diet, was able to achieve a remission of diabetes for at least 6 months in 40% of the participants.14 This needs to be investigated in large randomized trials. However, since very low energy diets are hard to maintain, intermitting fasting (for example the 5:2 diet) has become popular. This diet typically comprises several days of healthy eating per week interspersed with ‘fasting’ days, in which calorie consumption is restricted to around 600 kcal. Limited data from small, short-term studies (typically less than 6 months) in overweight and obese subjects suggest that intermittent fasting can improve insulin sensitivity15, 16 but there is insufficient evidence to recommend this diet as a preventative or management strategy.
Current guidelines supporting the use of a low-fat, high-carbohydrate diet are based on data from several large studies in adults.3-5 At present, there have been no studies on a similar scale for other dietary interventions, so recommendations are unlikely to change. However, the seriousness of the diabetes crisis has forced us to evaluate all of the available evidence, and several strategies, including the very low-carbohydrate diet, a very low energy diet, lower-GI diet, and an intermittent fasting diet, appear to be worthy of consideration. In particular, a growing body of evidence supports the use of low carbohydrate diets. While we await further evidence from large scale, high quality intervention studies, it is important to remember that other lifestyle interventions involving exercise should also form part of a structured diabetes management programme.17
Summary of evidence for dietary interventions in the prevention and management of type 2 diabetes
1. Fazeli Farsani S, van der Aa MP, van der Vorst MM, et al., Global trends in the incidence and prevalence of type 2 diabetes in children and adolescents: a systematic review and evaluation of methodological approaches, Diabetologia, 2013;56:1471-88.
2. Khavandi K, Amer H, Ibrahim B, et al., Strategies for preventing type 2 diabetes: an update for clinicians, Ther Adv Chronic Dis, 2013;4:242-61.
3. Carter P, Khunti K, Davies MJ, Dietary Recommendations for the Prevention of Type 2 diabetes: What Are They Based on?, J Nutr Metab, 2012;2012:847202.
4. Dyson PA, Kelly T, Deakin T, et al., Diabetes UK evidence-based nutrition guidelines for the prevention and management of diabetes, Diabet Med, 2011;28:1282-8.
5. WHO, Diet, nutrition and the prevention of chronic diseases, http://apps.who.int/iris/bitstream/10665/42665/1/WHO_TRS_916.pdf?ua=1 Accessed 25 August 2016, 2003;.
6. Feinman RD, Pogozelski WK, Astrup A, et al., Dietary carbohydrate restriction as the first approach in diabetes management: critical review and evidence base, Nutrition, 2015;31:1-13.
7. Gow ML, Ho M, Burrows TL, et al., Impact of dietary macronutrient distribution on BMI and cardiometabolic outcomes in overweight and obese children and adolescents: a systematic review, Nutr Rev, 2014;72:453-70.
8. Volek JS, Feinman RD, Carbohydrate restriction improves the features of Metabolic Syndrome. Metabolic Syndrome may be defined by the response to carbohydrate restriction, Nutr Metab (Lond), 2005;2:31.
9. Wycherley TP, Moran LJ, Clifton PM, et al., Effects of energy-restricted high-protein, low-fat compared with standard-protein, low-fat diets: a meta-analysis of randomized controlled trials, Am J Clin Nutr, 2012;96:1281-98.
10. Fung TT, Schulze M, Manson JE, et al., Dietary patterns, meat intake, and the risk of type 2 diabetes in women, Arch Intern Med, 2004;164:2235-40.
11. Barnard ND, Cohen J, Jenkins DJ, et al., A low-fat vegan diet improves glycemic control and cardiovascular risk factors in a randomized clinical trial in individuals with type 2 diabetes, Diabetes Care, 2006;29:1777-83.
12. Brand-Miller J, Hayne S, Petocz P, et al., Low-glycemic index diets in the management of diabetes: a meta-analysis of randomized controlled trials, Diabetes Care, 2003;26:2261-7.
13. Franz MJ, The glycemic index: not the most effective nutrition therapy intervention, Diabetes Care, 2003;26:2466-8.
14. Steven S, Hollingsworth KG, Al-Mrabeh A, et al., Very Low-Calorie Diet and 6 Months of Weight Stability in Type 2 Diabetes: Pathophysiological Changes in Responders and Nonresponders, Diabetes Care, 2016;39:808-15.
15. Harvie M, Wright C, Pegington M, et al., The effect of intermittent energy and carbohydrate restriction v. daily energy restriction on weight loss and metabolic disease risk markers in overweight women, Br J Nutr, 2013;110:1534-47.
16. Harvie MN, Pegington M, Mattson MP, et al., The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women, Int J Obes (Lond), 2011;35:714-27.
17. Knowler WC, Barrett-Connor E, Fowler SE, et al., Reduction in the incidence of type 2 diabetes with lifestyle intervention or metformin, N Engl J Med, 2002;346:393-403.