The effectiveness of medical nutrition therapy (MNT) in the management of diabetes has been well established (1). Previous reviews have provided comprehensive recommendations for MNT in the management of diabetes (2,3). The goals of MNT are to 1 ) attain and maintain optimal blood glucose levels, a lipid and lipoprotein profile that reduces the risk of macrovascular disease, and blood pressure levels that reduce the risk for vascular disease; 2) prevent and treat the chronic complications of diabetes by modifying nutrient intake and lifestyle; 3) address individual nutrition needs, taking into account personal and cultural preferences and willingness to change; and 4) maintain the pleasure of eating by only limiting food choices when indicated by scientific evidence (4).
The literature on nutrition as it relates to diabetes management is vast. We undertook the specific topic of the role of macronutrients, eating patterns, and individual foods in response to continued controversy over independent contributions of specific foods and macronutrients, independent of weight loss, in the management of diabetes. The position of the American Diabetes Association (ADA) on MNT is that each person with diabetes should receive an individualized eating plan (4). ADA has received numerous criticisms because it does not recommend one specific mix of macronutrients for everyone with diabetes. The previous literature review conducted by ADA in 2001 supported the idea that there was not one ideal macronutrient distribution for all people with diabetes. This review focuses on literature that has been published since that 2001 date (5). This systematic review will be one source of information considered when updating the current ADA Nutrition Position Statement (4). Other systematic reviews and key research studies that may not be included in this review will also be considered.
When attempting to tease out the role of macronutrients from other dietary and lifestyle factors, two critical components of MNT - energy balance and a healthful eating pattern - are not addressed. While both are critical components in the management of diabetes as well as the secondary prevention of complications and promotion of health, these topics are beyond the scope of this particular review. The following questions are addressed in this review:
1 . What aspects of macronutrient quantity and quality impact glycémie control and cardiovascular disease (CVD) risk in people with diabetes?
2. How do macronutrients combine in whole foods and eating patterns to affect health in people with diabetes?
3. Is there an optimal macronutrient ratio for glycémie management and CVD risk reduction in people with diabetes?
4. What findings and needs should direct future research?
Systematic review procedure- A search of the PubMed database was conducted using the search terms "diabetes" and one of a number of words (low-fat diet, low-carbohydrate diet, Mediterranean diet, Mediterranean eating pattern, vegetarian, vegan, glycémie index (GI), dietary carbohydrates, dietary protein, total fat, dietary fat, saturated fat, omega-3 fatty acid, dietary fiber, meats, legumes, nuts, fruit, vegetables, whole grains, milk) to identify articles published between January 2001 and October 2010. Certain terms relevant to nutrition therapy in the management of diabetes were not included in the search terms. These terms include trans fatty acids, monounsaturated fatty acids (MUFAs), polyunsaturated fatty acids (PUFAs), sucrose, and sugars. The literature search was limited to articles published in English, and multiple publications from the same study were limited to the primary study results article.
Studies included in the systematic review were conducted in people already diagnosed with diabetes; conducted in outpatient ambulatory care settings; contained a sample size of 10 or more participants in each study group; and one of the following study designs: clinical trials (controlled and randomized controlled [RCT]), prospective observational studies, cross-sectional observational studies, or case-control studies. Studies were excluded if they were published before January 2001 or after October 2010; were conducted in acute care or inpatient settings, in women with gestational diabetes, children under 2 years of age, or individuals without diabetes or at risk for diabetes; had less than 10 participants in any study group; were studies lasting only 1 or 2 days; or were not in one of the study designs previously listed.
In an effort to expand the research review, studies were not excluded based on retention rates; however, this information is included in Supplementary Table 1 and only studies with a retention rate of >80% are included in the key summary for each topic area. Weight loss is a confounder in some of the studies and is noted in Supplementary Table 1.
Meta-analyses published during the inclusion period of this systematic review were reviewed for studies meeting this systematic review's criteria. This information can be found in Supplementary Table 1.
An initial PubMed database search found 152 studies after excluding by title and abstract review. An additional 18 studies were found from bibliography review. Of these, 72 studies were excluded for not meeting inclusion criteria. The most common reasons for exclusion were for results not applicable to the research question, not published in a major journal, small sample size, review articles, and duplicates.
Challenges in evaluating macronutrient studies in diabetes management- Isolating the effects of dietary macronutrient composition on glycémie control and CVD risk is difficult due to confounding, especially by weight loss and medication changes. Furthermore, altering the level of one macronutrient affects the proportion of other macronutrients, making it difficult to isolate the true exposure. Additional study design issues include the difficulty blinding study participants, investigators, and clinicians. Finally, the lack of standardized definitions for terms such as "lowfat (or high-fat) diet," "low-carbohydrate (or high-carbohydrate) diet," and "lowGI (or high-GI) diet" makes comparisons among study results difficult. These issues were addressed by reporting the entire macronutrient composition of diet approaches and potential confounders when this information was available.
Question 1 : What aspects of macronutrient quantity and quality impact glycémie control and CVD risk in people with diabetes?
Carbohydrate amount- There is no consistent definition of "low- (or high-) carbohydrate diets" throughout the literature. Based on the studies in this systematic review, the following definitions are used:
* very-low-carbohydrate diet: 21-70 g/day of carbohydrate
* moderately low-carbohydrate diet: 30 to <40% of kcal as carbohydrate
* moderate-carbohydrate diet: 40-65% of kcal as carbohydrate
* high-carbohydrate diet: >65% of kcal as carbohydrate
These definitions are not all-inclusive (e.g., a 100-g/day carbohydrate diet may be <30% kcal), but they represent the typical definitions used by authors, and all published articles fit in one of these categories.
Many studies use the term "conventional" or "traditional" macronutrient distribution as a comparison group. Based on studies in this review, these terms refer to an energy contribution from the diet of 55-65% carbohydrate, <30% fat, and 10-20% protein. It should be noted that people with diabetes have been shown to consume an eating pattern that is about 45% of calories from carbohydrate (6-9). The comparison diets referred to as conventional or traditional throughout this review are higher in carbohydrate than those generally consumed by people with diabetes.
Lower (very low and moderately low) carbohydrate
Glycémie control. Eleven clinical trials examined the effects of lowering total carbohydrate intake on glycémie control in individuals with diabetes. The carbohydrate content goal of the diet was very low in 7 studies (10-16) and moderately low in 4 studies (17-20).
All studies included adults with type 2 diabetes, duration of follow-up ranged from 14 days to 1 year, and sample sizes ranged from 10 to 55 participants per study group. Designs included two feeding trials (one crossover clinical trial and one RCT) (10,18) and nine outpatient nutrition counseling interventions (two single-arm clinical trials, one crossover RCT, and six parallel RCTs) (11-17,19,20). All studies analyzed participants according to treatment assignment, eight studies were randomized (11-13,15-19), and for six studies, completion of follow-up was 80% or higher (10,12,13,17-19).
AlC decreased with a lowercarbohydrate diet in 6 of 10 studies in which it was measured (10,14-17,20). Three RCTs found no statistically significant changes in AlC with a very-lowcarbohydrate diet (11-13) and one found no difference with a moderately low-carbohydrate diet (19). Other glycémie parameters such as fasting blood glucose (FBG), 24-h blood glucose, 24-h insulin (10), and fasting insulin levels (18) decreased significantly, and insulin sensitivity increased significantly (10) on the lower-carbohydrate diet. Glucoselowering medications were decreased for individuals following the lower-carbohydrate diet (10-12,14,17) or were more frequendy decreased than in the comparison diet (16).
CVD risk. Each of the 1 1 clinical trials reported at least one serum lipoprotein. The most notable results were that HDL cholesterol increased significantly more in one very-low-carbohydrate diet group (16) and two moderately low-carbohydrate diet groups (18,20) compared with the higher-carbohydrate control diet. Also, triglycerides (TGs) decreased more in one moderately low-carbohydrate diet group (20) compared with the higher-carbohydrate control diet. Otherwise, mean changes in serum lipoproteins resulting from a lowercarbohydrate diet were typically beneficial but occurred without a comparison arm or
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