Table 2.1 Studies on Macronutrients (Carbohydrate, Protein, and Fat) Published After September 2009
MACRONUTRIENT DISTRIBUTION IN THE NUTRITION PRESCRIPTION
The ADA 2008 nutrition position statement concluded that it is unlikely that there is an optimal mix of macronutrients for the diabetic diet (ADA 2008). For guidance on macronutrient distribution, the Institute of Medicine’s dietary reference intakes (DRIs) for a healthy eating pattern for adults may be helpful (Institute of Medicine 2002). The DRI acceptable macronutrient distribution ranges for carbohydrate, fat, and protein are 45–65, 20–35, and 10–35% of total energy, respectively. The position statement also notes that regardless of the macronutrient distribution, total energy intake must be appropriate for weight management. The mix of macronutrients is adjusted to meet metabolic goals and individual preferences of the person with diabetes (ADA 2012).
A 2012 ADA systematic review of the literature regarding macronutrients, food groups, and eating patterns in the management of diabetes concluded that several different macronutrient distributions may lead to improvement in glycemic and/or cardiovascular disease (CVD) risk factors and that many different approaches to medical nutrition therapy and eating patterns effectively improve glycemic control and reduce cardiovascular risk among individuals with diabetes (Wheeler 2012).
The Acad Nutr Diet EBNPG reviewed a total of 18 studies using differing percentages of carbohydrate, fat, or protein and also concluded that research does not support an ideal percentage of energy from macronutrients in the food/meal plan for people with diabetes. It is recommended that registered dietitians encourage consumption of macronutrients on the basis of DRIs (Acad Nutr Diet 2008; Franz 2010).
The Dietary Guidelines for Americans, 2010 can be used to identify a healthy eating pattern that is not a rigid prescription, but rather includes options that can accommodate cultural, ethnic, traditional, and personal preferences as well as food costs and availability. Although healthy eating patterns that meet nutrient needs over time at an appropriate calorie level can be diverse, some key elements exist: an abundance of vegetables and fruits, an emphasis on whole grains, moderate amounts and a variety of protein foods, limited amounts of foods high in added sugars, and more oils than solid fats. Research is available on beneficial health outcomes from examples of healthy eating patterns, such as the Dietary Approaches to Stop Hypertension (DASH), a Mediterranean-style eating pattern, and a vegetarian eating pattern (U.S. Department of Agriculture and U.S. Department of Health and Human Services 2010).
CARBOHYDRATES AND DIABETES NUTRITION THERAPY
Carbohydrates consist of sugars, starches, and fibers. These are the preferred names for carbohydrate categories rather than simple or complex carbohydrates, since they are based on chemical composition (DGAC 2010). Along with the acceptable macronutrient distribution ranges, the DRIs set a recommended dietary allowance (RDA) for carbohydrates of at least 130 g/day for adults and children (Institute of Medicine 2002). This RDA is based on the estimated average requirement for carbohydrate ingestion that will provide the brain with adequate glucose without additional glucose from protein or triglycerides stored in the fat cells (100 g/day) and a coefficient of variation of 15% based on the variation in brain glucose utilization. The RDA is equal to the estimated average requirement plus twice the coefficient of variation to cover the needs of 97–98% of individuals. Therefore, the RDA for carbohydrate is at least 130% of the estimated average requirement, or at least 130 g/day of carbohydrate. The ADA notes the following: “Although brain fuel needs can be met on lower-carbohydrate diets, long-term metabolic effects of very-low-carbohydrate diets are unclear, and such diets eliminate many foods that are important sources of energy, fiber, vitamins, and minerals and that are important in dietary palatability” (ADA 2012).
Definitions of carbohydrate intake have not been well defined. A high-carbohydrate intake is often defined as a carbohydrate intake ≥55% of total energy. A low-carbohydrate intake may be defined as <25% of total energy or <130 g/day. A very-low-carbohydrate ketogenic diet is defined as <20 g/day. However, differing definitions for carbohydrate intake are used. For example, a meta-analysis used 9–45% of total energy as carbohydrate as a definition of low-carbohydrate intake (Kirk 2008). As a result of this definition, there was an overlap of carbohydrate intake in the low- and high-carbohydrate groups (carbohydrate in the high-carbohydrate group ranged from 40 to 70%). Of interest is a Mediterranean-style eating pattern in subjects with type 2 diabetes that was considered to be low carbohydrate, with <50% of daily calories from carbohydrate (actual intake ~44%) (Esposito 2009), whereas for most individuals with type 2 diabetes, this intake of carbohydrate would be considered a moderate-carbohydrate intake.
It is important to note that most individuals with diabetes do not eat a low- or high-carbohydrate diet but rather report a moderate intake; studies reported an intake of ~46% in individuals with type 1 diabetes (Delahanty 2009) and ~44% in individuals with type 2 diabetes (Vitolins 2009). Furthermore, it appears difficult for people with type 2 diabetes to eat a high-carbohydrate diet. In the U.K. Prospective Diabetes Study, despite receiving individual education from dietitians on the recommended carbohydrate intake of 50–55%, patients reported a carbohydrate intake of 43% energy intake, which was similar to the general public (Eeley 1996).
Carbohydrate and Insulin Resistance
If consuming a high-carbohydrate intake contributed to insulin resistance, carbohydrate intake should be reduced, especially in people at risk for or with type 2 diabetes. Although evidence is limited, available evidence does not report an adverse effect on insulin sensitivity from carbohydrate intake; instead, carbohydrate may improve insulin sensitivity. A review compared short-term intervention studies with higher (>50% of total energy) versus lower carbohydrate intake in subjects with and without diabetes (McClenaghan 2005). Of 11 studies in subjects without diabetes, 7 reported an increase in insulin sensitivity from the higher-carbohydrate diet and 5 reported no differences. Of eight studies in subjects with diabetes, five reported improvement in insulin sensitivity from the higher-carbohydrate diet and three reported no difference. The author concluded that higher-carbohydrate diets do not adversely affect insulin sensitivity and may offer some benefits. Longer-term clinical trials and epidemiological studies in people without diabetes have also reported no adverse effects on insulin sensitivity from higher-carbohydrate diets (Ard 2004; Bessesen 2001; Howard 2006).
However, examining the effect of carbohydrate on insulin action is difficult because any change in one component of the diet is accompanied by changes in other components of the diet. Therefore, as carbohydrate intake is increased, fat is generally decreased, and vice versa. Chronic consumption of foods high in fat, especially saturated fats, as will be reviewed later, is reported to increase insulin resistance. Therefore, it is unknown if the benefit on insulin sensitivity is due to the higher carbohydrate intake or the lower fat intake.
Carbohydrates and Glycemia
The balance between digestible carbohydrate and available insulin is a major determinant of postprandial glucose levels. However, other intrinsic and extrinsic variables also influence the effect of carbohydrate on glucose levels. Continuous glucose monitoring systems can be used to better understand the postprandial effects of carbohydrate. For example, in people with type 2 diabetes, a lunch meal containing double the carbohydrate content did not double the glycemic response (Powers 2010). Under debate is what amount of carbohydrate intake best facilitates glycemic control in people with diabetes.
Type 1 Diabetes
In people receiving intensive treatment in the Diabetes Control and Complications Trial, a lower carbohydrate (37%) intake and higher total