Understanding the Effects of Carbohydrate Type and Amount of Intake on Glucose Homeostasis

Type 2 diabetes (T2DM) is characterized by inadequate glycemic control and is associated with many health complications. Its ever-increasing prevalence in the United States and around the world calls for effective prevention and management strategies. Evidence suggests that achieving weight loss and reducing postprandial glycemia are two viable options for these purposes. The type and amount of dietary carbohydrate (CHO) intake play important roles in weight management and postprandial glucose homeostasis. First, even moderate levels of weight loss (5-10%) can improve glycemic control. Evidence from intervention trials suggests that low-CHO diets, especially very low-CHO ketogenic diets (VLCKDs), are superior to traditional calorie-restricted normal-CHO/low- fat diets for weight loss. However, it is not clear whether low-CHO intake can improve the glycemic control achieved by weight loss alone. To fill this gap in knowledge, we studied the effects of diets with varying CHO contents on weight loss-induced changes in indices of glycemic control using a systematic review and meta-regression approach. Diets were categorized into normal-CHO (NCD, ≥ 45 to ≤ 65% of total energy intake from CHO), moderate/low-CHO (MLCD, \u3c 45% of total energy intake from CHO and non-ketogenic), or VLCKD (diets with initial CHO intake \u3c 50 g/d). We performed meta-regression analyses on changes in fasting glucose and insulin concentrations using data obtained from 45 articles. Our results show that greater weight loss was associated with greater reductions in fasting glucose concentrations independent of CHO content of the diets. In contrast, greater weight loss was associated with greater reductions of insulin concentrations for NCDs and MLCDs, but not VLCKDs. In addition, VLCKDs were associated with greater reductions in fasting insulin concentrations with limited weight loss in comparison to NCDs and MLCDs. However, this superiority diminished progressively as more weight loss was achieved, reaching equivalence when weight loss was ~11%. In conclusion, our systematic review and meta-regression analysis underscores the importance of weight loss, despite of varying carbohydrate intakes, on progressive improvement in fasting glycemic control. Also, we showed the potential for ketosis-inducing diets to more effectively improve insulin resistance with moderate, but not substantial weight loss. Second, beverages consumed by the U.S. adults are one of the major sources of daily carbohydrate and added sugar intake. Due to the variations in carbohydrates and other nutrients of beverages consumed, it is expected that they could elicit differential postprandial glucose and insulin responses. In chapter 3, we examined the effects of commonly consumed breakfast beverages with equal carbohydrate quantity and volume on postprandial glucose and insulin responses when consumed with a meal. Individuals who were overweight and obese without type 2 diabetes were studied. The beverages used were water, sugar-sweetened coffee, reduced-calorie orange juice (OJ), and 1% milk. Among 46 participants, we observed that when consumed with a meal, sweetened coffee caused the highest glucose response compared to water, OJ, and milk, while insulin responses for sweetened coffee were comparable to milk but were higher than OJ and water. We then compared 3 varieties of milk that differed in their fat content (skim milk, 1% milk, and whole milk). Our data revealed no differences in the glucose and insulin responses among them. In conclusion, sugar-sweetened coffee is a poor beverage choice compared to water, OJ, and fluid milks with regards to postprandial glycemic control of obese individuals without T2DM. Additionally, the fat content of milk did not significantly influence postprandial glycemic control. Lastly, individuals with T2DM are characterized by pancreatic beta-cell dysfunction and insulin resistance, and are more susceptible to postprandial hyperglycemia. Thus, beverage choices should be considered as part of their meal planning for blood glucose management. In chapter 4, we reported pilot investigations of the same research questions as chapter 3 but among individuals with T2DM. In contrast to results from chapter 3, the pilot data obtained from 7 participants did not support differential effects of coffee, OJ, and 1% milk on postprandial glucose responses. Insulin responses were only elevated after 1% milk consumption. In addition, similar to chapter 3, we found no differences in postprandial glucose or insulin responses among the 3 varieties of milk. Due to the pilot nature of this investigation, no recommendations can be made regarding beverages choices for individuals with T2DM, however, the data can be used for future explorations on the effects of beverage consumption on postprandial glucose homeostasis