Background and Significance: The incidence of type 2 diabetes mellitus (T2DM) and pre-diabetes continues to increase. Many individuals are not aware of having the disease or early stages of the disease, pre-diabetes, until symptoms develop and it is of particular importance to try to prevent the development of T2DM during the pre-diabetic stage. T2DM is a major contributor to morbidity and mortality due to its increased risk of cardiovascular disease (CVD), where more than half of individuals with T2DM will die from complications due to CVD. Diet and lifestyle modifications play a critical role in the development and prevention of pre-diabetes and T2DM. Much focus has been given to limiting carbohydrate consumption within this population, yet there are no established recommended dietary allowance (RDA) for pre-diabetes and T2DM for protein consumption. The intake of protein above the RDA may play a role in prevention or progression of T2DM and risk factors associated with CVD. There is a sparsity of studies examining the role of protein on glycemic control as well as CVD in people with pre-diabetes and T2DM. Therefore, the purpose of this cross-sectional study was to examine the relationship between protein consumption as well as animal versus plant protein on glycemic control and indices of cardiovascular health in individuals with pre-diabetes and T2DM. Methods: A total of 62 men and women with pre-diabetes or T2DM that were overweight or obese between the ages of 45-75 years old were in included in this study. Participants were stratified based on their intake from a three-day food record into the following three groups based on their protein intake: protein levels of less than 0.8 grams per kilogram body weight (g/kg/bw), between 0.8 to 1.0 g/kg/bw and greater than or equal to 1.0 g/kg/bw as below-, meeting-, and above recommended-levels, respectively. During the study visit, after an overnight fast, participant's blood was drawn, anthropometric parameters were assessed, resting brachial blood pressure was taken, duel-energy x-ray absorptiometry (DXA) scans were completed, and three-day food records were assessed. Results: Weight, body mass index (BMI), and hip circumference (HC), were higher in participants who consumed below and meeting the recommended levels of protein (P≤0.01 for all values and P<0.05 for all values, respectively) than those in the above recommended protein group. Waist circumference (WC) was greater (P≤0.01) for participants who consumed below recommended protein levels versus those who consumed above the recommended level. There were differences (P<0.05) in total energy, monounsaturated fat, polyunsaturated fat, cholesterol, carbohydrate, and fiber intake, as well as animal and plant protein consumption amongst groups. When groups were compared, intake of animal protein, similar to total protein was different amongst groups (P≤0.01). Whereas plant protein intake was only greater in participants consuming above the recommended protein level (P≤0.01, P=0.02, below and recommended protein levels, respectively). The results of this study suggest that the higher the animal-to-plant protein ratio the higher the insulin secretion as indicated by the HOMA- β equation. Our findings also demonstrated that the higher the plant protein intake the lower the high density lipoprotein (HDL)-cholesterol concentration. Furthermore, a higher animal protein intake was associated with lower triglyceride (TG) concentrations. Participants who consumed above recommended protein had lower (P≤0.01) fat mass (FM) and higher lean mass (LM) (P<0.05) in all sites assessed when compared to participants who consumed below or recommended protein levels. Fat-to-lean mass ratio (FM/LM) ratio was lower in participants who consumed above the recommended protein level when compared to participants who consumed below (P=0.03) and recommended protein (P≤0.01) levels. Total-, low density lipoprotein (LDL)-, and HDL-cholesterol were significantly higher in participants who met the recommended protein intake (P=0.01, P≤0.01, P=0.04, respectively) versus participants who consumed below the recommended protein. Conclusion: Findings of this study suggest that consumption of greater than or equal to 1.0 g/kg/bw protein is associated with lowered insulin resistance in addition to BMI, HC, WC, and FM without having adverse effects on lipid profiles in overweight and obese individuals with pre-diabetes or T2DM. Additionally, increased animal protein was associated with better β-cell function and lower TG levels.
