Glycemic response to carbohydrate and the effects of exercise and protein

Abstract

Objective

The purpose of this study was to investigate the effects of pre-exercise and protein coingestion on the glycemic response to carbohydrates.

Methods

Twenty-one volunteers (13 males and 8 females) aged 22 y (± 3.8 y) participated in four trials in random order. These included: 1) glucose ingestion at rest (RG), 2) glucose and protein ingestion at rest (RGP), 3) glucose ingestion after exercise (EG), and 4) glucose and protein ingestion after exercise (EGP). Exercise consisted of 45 min of cycling at 60% of participants’ age-predicted maximum heart rate. Test drinks contained 50 g glucose or 50 g glucose with 20 g whey protein. Venous blood samples were taken at baseline and subsequently every 15 min for 2 h after drink consumption. Blood plasma was subsequently analyzed for glucose and insulin.

Results

Plasma glucose concentration was significantly lower in the RGP group than in the RG group at 30, 45, 60, and 75 min and in the EGP group than in the EG group at 30, 45, and 60 min (P < 0.05). Plasma insulin area under the curve was significantly higher in the RGP group than in the RG group and in than in the EGP group than in the EG group (P < 0.05). No significant effect of exercise was seen on glycemic or insulinemic responses.

Conclusions

Coingestion of protein with carbohydrate reduces glycemic response and increases insulinemic response in healthy subjects, whereas pre-exercise seems to have no effect.

Introduction

Type 2 diabetes mellitus (T2DM) is a largely preventable lifestyle disease, yet remains extremely prevalent in our society. Controlling blood glucose levels is important in the prevention and management of T2DM, as chronic hyperglycemia may increase the risk for insulin resistance and T2DM, and is the main cause of diabetes-associated complications [1], [2]. Carbohydrate-containing foods have a large influence on blood glucose levels, which have been a focal point of significant research [3], [4], [5], [6].

It has been suggested that the overall glycemic index (GI) and glycemic load (GL) of the diet are independently related to the risk for developing T2DM [7], [8]. Studies have found that low GI or GL diets may result in reduced insulin release, improved glycemic control, and increased insulin sensitivity, and ultimately might help prevent T2DM; whereas high intakes of high GI and GL carbohydrates over short-term and long-term periods may produce greater insulin resistance and increase the risk for developing T2DM [7], [8], [9]. Understanding ways to control the glycemic response to carbohydrate is important in preventing T2DM and reducing the risk for complications associated with existing diabetes.

The glycemic response to carbohydrate is typically thought to be dependent on the amount and type of carbohydrate consumed [10]. However, many other factors also might influence the glycemic response, including age, sex, metabolism, exercise status, and coingestion of other macronutrients [11], [12], [13].

Exercise also might play a large role in mediating the glycemic and insulinemic responses to carbohydrates [14]. Long-term exercise training over periods of at least 12 wk has been shown to improve insulin sensitivity and reduce glycemic response [11], [13], [15], [16], [17]. However, there is some discrepancy in the literature regarding the acute effects of exercise on postprandial glycemic response. Some studies have reported impairment in glucose tolerance immediately after aerobic exercise [17], [18], [19], whereas others have observed improvements [11], [20]. The use of different exercise intensities, modes (e.g., aerobic versus resistance), durations, timing of exercise relative to feeding and methods of testing glucose tolerance (e.g., oral glucose tolerance test, frequently sampled intravenous glucose tolerance test) within these studies may be the cause of these inconsistent findings.

Another possible mechanism for reducing the glycemic response to a carbohydrate is the coingestion of protein [21]. The addition of between 5 g and 50 g of protein to a carbohydrate load has been shown to reduce the area under the blood glucose curve in both healthy and diabetic patients in a dose-dependent manner [21], [22], [23]. The proposed mechanisms by which protein mediates these effects on blood glucose are either by stimulating insulin release, and/or the presence of protein in the gut, which may slow the digestion and absorption of carbohydrates [3], [24], [25]. It is believed that the amino acid profile of proteins also might affect the insulinemic response [26], [27].

The simple combination of lifestyle and dietary strategies (adding protein to a carbohydrate meal after exercise) has the potential to significantly lower the postprandial glycemic response. The magnitude and potential interaction of these effects have not been compared within the same group of individuals. A more thorough understanding of these interactions might enable more specific and appropriate lifestyle recommendations targeted at reduced glycemic responses to possibly assist in the prevention of T2DM. This study aims to investigate the individual and combined effects of prior aerobic exercise and protein coingestion on the glycemic and insulinemic responses to carbohydrates in healthy individuals.