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Article Excerpt
KEY POINTS
* Dietary protein is more satiating than carbohydrate or fat and has been shown to reduce food intake after controlled liquid preloads and meals.
* High-protein, low-energy weight-loss diets may assist compliance by increasing satiety up to three hours after a meal and providing a lower dietary variety, which has been shown to be associated with lower food intake.
* High-protein diets in ad libitum studies show greater weight loss than high-carbohydrate ad libitum diets.
* Isocaloric high-protein and high-carbohydrate diets in energy restriction achieve similar weight loss, but diets with a higher protein-carbohydrate ratio achieve greater loss of fat to lean tissue.
* High-protein low-carbohydrate diets lower triglycerides and glycosylated haemoglobin (HbA1c) more than high-carbohydrate diets.
* High-protein, low-energy weight-loss diets are more nutrient dense than normal-protein, high-carbohydrate weight-loss diets, which may not meet the recommended dietary intake, particularly for calcium, but also iron and zinc for some groups.
* Current recommendations for protein intakes may be lower than optimal for weight management to optimise satiety, body composition and micronutrient nutrition.
INTRODUCTION
To what extent does the composition of the food we eat influence how much of it we eat on an occasion, or to what extent it satisfies us sufficiently to delay the next eating occasion? These are two separate attributes--the former being termed 'satiation' and the latter 'satiety'. Much of the research that has been conducted on food composition has generally focused on 'satiety'--subjectively defined as the feeling of fullness or satisfaction that follows eating. It is generally measured by questionnaire using a visual analogue scale after a food has been consumed. In addition, some studies combine this approach with exposure of the participants to a buffet meal and measure food consumed, which represents a more objective measure of satiety. Satiety appears to be influenced by a wide variety of factors, including macronutrient profile, palatability, food mass, energy density, fibre and glycaemic index (GI). When using real foods, it is almost impossible to control for all of these influences at the same time, and if these factors are controlled, the relevance to real-life foods and diets can be questionable. In addition, the context in which the food is eaten can have a considerable effect on how much food is consumed, which may override perceived satiety. Therefore, an assortment of methodologies is important to understand how different food and diet attributes affect satiety but also food intake and, ultimately, energy balance. This paper reviews studies undertaken to demonstrate the effect of high-protein meals and diets on satiety, weight loss and diabetes management.
PRELOAD STUDIES
Several studies have compared satiety after high-protein or high-carbohydrate or high-fat meals. Typically, these studies compare satiety after different test meals in the same individual in a crossover design. In some studies, the amount of food consumed at a buffet, usually three hours after the test meal, is also assessed. In a recent review of such studies, high-protein meals were more satisfying, with 11 of the 14 studies that compared high protein with at least one other macronutrient finding the protein preload significantly increased subjective ratings of satiety. (1) Few of these studies were able to control for potentially confounding variables. However, the test meals differed widely in physical and sensory properties, so it cannot be concluded that it was the protein conferring these effects. Latner designed a study so that the sensory properties of the meals were exactly the same. (2) In 12 lean female students, 31% more calories were eaten at a subsequent dinner after a high-carbohydrate liquid lunch (450 kcal, 99% carbohydrate from polycose) than high-protein liquid (71% protein) meal or a 50%-protein, 50%-carbohydrate lunch. The protein was a dried powder mix derived from whey.
When protein is provided as a 50-g dose in the form of a beverage and compared with an isocaloric, isovolumetric and palatability-matched carbohydrate beverage, protein has also been shown to be more satiating than glucose. Bowen et al. compared liquid preloads (1.1 MJ, 450 mL) containing 50-g whey, soy, gluten or glucose. (3) Energy intake at the buffet three hours after the preload was 10% lower for all protein preloads compared with the glucose treatment (P < 0.05). Different protein sources behaved similarly. The present study also demonstrated that the effect of protein on satiety appears independent of body mass index (BMI) status, which is an important finding as almost all previous studies had been conducted in lean individuals.
HIGH-PROTEIN DIETARY PATTERNS AND WEIGHT LOSS
Fryer et al. (4) found that feelings of hunger were lowest on a high-protein diet for 12 male students with nine-week dietary periods. Several additional...
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