Scientific evidence from the nutritional literature supports the contention that protein is an anti-obesogenic agent. The weight loss effects of protein are well reported, and consistently high protein diets are shown to be superior to high carbohydrate diets in this regard. The reason for the benefits of high protein diets may be multifactorial, but their satiety inducing effects are certainly of significance. The appetite inhibiting effects of high protein diets is interesting because it is a relative phenomenon and is therefore difficult to asses. For example, high protein diets increase satiety compared to high carbohydrate diet, but it is difficult to ascertain whether it is the high protein diets decreasing appetite or the high carbohydrate diet increasing appetite. Certainly there is good evidence that refined carbohydrates and sugar affect the neurochemistry of the brain, and induce the release of factors that may act in a stimulatory way towards further food intake. Irrespective of this nuance, high protein diets appear to be effective at inducing satiety.
The protein leverage hypothesis states that an organisms appetite is based on its protein intake, in order to fulfill its requirement for amino acids. Foods containing lower amounts of protein stimulate further eating until this protein requirement has been met. High protein diets more rapidly fulfill the amino acid requirement of the organisms and this then inhibits further food intake. Protein is important in organisms because it is required at a minimum level to maintain nitrogen balance. As this requirement increases through factors such as stress, exercise or illness, the protein requirements of the organisms increases concomitantly. Further, protein is the determining factor in the release of chyme from the stomach and in this way slows digestion and has short term satiety effects by the stimulation of stretch receptors in the stomach wall. Undigested peptides detected in the small intestine cause the release of cholecystokinin to the plasma and this inhibits the release of protein from the stomach by contraction of the pyloric sphincter.
Because protein is involved in the regulation of satiety, skipping meals may increase the risk of hunger in the subsequent period. Such hunger is detrimental because it increases the likelihood that low quality snack foods will be consumed. Because of the protein hunger connection, researchers have investigated the effects of a high protein breakfast in comparison to a lower protein breakfast and no breakfast in healthy, but overweight or obese adult women1. All of the 20 subject consumed each breakfast type in a random order for 6 days, before completing a questionnaire based on their experiences. Consumption of breakfast reduced the hunger experienced later in the day with the high protein diet increasing the feeling of fullness to a greater extent than the lower protein breakfast. Measurements of the hunger inducing hormone ghrelin were reduced, and the satiety inducing peptide YY increased, only in the high protein breakfast group. The high protein breakfast also reduced evening snacking.
These results support the contention that protein reduces appetite and this may explain its weight loss effects. The high protein diet in this study comprised of 350 kcal and included a supplements of egg and beef rich protein. This provided 35 grams of protein, with an energy ratio of 40 % protein, 40 % carbohydrate and 20 % fat. In contrast, the lower protein diet contained its 350 kcal in an energy ratio of 15 % protein (13 grams of protein) with 65 and 20 % of the remaining energy coming from carbohydrate and fat, respectively. Because the corticolimbic system of the brain is thought to be involved in the desire for food, the researchers used an magnetic resonance imaging scan to determine the activity of particular brain regions following feeding. Both breakfasts were able to reduce pre-dinner amygdala, hippocampal and midfrontal corticolimbic activation compared to no breakfast, suggesting that the neurochemistry within the brain had been modified to inhibit the region of the brain identified as playing a role in food desire and feeding.
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