Protein is a weight loss food. Evidence from the nutritional literature suggests that eating a high protein diet causes weight loss in those who are overweight. The reason for this in not clear, but the weight loss effects of protein may result from a number of different mechanisms. Firstly, protein is the rate limiting step in the passage of food from the stomach to the small intestine. Protein leaving the stomach in an undigested state signals the I cells in the mucosa of the duodenum to release cholecystokinin, and this enters the circulation where it signals the stomach to close the pyloric sphincter thus trapping the protein in the stomach until it is completely digested to shorter peptides. This may have beneficial glycaemic effects as any carbohydrate consumed with the protein will also remain in the stomach for longer, ultimately slowing the digestion of starch. Protein may therefore slow the absorption of glucose and other sugars, with the resultant glycaemic effects having favourable effects on insulin sensitivity.
Protein may also have effects on the central nervous system as an appetite suppressant. This role relates to the protein leverage theory and may be applicable to many different animals, not just humans. This theory states that each organisms has a minimum threshold for protein intake. Eating high protein foods satisfies the protein threshold and turns off the appetite of the organisms. However, eating low protein foods, requires the consumption of more energy because the density of the protein in the food is much lower. Eating lower protein foods therefore results in overeating, as the organism tries to satisfy its minimum protein threshold. It is unclear how this mechanism works but may relate to the release of peptides from the enterocytes of the gut acting centrally on the hypothalamus to relay information about the protein content of the diet. This would explain the effect of protein on the release on many gut peptides that appear to have appetite regulatory effects.
Fibre is another weight loss food that has been shown to be beneficial to overweight individuals. Fibre is a heterogenous group of plant polysaccharides that possess glycosidic bonds that are not digestible to human enzymes. The reason for the weight loss effects of fibre are not fully understood but it is thought that soluble fibre may have beneficial glycaemic effects. In this regard the fibre mixes with water and forms a viscous get. This gel then physically prevents starch digesting enzymes coming into contact with the starch, and also inhibits the transport and absorption of the resultant glucose by forming a barrier in the unstirred layer of the brush border near the enterocytes. Fibre may also stimulate the release of certain peptide from enterocytes in the gut which have appetite inhibitory effects. Also certain fibres have prebiotic effects in the gut, stimulating the growth of beneficial bacterial colonies in the colon. This may produce short-chain fatty acid which are absorbed and may have appetite regulatory effects.
High protein and high fibre diets may therefore be beneficial at causing weight loss, and this effect may be greater if the two macronutrients are combined. The effects of increasing protein and fibre intake through addition of whey protein and polydextrose (a synthetic non-digestible polysaccharide) to the diet of 10 lean men has been tested1 Subjects consumed either a whey protein and polydextrose snack back or an isoenergetic alternative which replaced the protein with carbohydrate and removed the fibre. After 14 days of consuming the snack bar as a mid morning snack, daily energy intake was significantly lower in those men consuming the high protein high fibre snack bar compared to the controls during both free living conditions (2426 versus 2851 kcal, respectively) and during experimental laboratory visits (1889 versus 2161 kcal, respectively). Manipulating protein and fibre intakes upward using functional foods therefore appears an effective strategy at controlling energy intake in lean subjects.
Dr Robert Barringtons Nutritional Recommendation: This study is interesting in that it did not involve overweight or obese subjects but lean men. It is unclear if this effect would be transferred to overweight individuals, but based on studies using both high protein and high fibre diets with obese subjects, it is likely that similar effects would be observed. Consumption of the high protein high fibre snack bar caused a significant decrease in blood glucose response in response to a test meal suggesting that blood glucose control had been improved. This supports the previous studies showing beneficial glycaemic effects for protein and fibre. In addition, the high protein high fibre snack decreased ghrelin release, a hormone associated with appetite stimulation. Peptide YY (peptide tyrosine-tyrosine) and glucagon-like peptide I levels also increased following consumption of the high protein high fibre snack bar, these hormones being responsible for inhibiting gut motility and appetite, respectively.
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