When it comes to protein consumption as in body building or exercise, thoughts do not often turn to managing weight or even using it to help lose weight. However, there are many studies since the turn of the century that have examined how protein might aid weight management.

Most studies investigating protein impact on weight loss have increased the content from 10-15% which is consistent with surveys of protein consumption, to above 25% and in some cases by 40 to 70%. There is no evidence of any long term benefit for using animal versus vegetable or plant protein. There may be a short term or 24 hour creation of satiety when animal protein is rapidly digested compared to more slowly digested protein from plant or animal (Anderson and Moore, 2004). Studies investigating weight loss when diets are ad libitum show significant results in this measure which is partly due to reduced energy intake.  By comparison, isoenergetic diets show no significant changes in weight loss but improvements to body profile (Westerterp-Plantenga, 2003).

There is reasonable evidence for weight loss over 6 months from recent studies but further work is needed to determine the optimal dose, type of protein, and the functional effects of different protein elements (Anderson & Moore, 2004). To make claims for products in this category, they must contain 12g protein per quantity that can be reasonably consumed per day, and either 12% energy from protein for a “contains” claim or 20% energy from protein for a “rich in” claim based on the current UK/EU legislation.

What are the potential mechanisms  ?

Data supports enhanced satiety from protein when compared to carbohydrate and fat ingestion over 24 hours. The mechanism requires further elaboration however which may include triggering or maintaining the release of cholecystokinin (Dye & Blundell, 2002). A protein content of about 50g in a food or meal will lead to satiety but there are no dose response studies (Anderson & Moore, 2004) to confirm this finding. It needs to be pointed out that these studies did not always control for other dietary influences such as fibre intake.

Reduced energy intake at meal times may be achievable through protein ingestion, however the amount consumed in many of these studies exceeds 35% and may not be relevant to most dietary patterns (Eisenstein et al, 2002).

There is data to support a greater thermogenic energy expenditure for protein in comparison to carbohydrate or fat.  However, increasing protein content from 15% to 30% would actually result in relatively small overall energy expenditure and thus weight loss (Eisenstein J et al, 2002).

There are potential adverse affects with excessive protein intake. Excessive protein intake, say two or three times the USA recommended daily intake may be detrimental to calcium homeostasis and thus bone mass (Eisenstein et al, 2002). There are harmful effects on kidney function too.

References

Anderson, G.H., Moore, S.E. (2004) Dietary proteins in the regulation of food intake and body weight in humans J. Nutrition  134: 974S-979S

Dye, L., Blundell, J. (2002)    Functional foods: psychological and behavioural functions     Br. J. Nutr. 88: S187-S211

Eisenstein, J,. Roberts, S.B., Dallal, G., Saltzman, E. (2002) High-protein weight-loss diets: are they safe and do they work?  A review of the experimental and epidemiologic data Nutr Rev. 60 (7) pp.189-200

Westerterp-Plantenga, M.S. (2003) The significance of protein in food intake and body weight regulation.  Curr. Opin. Clin. Nutr. Metab. Care 6 pp. 635-638