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How much Protein should you eat?

Human Protein Requirements
Human protein requirements vary significantly

A recent study showed that lower protein diets extended the life of fruit flies.[1] Proponents of low protein diets feel that this study provides justification for lower protein in human diets. Unfortunately, this is not a valid conclusion because different species have different nutritional requirements. Nutrition is not an ideology. Nutrition is a science and any conclusions about what is good and what is bad should be based on scientific evidence.

The study on fruit flies, conducted at the University of Sydney, showed that the highest longevity was achieved by a protein-to-carbohydrate (P:C) ratio of 1:16, whereas the egg-laying rate was maximized at a P:C ratio of 1:2. The maximum lifetime egg production, which corresponds to the optimum nutrition for fruit flies, was attained with a P:C ratio of 1:4.

Human protein requirements depend on many factors, including the degree of physical activity. The chart above shows this variability. The Institute of Medicine, the scientific body which establishes national nutritional policies, recommends a minimum of 0.8 grams of protein per day per kilogram of body weight.[2] This corresponds to 54 grams of protein for a person weighing 150 pounds (68 kg). So how does this compare with the fruit fly diet?

The Zone diet promoted by Dr. Sears has a a proportion of 30% Protein, 40% carbohydrate, and 30% fat. The Zone diet P:C ratio is 1:1.3 which is considerably higher in protein than any of the fruit fly diets. However, the standard USDA diet recommends a proportion of 15% Protein, 55% Carbohydrate, and 30% fat. The P:C ratio of the USDA diet is 1:3.7 which is very close to the optimal 1:4 ratio for the fruit fly. Maybe we are not that different from fruit flies after all.
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[1] Lee KP, Simpson SJ, Clissold FJ, Brooks R, Ballard JW, Taylor PW, Soran N, Raubenheimer D., Lifespan and reproduction in Drosophila: New insights from nutritional geometry. Proc Natl Acad Sci USA. 2008 Feb 11; PMID: 18268352

[2] Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients).

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