The Nutrition Debate #171: “Dietary Protein and its Impact on Obesity”

“Dietary Protein and its Impact on Obesity” is the title of this précis in Diabetes in Control, a website for physicians that I monitor. It is another “take” on the study published online in Obesity Reviews I reported on here in The Nutrition Debate #170, “Your instinctive ‘appetite for protein’.” Medicalese notwithstanding, the takeaway was the same: “Analysis of percent protein in diet versus total energy intake showed that when a person’s diet was decreased from 20% protein to 10% protein, there was a significant increase in non-protein energy consumption and vice versa.”

“Age, study duration, and baseline BMI had no impact in dietary percent protein versus non-protein energy intake, but sex, however, did,” this analysis reported. “Men tended to have a higher dietary protein intake as compared to women.” This correlates with an early meta analysis I read years ago that reported that men averaged 16% dietary protein vs. 15% for women. Interestingly and I think very significantly, the Standard American Diet (SAD) recommends only 10% dietary protein (50g RDA of protein = 200kcal or10% of a 2,000kcal daily intake for a woman, as stipulated on the government’s mandated Nutrition Facts panel on processed food packaging. This very low 10% recommendation for protein is very problematic for me and ties in nicely with the hypothesis of this study, as we shall see.

“The study also analyzed high protein intake, but diets with >20% protein did not show significant correlation to decline in energy consumption,” the Diabetes in Control piece reported. “According to the authors, maintaining proper proportions of macronutrients is…important not only for our muscles and cellular building blocks, but also to keep overall non-protein (carbohydrate and fat) energy intake down. The authors also note that the study did not show any great benefit to high protein diets (>20%) which can come with other problems such as kidney failure and/or high cholesterol depending on the protein source.” I have a problem with that last clause, but will not quarrel with it here.

The “Practice Pearls” for this Diabetes in Control piece are as follows:

• Persons who maintain diets with 15-20% protein intake tend to intake less energy from carbohydrates and fats.
• Macronutrient energy intake should be calculated as a percentage of total diet; actual protein amount (i.e. grams) doesn't matter as much if it is diluted by the amount of carbohydrates and fats.
• Persons who fall in low socioeconomic status and women tend to eat less protein.

The original research, “Protein leverage and energy intake,” is copyrighted by the authors, Gosby, A. et. al., Obesity Reviews, and the International Association for the Study of Obesity (IASO). Unfortunately (from the point of view of the wide dissemination of knowledge), the full text is only available by subscription or purchase. But the Summary, a “lay” excerpt of which I quote below, is very well reasoned and illuminating:

“Increased energy intakes are contributing to overweight and obesity. Growing evidence supports the role of protein appetite in driving excess intake when dietary protein is diluted (the protein leverage hypothesis). Understanding the interactions between dietary macronutrient balance and nutrient-specific appetite systems will be required for designing dietary interventions that work with, rather than against, basic regulatory physiology. (bold added) Data were collected from 38 published experimental trials measuring ad libitum intake in subjects confined to menus differing in macronutrient composition. Collectively, these trials encompassed considerable variation in percent protein, carbohydrate and fat. The data provide an opportunity to describe the individual and interactive effects of dietary protein, carbohydrate and fat on the control of total energy intake. Percent dietary protein was negatively associated with total energy intake irrespective of whether carbohydrate or fat were the diluents of protein. The analysis strongly supports a role for protein leverage in lean, overweight and obese humans. A better appreciation of the targets and regulatory priorities for protein, carbohydrate and fat intake will inform the design of effective and health-promoting weight loss diets, food labeling policies, food production systems and regulatory frameworks.”

So, the “protein leverage hypothesis” proffers that 1) increased energy intakes has contributed to overweight and obesity; 2) that protein as a macronutrient in the human diet has been diluted by either or both carbohydrates or fat by the excess intake of one or both of them; and 3) that the “protein appetite” is driving this excess intake of either or both carbohydrate and fat. And the study concluded that the “right” amount of protein is 20%, and that this “protein leverage” applies to “lean, overweight and obese humans.”

Conclusion: “…when a person’s diet was decreased from 20% protein to 10% protein, there was a significant increase in non-protein energy consumption and vice versa.” Reminder: the SAD prescribes an RDA of 10% protein, 30% fat and 60% carbohydrate. Could it be that our government’s recommendations, the Dietary Guidelines for Americans promulgated in 1977 and repeated every 5 years since 1980, are what is making us fat*? It is comforting for me that my target macronutrient distribution is 20% protein, 75% fat and 5% carbohydrate. What’s yours?   * hat tip to Authority Nutrition for the obesity curve table.