There’s the common view of obesity that it’s due to increased fat and/or carbs in the American diet. And the statistics bear out that increase (Gregory L Austin, Lorraine G Ogden, James O Hill; Trends in carbohydrate, fat, and protein intakes and association with energy intake in normal-weight, overweight, and obese individuals: 1971–2006, The American Journal of Clinical Nutrition, Volume 93, Issue 4, 1 April 2011, Pages 836–843):
The prevalence of obesity increased from 11.9% to 33.4% in men and from 16.6% to 36.5% in women. The percentage of energy from carbohydrates increased from 44.0% to 48.7%, the percentage of energy from fat decreased from 36.6% to 33.7%, and the percentage of energy from protein decreased from 16.5% to 15.7%.
There’s an interesting note:
In NHANES 2005–2006, a 1% increase in the percentage of energy from protein was associated with a decrease in energy intake of 32 kcal (substituted for carbohydrates) or 51 kcal (substituted for fat).
What is the Protein Leverage Hypothesis?
The central claim is that protein is being displaced by increasing amount of carbs and fat. From this paper (Alison K. Gosby , Arthur D. Conigrave, Namson S. Lau, Miguel A. Iglesias, Rosemary M. Hall, Susan A. Jebb, Jennie Brand-Miller, Ian D. Caterson, David Raubenheimer, Stephen J. Simpson. Testing Protein Leverage in Lean Humans: A Randomised Controlled Experimental Study. PLoS ONE 6(10): e25929.):
The ‘protein leverage hypothesis’ proposes that a dominant appetite for protein in conjunction with a decline in the ratio of protein to fat and carbohydrate in the diet drives excess energy intake and could therefore promote the development of obesity.
The study found:
In our study population a change in the nutritional environment that dilutes dietary protein with carbohydrate and fat promotes overconsumption, enhancing the risk for potential weight gain.
Here’s the chart showing the differences:
From the study:
Simpson and Raubenheimer (Simpson, S. J. and Raubenheimer, D. (2005), Obesity: the protein leverage hypothesis. Obesity Reviews, 6: 133-142.) used data from the FAOSTAT  nutrient-supply database to show that an estimated decrease in percent dietary protein from 14% to 12.5% between 1961 and 2000 in the USA was associated with a 14% increase in non-protein energy intake, with absolute protein intake remaining almost constant.