Chew on this excerpt from [full text available] "Isocaloric diets: effects of dietary changes" by Leveille and Cloutier:
"Forbes and colleagues (8, 9) published several papers in 1946 on the relationship between the level of fat in the diet of adult rats and the efficiency of use of dietary energy. In those studies dietary fat varied from 2% to 30%, while intake of gross energy, protein, and essential nutrients remained constant. Researchers
observed that as the dietary fat content increased, relative heat production decreased. Thus, with increasing fat intake the apparent efficiency of food use increased, allowing for more energy to be stored in the animal carcass (see Fig 2). In essence, more energy is stored as fat in the body as the proportion of fat to carbohydrate increases in the diet.
"Increasing the protein level of the diet at the expense of carbohydrate also results in changes in apparent energy efficiency. Donald et al (10) observed that adult rats fed a high-protein diet gained more weight than a similar group fed low-protein diet when the content of dietary fat was maintained at a constant level and levels of protein varied from 5% to 25%. The animals had free access to food and water. Absolute food consumption, ie, g/rat, was not significantly different between the 5% and 25% groups. Yet total body weight gain and body fat were higher in the 25% group (Table 2).
"Humans appear to respond in much the same way as rats to changes in dietary composition. Danforth (11) noted that lean subjects gained weight relatively easily when overfed fat but not when fed a mixed diet of carbohydrate and fat. In an earlier study by Miller and Mumford (12), researchers observed that students overfed a high-protein diet gained more weight than a group of students fed a low-protein diet with a similar number of calories. The weight gain for both diets was less than the predicted value."
These studies showed that when we hold the caloric content of a human or rat diet constant, but vary the macronutrient composition, increasing either fat or protein at the expense of carbohydrate leads to greater gain of fat.
Here's table 2 from the paper:
Although fed the same number of calories, the rats fed a high (25%) protein diet weighed 23% more than those fed the low (5%) protein diet, and had 50% greater body fat percentage.
The European upper paleolithic diet was high in protein and fat and low in carbohydrate, which according to this type of research, has the highest food efficiency and promotes the highest body fat percentage.
Maybe that's why Venus was obese, maybe that's why people eating 'primal' and supposedly 'paleo' diets have trouble losing fat, maybe that's why only 3% of Japanese are obese.