Nutrient Overfeeding in the Metabolic Chamber: Protein, not Fat Triggers Compensatory Increase in Energy Expenditure

In case you are asking yourselves why the subjects didn't live in the metabolic chamber for the whole study period, take a look at the photo of the metabolic chamber of HealthQ as an example for the "ample" space in a research instrument like this.

Scientist from the Pennington Biomedical Research Center have recently conducted a sstudy to quantify the effects of excess energy from fat or protein on energy expenditure of men and women living in a metabolic chamber.

A metabolic chamber is a small room a person can live in for a 24 hour period. It is used to measure the metabolic rate of said individual during meals, sleep, and light activities. To do so, scientists measure the heat released from a person's body to determine the energy expenditure of the individual continuously. Compared to "regular" VO2 based short-term measurements studies like the one at hand will therefore provide significantly more reliable information about the effects of certain foods, exercise and - as in this case - macronutrients on metabolic parameters.
In the study at hand, Bray at el. put 25 sedentary, but healthy subjects aged 18–35 years (BMI 19.7–29.6 kg/m²) on iso-caloric diets containing an extra 40% of energy for not just one, but 56 days.

Figure 1: Graphical representation of the study design. The figure shows the baseline period followed by the 8 wk of overfeeding and the timing of the experimental procedures (Bray. 2015).

The effects of the diets that varied in their protein content from 5%, to 15%, and 25% was measured on four occasions. On day 1, 14, and 56 of overfeeding and on day 57 while consuming the baseline diet. In addition, the scientists measured metabolic and molecular markers of muscle metabolism via skeletal muscle biopsy specimens.

"All food was prepared by the metabolic kitchen and provided to the participants in a 5-d rotating menu over the entire inpatient stay. Overfeeding diets provided 40% of energy above the final caloric prescription determined at baseline to maintain weight on the metabolic ward. Menus were prepared in duplicate and food composites analyzed for nutrient composition.  To ensure feeding compliance, all meals were consumed under supervision by inpatient personnel" (Bray. 2015).

When expressed per kilogram of weight, the 5% low protein group (LPD) consumed 0.68 +/- 0.07 g/kg per day, the NPD group consumed 1.80 +/- 0.25 g/kg per day, and the HPD group consumed 3.01 +/- 0.30 g/kg per day during the overfeeding period.

Figure 2: Macronutrient composition (g/day) of the diets before and during the intervention (Bray. 2015).

As Bray et al. point out in the corresponding paper in the American Journal of Clinical Nutrition, the carbohydrate intake in grams per day was maintained throughout the overfeeding period with the balance of energy as fat. Practically speaking this meant that carbohydrates provided 60% of calories at baseline and 42–43% in all 3 overfeeding diets. Since the rest of the energy came from fat, this means that the subjects in the three study groups consumed 49% in the LPD, 42% in the NPD, and 30% in the HPD.
The results of the study are not really surprising. As it was to be expected based on previous studies which highlight that high fat overfeeding is the most, high protein overfeeding the least obesogenic way of stuffing yourself:

Figure 3: Relative changes in muscle mass and body fat (%) in the different groups (Bray. 2015).

• The low-protein and thus higher fat diet group did not experience an increase in 24EE and SleepEE and did thus gain the most amount of body fat
• The normal protein and the high protein groups on the other hand saw increases in both 24EE and SleepEE increased in relation to protein intake (r = 0.50, P = 0.02).
• All three diets increased the amount of adipose tissue (body fat), but only the normal and high protein diets lead to concomitant increases in muscle mass even in the absence of resistance training (see Figure 3).

It is also not exactly surprising that the scientists were able to show that the increase in energy expenditure over 8 weeks was correlated with protein intake (r = 0.60, P = 0.004). The amount of energy the subjects consumed, however, showed no significant correlation with the effect the diets had on 24hEE and SleepEE.

Figure 4: Scatter plots of the individual increases in 24h (left) and sleeping energy expenditure (middle) as well as the changes in energy expenditure in selected organs (right | Bray. 2015).

Similarly worth mentioning are the facts that (1) protein and fat oxidation were reciprocally related during overfeeding (meaning the more fat was oxidized, the less protein had to be burned as fuel) and that (2) the observed 24EE was higher than predicted on days 1 (P # 0.05), 14 (P = 0.0001), and 56 (P = 0.0007) and (3) it was not the muscle tissue but other organs that burned the extra energy (you can see the individual contributions in Figure 3, right).
References:

• Bray, George A., et al. "Effect of protein overfeeding on energy expenditure measured in a metabolic chamber." The American Journal of Clinical Nutrition (2015): ajcn-091769.