It's an important review however, bringing together a comprehensive and impressive set of data - 87 studies made the cut and were included in the analysis. Oh, and I think it's important to note, the researchers in this study - from the Department of Food Science and Human Nutrition and of Statistics, University of Florida - none have previously published studies investigating low-carb diets or low-fat diets. Noteable too is the conflict-of-interest disclosure - the paper was not the product of any outside funding - in fact, the paper states the lead researcher, James W Krieger, funded the effort.
So, then, let's get to the good stuff - what the researchers wanted to know and what they found when they started looking at all this data! They reviewed a number of factors:
- Body mass change
- Fat-free mass change
- Percentage changes in body fat
- Fat mass changes
After this comprehensive review and analysis, their conclusion was "Low-carbohydrate, high-protein diets favorably affect body mass and composition independent of energy intake, which in part supports the proposed metabolic advantage of these diets."
Body mass change
Diets with carbohydrate intake in the lowest quartile were associated with a 1.6–1.7kg greater body-mass loss than were diets with carbohydrate intake in the highest 3 quartiles. When carbohydrate intake was categorized as low or high, the significant effect in the low-carbohydrate intake group remained.
Fat-free mass (FFM) change
The amount of FFM retained tended to increase with each successive quartile of protein intake, with a significant difference existing between the upper 2 quartiles (greater than 1.05g/kg) and the first quartile (0.7g/kg). Specifically, the third quartile ( greater than 1.05 and 1.2g/kg) was associated with 0.78kg additional FFM retention and the fourth quartile (greater than 1.2g/kg) was associated with 0.96kg additional FFM retention.
Compared with carbohydrate intake in the lowest quartile, the carbohydrate intake in the highest quartile was associated with 0.98kg greater FFM retention. Carbohydrate intake in the second and third quartiles tended to be associated with 0.62–0.65kg more FFM retention.
In their discussion, the researchers state "Protein intake was a significant predictor of FFM retention. A daily protein intake of greater than 1.05g/kg was associated with a greater FFM retention than was a protein intake closer to the RDA. The magnitude of this effect increased when studies of greater than 3-mo duration were analyzed. Thus, the protein RDA may not be optimal for FFM retention during energy restriction, particularly during prolonged periods of dieting. Energy restriction can decrease nitrogen balance and thus decrease the amount of protein and FFM retained by the body. An increase in protein intake would increase nitrogen balance and thus increase the amount of FFM retained."
Percentage changes in body fat (BF)
Diets with a carbohydrate intake in the lowest quartile were associated with a 1.32–1.48% greater decrease in percentage BF than were diets with carbohydrate intake in the highest 3 quartiles. When carbohydrate intake was categorized as low or high, the significantly greater decrease in percentage BF in the low-carbohydrate intake group remained.
Fat mass (FM) changes
Diets with carbohydrate intake in the lowest quartile were associated with a 1.79–2.32kg greater loss of FM than were diets with carbohydrate intake in the highest 3 quartiles. When carbohydrate intake was categorized as low or high, low-carbohydrate diets were associated with a greater loss of FM than were high-carbohydrate diets.
One claim often made by those who insist that low-carb diets are not effective is the contention that weight loss is just water loss. Here, the researchers are clear - "Compared with higher carbohydrate intakes, low-carbohydrate diets increased the loss of body mass, BF, and percentage BF, even after control for energy intake as a covariate in the regression analyses. The mean total carbohydrate intake in the low-carbohydrate studies ranged from 79g–97g, depending on the analysis. Typically, a carbohydrate intake of less than 100g will cause ketosis. These results support the apparent metabolic advantage of low-carbohydrate, ketogenic diets. The additional body mass change is not likely due to water loss, because the duration of the diet periods (6–24 wk) was too protracted and estimations of total body water tend to be similar between low-carbohydrate and low-fat diets after 2-wk. The similar results of the analyses on body mass and BF also supports the concept that the effect on body mass of low-carbohydrate diets is an effect on FM rather than on body water."
At the end of their discussion, the researchers - a second time - make a bold statement - In conclusion, low-carbohydrate diets may increase the loss of body mass, FFM, FM, and percentage BF during weight reduction compared with traditional diets. The RDA for protein may be insufficient for optimal FFM retention during weight loss; high protein intakes (greater than 1.05g/kg) may improve FFM retention.
The researchers did a good job of selecting studies and weeding out those that did not meet their inclusion criteria. If you recall, last week I wrote about how poor data results in useless analysis in my article Comparing Low-Carb to Low-Fat: Analyzing a review. A quality review requires quality data - quality data comes from participants actually sticking with a particular dietary approach.
More importantly, this particular review highlights the importance of protein intake. The researchers took the additional step to analyze not percentage of calories, but actual gram intake in each study. This approach allowed us to gain a better understanding of how critical protein is in our diet. Those consuming protein at the RDA did much worse than those consuming a higher protein intake.
The review puts the spotlight on just how important protein consumption is when you're following a weight loss diet - higher in carbohydrate or lower in carbohydrate - protein is a critical, essential nutrient and if you reduce your intake of protein, you'll lose more lean body mass. This fact is all the more obvious in this review and calls into question, once again, the standard recommendation to consume protein as a percentage of calories (10-15%) while trying to lose weight. As I've pointed out a number of times - following that advice reduces protein intake dramatically - something this paper clearly shows is detrimental to lean body mass in weight loss regardless of carbohydrate intake.
Let's not forget, protein isn't just an essential nutrient - it also offers a higher degree of satiety when one is trying to lose weight too!Take away message from this review - low-carb diets work, they do have a metabolic advantage and it's critical to make sure you're eating enough protein each day while keeping your carbohydrate intake low!