Effect of Protein in a Low-Carb Diet - Good or Bad?

In the July issue of Current Opinons in Clinical Nutrition and Metabolic Care an interesting review was published - Effects of dietary protein on glucose homeostasis [Carbohydrates]. Within the abstract the authors write, "Recent intervention trials revealed that, in the short-term, the intake of proteins at the expense of carbohydrates increases satiety and thereby lowers intake of calories. High protein intake augments prandial insulin secretion and might thereby improve glycaemic control in type 2 diabetic patients."

Then they turn to the predictable - issue a caution and the reasons why such a diet should be avoided. "On the other hand, epidemiological studies suggest that chronic high dietary protein intake is associated with increased incidence of type 2 diabetes. Furthermore, a short-term increase in plasma amino acid concentrations has been shown to directly induce insulin resistance in skeletal muscle and stimulate endogenous glucose production."

So what is one to make of this? Does a low-carb diet offer a benefit or not?

To answer that we have to determine if the authors provide justification for their position. It's important to keep in mind when reviewing a study or review of studies that it is the obligation of the author to build an explicit arguement for their claim using appropriate data. It is our obligation as readers to determine if the claims are warranted.

The big question here is, are the studies cited to highlight potential adverse effects similar in context? Basically, is existing data about potential adverse effects from other studies transferable and thus appropriate for extrapolation?

In this case, the answer is a firm "no."

The reason is that the studies used to support the beneficial effects of low-carb diets are well-controlled clinical trials that specifically investigate a low-carb diet; those used to claim potential adverse effects are epidemiological studies and data from studies where amino acids are directly inflused into the blood stream. Neither specifically include low-carb dieters or sub-groups consuming a carbohydrate restricted diet.

This is an important point. Especially when we're mindful of the fact that it is incumbent upon the authors to articulate their position and also reference data in the right context. In this case, the context is a low-carbohydrate diet. Where data is cited for potential detriment, the findings must be in the context of a low-carbohydrate study population, or the authors must acknowledge their extrapolation is beyond the specifics studied in those references they do use and justify why using studies out-of-context is warranted in their review.

They do neither. And we find the epidemiological studies are problematic mainly due to the fact they are not in the proper context. That is, they're investigating the effect of dietary habits of a large population consuming a carbohydrate-rich diet. So, while the finding that one is at an increased risk for diabetes when they consume high amounts of red meat and processed meat applies to a population consuming a carbohydrate-rich diet, it cannot be extrapolated to apply to a population consuming a low-carbohydratre diet. It's out of context and beyond the scope of the data available.

We find similar problems with the data presented from studies where subjects were infused with amino acids directly into their blood stream. They too were consuming a typical diet before the testing. But more importantly, humans don't typically have amino acids infused into them. So the delivery of the amino acids makes the extrapolation of this data subject to suspicion as a justification to say the data should apply to those following a low-carb diet. In this case, it's not even comparing apples to oranges, it trying to compare apples to rocks.

In this study, the authors do not meet their obligation to build an explicit arguement for their claim. And, it seems, they missed the opportunity to highlight an area of research where data is slim - the metabolic effect of protein intake in the long-term while consuming a carbohydrate restricted diet.