Monday, April 6, 2009


Casein is the main protein in cheese made from cow milk. (Photo by cwbuecheler)

The war over weight loss is usually fought between the low-carb and the low-fat folks, both of whom are absolutely convinced they're right and the other side is wrong. If you've read diet forums, you know what I'm talking about (and if you've read my blog, you know that it's really the low-fat people who are wrong).

Protein, the third macronutrient, is often forgotten in the heat of battle. Putting the question of fat intake aside for a moment, which is better for weight loss, carbohydrates or protein? That is the question Claessens et al. set out to answer in their recent study. They first put obese people on a low-calorie diet to induce weight loss and then divided them into two groups: the high-carbohydrate group and the high-protein group. The idea was to see whether they would gain back the weight they lost during their calorie restriction diet.

Composition of diets

During the first 5 weeks of the study, all of the subjects consumed a liquid diet providing 500 kcal per day. In addition, they were allowed to eat an unrestricted amount of vegetables, except for pulses. All participants lost weight during this period.

During the next 12 weeks, the subjects were told to eat as much as they like but maintain a fat intake of about 30%. The high-carbohydrate (HC) group was told to consume at least 55% of their energy intake as carbs, while the high-protein group was told to eat a diet with at least 25% protein. The HC group was also given a 25 g maltodextrin supplement twice per day.

To compare the effects of so-called slow and fast proteins, the high-protein was further split into a whey protein group and a casein protein group. The distinction between slow and fast proteins is made based on their absorption pattern. Whey protein is quickly absorbed and thus a fast protein, while casein is slowly absorbed and therefore a slow protein. The subjects in the high-protein group were given either 25 mg casein (HPC group) or 25 mg whey (HPW group) twice per day.

At the end of the study, the HC group consumed 63% carbs, 16% protein and 21% fat. Mean total energy intake was 1868 kcal. Both HP groups reported consuming 42% carbs, 35% protein and 23% fat. According to urinary nitrogen excretion, however, the protein intakes of the HP groups were between 27-28%, which means that the difference in protein intake between the groups was probably not as big as one might hope, though it's still significant. Mean total energy intakes were 1848 and 1812 in the HPC and HPW groups, respectively.

Effects on body weight & body fat

During the 12 weeks of ad libitum eating, subjects in the HC group gained weight, while those in the HP groups kept losing weight. Fat mass and waist circumference also increased in the HC group, but decreased in the HP groups. The graphs for body weight and body weight change are shown below (the upward lines are the high-carb group and the downward lines are the high-protein groups).

High-protein vs. high-carbohydrate diet and body weight
Thus, despite similar energy intakes, those who ate more carbohydrates gained weight, while those who ate more protein lost weight. Furthermore, despite losing weight, the HP group actually gained fat-free mass.

Effects on blood pressure & cholesterol

Systolic and diastolic blood pressure increased slightly in the HC group, but the increase was not statistically significant. In the HP groups, however, a modest but significant decrease in blood pressure was seen.

Both groups modestly increased their HDL and LDL levels during the 12 weeks. No significant differences were seen between the groups. Triglycerides increased in the HC group but remained constant in the HP group.

Effects on insulin and glucose

Fasting insulin increased in the high-carbohydrate diet group and decreased in the high-protein diet group, but the changes were not statistically significant. Fasting blood glucose increased in both groups, with the increase reaching significance only in the HP groups. With glucagon, the situation was the opposite: the increase reached significance only in the HC group.

It's not entirely clear why the high-protein diet increased fasting blood glucose more than the high-carbohydrate diet. The authors comment:

This may be related to the gluconeogenesis-stimulating and/or glucagon-stimulating effect of high protein intake, which both will lead to increased hepatic glucose output.
-- Although impaired fasting glucose is a risk factor for the development of impaired glucose tolerance and type 2 diabetes in a high-risk population, there is no evidence that elevations of fasting plasma glucose within the normal range, as in the majority of our subject population, are associated with increased risk. Furthermore, no adverse effect of the high-protein diet on HbA1c was found in our subjects.

So, while this is an interesting result that deserves a closer look, I wouldn't be too worried at this point about high-protein diets and glucose levels it as long as your insulin levels are fine.


After a 5-week low-calorie diet, a high-protein ad libitum diet resulted in further weight loss compared to a high-carbohydrate ad libitum diet, which resulted in weight gain. Despite a decrease in body weight, body fat percentage and fat mass, fat-free mass increased in the high-protein groups. Energy intakes were similar in all groups.

Cholesterol levels increased similarly in both groups, but triglycerides increased only in the HC group. Fasting blood glucose increased in both groups, with a larger increase seen in the HP groups. Blood pressure decreased slightly in the high-protein groups.

No significant differences were seen between those eating a casein (slow) protein or a whey (fast) protein supplement.

For more information on carbohydrates, protein and fat in diets, see these posts:

The Effects of a High-Fat Diet on Health and Weight - Experiment Conclusion
Caloric Restriction Improves Memory in the Elderly

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