Are moderate-protein diets better than low-protein diets for people with metabolic syndrome?

Moderate carbohydrate, moderate protein weight loss diet reduces cardiovascular disease risk compared to high carbohydrate, low protein diet in obese adults: A randomized clinical trial

Original source: Nutrition & Metabolism 2008, 5:30
Authors: Denise A Walker Lasker (corresponding author), Ellen M Evans, and Donald K Layman
Publication date: 7 November 2008
DOI (digital document ID): 10.1186/1743-7075-5-30
Requires subscription? No
Publicly-available abstract: Yes

Synopsis:
This study compares the effectiveness of the standard USDA Food Pyramid diet with a higher-protein, lower-carbohydrate diet for weight loss in general, for fat loss in particular, and for lowering the blood-based indicators of cardiovascular risk and insulin resistance. A total of 50 people completed the testing, split between the two dietary approaches.

Both diets under test had the same amount of calories, with about 30% of total calories coming from fats. Carbohydrates supplied about 55% of the Food Pyramid diet group's caloric intake, and protein, the remaining 15%. The people assigned to this group were instructed to eat whole grains for their bread products. About 30% of the high-protein group's calories came from protein, with the remaining 40% coming from carbohydrates. (This is the same protein-carbohydrate-fat distribution advocated by the Zone Diet.) This group was instructed to eat primarily "high quality protein", including lean meats, dairy, and eggs. Both groups had similar amounts of fruits and vegetables in their daily diets. All subjects were instructed on portion sizes, dietary substitutions, how to weigh foods, and how to keep a food diary. There was no difference in exercise between the two groups.

Baseline measurements (weight, body fat percentage, cholesterol, and postprandial insulin) were taken at the start of the experiment. The subjects followed their assigned diet plans for four months, at which time the same measurements were repeated.

• The moderate-protein group lost about 5 pounds more per person than the USDA group, and more of their weight loss was from fatty tissues.
• Fasting glucose levels were unchanged, but fasting insulin levels decreased in the moderate protein group, while it increased in the USDA group. In addition, postprandial insulin levels dropped 85% for the protein group, but only 50% for the Food Pyramid group. (High plasma insulin levels are indicative of insulin resistance, which is often a predecessor to Type 2 diabetes.)
  
• There was a greater drop in total cholesterol in the USDA group than in the protein group. LDL ("bad") cholesterol dropped in the USDA group but rose in the protein group; on the other hand, HDL ("good") cholesterol dropped in the USDA group but rose in the protein group. The protein group also showed greater improvement in triglyceride levels than the USDA group. (High triglyceride levels are associated with increased risk of heart attack.)
  

Reliability:
The study was small scale, with only 50 people completing the four-month study. While the two groups of subjects were selected to be closely-matched, there is still opportunity for the results to be skewed. An initial run-in period put all subjects on as close to an even footing as possible before the trial started.
The authors are associated with an accredited university; the paper was published in a peer-reviewed journal.
Longer-term studies of larger groups may be needed to determine the long-term safety and effectiveness of high-protein diets.

The principal investigator (Denise A Walker Lasker) received funding from the National Cattleman’s Beef Association, The Beef Board and Kraft Foods. Both Beef associations have a vested interest in increasing protein consumption (in the form of beef).
The investigators have made no comparison between a higher-fat restricted-carbohydrate diet and a higher-protein restricted-carbohydrate diet.

Practical Applications:
The 30/40/30 protein/carb/fat (PCF) ratio is the basis of the Zone Diet. While this diet is higher in protein than traditionally advised for people with diabetes, recent publications suggest that this is not as high a risk as formerly perceived.

For further reading:
WebMD evaluates the Zone Diet
Wikipedia entry on the Zone Diet

This synopsis was based on the entire article.

Prenatal diet may affect insulin resistance for multiple generations

Insulin sensitivity is normalized in the third generation (F3) offspring of developmentally programmed insulin resistant
(F2) rats fed an energy-restricted diet
Original source: Nutrition and Metabolism 2008, 5:26
Authors: Daniel C Benyshek (corresponding author), Carol S Johnston, John F Martin and William D Ross
Publication date: 17 October 2008
DOI (digital document ID): 10.1186/1743-7075-5-26
Requires subscription? No
Publicly-available abstract: Yes

Synopsis:
A number of studies have shown that the way animals develop before birth can affect the birth weights, blood pressure, and glucose metabolisms of their children, grandchildren, and so on. Most research to date has only gone two generations beyond the original parents. This study takes observation to the third generation (great-grandchildren) and focuses on impairments to the animals' glucose metabolisms. A parent generation of female rats were fed protein-deficient diets while pregnant and nursing, and both they and three generations of their offspring were fed low-calorie diets, to see how this affected the insulin sensitivity of multiple generations of their offspring. A group of control rats was fed normal-calorie diets over the same number of generations.

The first generation of offspring (children) tended to be of low birth weight, while the second and third generations (grandchildren and great-grandchildren) were of normal birth weight. All generations were of lower than normal weight as adults, due to the caloric restriction. The first=generation offspring had normal insulin metabolism, While the second generation, as adults, had reduced insulin sensitivity and reduced insulin production -- a set of symptoms consistent with impaired glucose tolerance. The third generation did not show insulin resistance -- indeed, the males were even more insulin-sensitive than the control animals, whether they were fed calorie-restricted diets or allowed to eat at will.

Previous studies of this type showed insulin resistance in the first-generation offspring, which was not found in this particular study; however, the second-generation female offspring in this study were insulin-resistant.

The investigators are unsure why the third-generation offspring did not show impaired glucose tolerance or insulin resistance, even though both of those traits are considered to be adaptive traits that show up in "nutrient poor environments" (reduced protein, reduced calories). They suggest that one reason for this may be that the second-generation mother's insulin resistance left more energy in the bloodstream to be used by the fetus. On the other hand, the relative insulin sensitivity of previous-generation mothers (who had not been fed sufficient calories after weaning) made fewer nutrients available for the developing fetus, resulting in insulin resistance in the offspring.

The investigators suggest that more research is required to determine how pregnancy and nursing diets, and maternal insulin sensitivity, affects the insulin sensitivity of her offspring.

Reliability:
The authors are from accredited institutions, across a number of medical and social disciplines. The research was published in a peer-reviewed online journal. References are all from peer-reviewed journals. Therefore the experiment design is reliable. Since only four female rats from each child generation were bred, the study was small in size and therefore subject to skew. The authors did note that further research was warranted to find out why they got the results they did.

Practical Applications:
The normal birth weight and increased insulin sensitivity of child generations born to insulin-resistant mothers suggests a parallel mechanism for the large baby size common for human mothers with diabetes. While this study was conducted on rats, the rats are intended to be a shorter-lived and expendable model for human childbirth and development. A closer understanding of which maternal nutrients affect insulin resistance may lead to improved recommendations for pregnancy and nursing diets and supplements, and will ideally prevent insulin resistance in future generations of human adults.

This synopsis was based on the entire article.