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"Study suggests that [low-carb, high-protein] diet regimen may have adverse effect on body's restorative capacity."  HarvardScience.com

Good—and Bad—News for Low-Carb, High-Protein Dieters

Many articles about the Atkins-type diet appear on this site, most with a negative slant. See “Could Atkins Be Right After All?” http://www.cbass.com/Atkins.htm “Atkins Controversy Continues” http://www.cbass.com/atkins2.htm  “Low-Carb Diet Is Back—Again” http://www.cbass.com/LowCarb.htm . As those titles suggest, the low-carb, high-protein diet keeps coming back. Actually, that’s true in spades. Diet historians tell us it was around long before Dr. Atkins came along; it first became popular with the publication in 1825 of The Physiology of Taste, one of the most famous books ever written about food. See “Rethinking Thin” http://www.cbass.com/RethinkingThin.htm .

The Atkins-type diet produces rapid weight loss; no doubt about it. That's why it keeps coming back. Dieters want fast results. Amazingly, new wrinkles on the very old story continue to pop up.

Recent evidence indicates that the diet reduces insulin resistance, lowers triglycerides, and raises HDL “good” cholesterol. What’s more, we have a new study showing that a plant-based low-carb, high-protein diet reduced LDL “bad” cholesterol.

Unfortunately, another new study links the low-carb diet to atherosclerosis, a disorder that damages blood vessels and restricts or blocks blood flow.

This is cutting-edge science. Let’s take a hard look at both studies, beginning with the plant-based “Eco-Atkins” diet. (I learned about this study in the August 2009 Tufts Health & Nutrition Letter. The full study appeared in the Archives of Internal Medicine June 8, 2009.)

Eco-Atkins Advantage

A major disadvantage of meat-based low-carb, high-protein diets, according to the researchers, is that they tend to raise LDL “bad” cholesterol. Led by David J. A. Jenkins, MD, of St. Michael’s Hospital and the University of Toronto, they decided to compare the effect of a low-carb diet “high in vegetable proteins from gluten, soy, nuts, vegetables, cereals, and vegetable oils…with a high-carbohydrate diet based on low-fat dairy and whole grain products.” (Emphasis mine)

Would these diets bring about weight loss and lower LDL cholesterol? Yes, but there’s more to the story

The study protocol was as follows.

Overweight men and women with high LDL were randomly assigned to one of the two diets: “Eco-Atkins” (26% carbs, 31% protein, and 43% fat) or lacto-ovo vegetarian (58% carbs, 16% protein, and 26% fat). Both diets cut calories by 40% and lasted 4 weeks. Forty-four participants completed the study, 22 in each dietary regimen.

To insure compliance, prepared meals were provided to the participants. The low-carb diet provided the “minimum level of carbohydrates currently recommended.” The high-carb diet included liquid egg whites or egg substitute to insure a low-saturated fat and low-cholesterol intake.

Here’s what happened.

Weight loss was essentially the same for both groups, about 8.8 pounds, confirming that calories are what count in weight loss. (See “Calories Matter” http://www.cbass.com/CaloriesMatter.htm )

Both groups saw a reduction in LDL cholesterol, but the low-carb, Eco-Atkins dieters dropped 20.4% compared to 12.3% for the high-carb group. The low-carb group also showed more improvement in Cholesterol/ HDL ratio and blood pressure. 

The researchers concluded: “A low-carbohydrate plant-based diet has lipid-lowering advantages over a high-carbohydrate, low-fat weight-loss diet in improving heart disease risk factors not seen with conventional low-fat diets with animal products.” In short, getting rid of the saturated fat and cholesterol in low-carb diets appears to provide weight loss without the increased health concerns seen in meat-based versions of the diet.

An accompanying editorial by Katherine R. Tuttle, MD, and Joan E. Milton, MS, RD, CD, however, raised cautions. First, they observe that “superior weight loss is not maintained with high-protein diets compared with other dietary strategies in the long run.” In other words, long-term weight maintenance has typically not been good with low-carb, high-protein diets.

Secondly, and perhaps more important, they say, “It is premature to recommend the ‘Eco-Atkins’ as a weight loss diet choice without confirmation of its efficacy in larger studies of more diverse and higher-risk individuals. Long-term studies are also essential to ascertain both sustainability and safety.”

Speaking of safety. The next study reveals a previously unrecognized health concern.

Link to Athero

A new study reported Online in the Proceedings of the National Academy of Science (August 24, 2009) provides some of the first information about the effect of the low-carb, high-protein diet on the health of blood vessels. The story of the study is told in an article by Bonnie Prescott on the Harvard Science website.

Conducted by a team of scientists at Beth Israel Deaconess Medical Center (BIDMC), the study found that mice placed on a 12-week low-carb, high-protein diet showed a significant increase in atherosclerosis—and an impaired ability to restore blood vessels. 

Senior author Anthony Rosenzweig, MD, Director of Cardiovascular Research at BIDMC and Professor of Medicine at Harvard Medical School, and lead author Shi Yin Foo, MD, a clinical cardiologist in the Rosenzweig laboratory at BIDMC, conceived the mouse study in a very human manner.

Dr. Foo began thinking about the possible dangers of low-carb, high-fat diets after seeing heart-attack patients who were on the diet—and after Rosenzweig himself went on the diet.

“Over lunch, I’d ask Tony how he could eat that food and would tell him about the last low-carb patient I’d admitted to the hospital,” Foo related. “Tony would counter by noting that there were no controls for my observations.”

“Finally,” Rosenzweig chimed in, “I asked Shi Yin to do a mouse experiment—so we could know what happens in the blood vessels and so I could eat in peace.”

The doctors and their colleagues fed mice know to develop atherosclerosis in the same manner as humans one of three diets: a standard mouse diet (65% carbs, 15% fat, 20% protein), a “Western diet” in line with the average human diet (43% carbs, 42% fat, 15% protein), and low-carb, high-protein diet (12% carbs, 43% fat, 45% protein). All three diets contained the same amount of calories. The last two diets contained the same amount of fat and cholesterol.

“We had a diet specially made that would mimic a typical low-carb diet,” Dr. Foo explained. As in most such diets, carbs were replaced with protein.

The mice were weighed after six weeks, and again at 12 weeks. Contrary to what happened in the Eco-Atkins diet study, the mice fed the low-carb diet gained 28% less weight than mice fed the Western diet. (The mice were male pups put on the diets one week after weaning, so it was expected that they all would gain weight.)

Significantly, the low-carb mice also exhibited a greater degree of atherosclerosis, as measured by plaque accumulation: 15.3% compared to 8.8% for the Western diet group. (As expected, the mice on the lab chow showed minimal atherosclerosis.)

“Our next question was, ‘Why do the low-carb mice have such an increase in atherosclerosis,’” Foo said.

Importantly, Foo and her colleagues found little or no difference in the standard markers of vascular disease, such as cholesterol, triglycerides, inflammation, etc. As predicted above, if anything those markers slightly favored the low-carb group. “None of these results explained why the animals’ blood had more atherosclerotic blockages and looked so bad,” Foo said.

Being that the Western diet and low-carb diet contained the same amount of fat and cholesterol—the traditional risk factors—other things were obviously at work.

Finally, the researchers looked at the animals’ endothelial or vascular progenitor cell (EPC) counts. Derived from bone marrow, the EPC cells are believed to play a role in blood vessel repair and regeneration following injury. “Examination of the animals’ bone marrow and peripheral blood showed that the measures of EPC cells dropped fully 40% among the mice on the low-carb diet—after only two weeks,” Rosenzweig said.

This finding suggests that the low-carb, high-protein led to an impaired ability to form new blood vessels in tissues deprived of blood flow, as occurs with atherosclerosis. The researchers hypothesized that the Western diet and the low-carb/high-protein diet caused “comparable injury and inflammation.” The low-carb group, however, suffered an impaired ability to repair and restore blood vessel function.

Rosenzweig  continued: “Although the precise nature and roll of [EPC] cells is still being worked out—and caution is always warranted in extrapolating from effects in mice to a clinical situation—these results succeeded in getting me off the low-carb diet.” (Emphasis mine)

Rosenzweig stressed the apparent disconnect between weight loss and standard blood markers—where the low-carb diet worked—and vascular health. It appears that blood vessel health can be affected by more than fat and cholesterol—in this case, carbs and protein. Since both carbs and protein were altered simultaneously—mimicking the typical low-carb diet—blame can’t be put on one or the other macronutrient.

Nevertheless, protein—the bodybuilder’s favorite macronutrient—may actually be an offender. Too much protein (and too little carbohydrate) may damage blood vessels and/or prevent restoration. No one knows for sure, of course. All we have at this point is a hypothesis for scientists to explore.

“For now,” says Rosenzweig, “it appears that a moderate and balanced diet, coupled with exercise, is probably best for most people.”

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