Taking the Pressure Off
Why wasn't the higher-carb diet best?
One possibility is that its extra sugars-say, a Peppermint Pattie, Fig Newton, sweetened yogurt or slice of angel food cake each day-curbed the drop in blood pressure. "We don't know if carbs raise blood pressure or if protein and unsaturated fat lower it," says Appel.
Researchers had earlier clues that protein would lower blood pressure. "In the mid-1980s, Jeremiah Stamler recognized that populations that eat more protein had lower blood pressures," says Sacks. Stamler, now emeritus, was chair of preventive medicine at Northwestern University Medical School in Chicago and served on the planning committees for both DASH and OmniHeart.
The mechanism is still a mystery. "Certain proteins may relax the muscles in the blood vessel wall," says Appel. When muscles relax, the blood vessel dilates, which lowers blood pressure. Unsaturated fat may do the same.
Another possibility: "Certain amino acids in protein, like L-arginine, are metabolized to nitric oxide, which relaxes blood vessels," explains Appel.
But researchers never anticipated that the higher-protein diet would lower HDL ("good") cholesterol. "That was a complete surprise," says Sacks. In contrast, it was no surprise that HDL would fall on the higher-carb diet and stay steady on the higher-unsaturated-fat diet.
However, experts aren't certain that a drop in HDL is always bad. "Some populations with low HDL levels have no heart disease," notes Appel. "HDL is so complex that we try to base our advice to the public on LDL and blood pressure."
And, he adds, "we still saw a substantial net reduction in overall heart disease risk on the higher-protein diet," largely because it slashed triglycerides the most. "The effect was huge."
The higher-protein diet might also have had an impact on the bathroom scale if the researchers hadn't insisted that people eat enough to keep their weight from falling.
"The higher-protein diet was the hardest to eat because some people found it too filling," explains Sacks.
Pick Your Diet
To some experts, the challenge is getting people to follow any of the three OmniHeart diets.
"These are all good diets," says Alice Lichtenstein of Tufts University in Boston, who served on the committee that oversaw OmniHeart. "The differences between them are quite modest."
She's more concerned that people will misinterpret the advice to eat more protein. "When most people think of high-protein diets, they think pork rinds, cheese and bacon. Anyone in the U.S. who ups protein in the diet has to be very vigilant because one or two poor choices can send saturated fat skyrocketing."
For example, the OmniHeart dietitians made sure that on days the participants got beef, pork or ham, it was a small, lean portion. "They increased protein largely from vegetable sources," not sausage and cheese, says Lichtenstein. That meant an extra two servings of beans, nuts and tofu (and an extra serving of poultry) each day.
Lichtenstein is less worried that people will misunderstand the higher-unsaturated-fat diet. "Most people who want to increase unsaturated fat will probably add oils, which are readily available and a logical choice for cooking or salad dressings," she notes.
But keeping calories in check is still critical. It's not just a matter of adding oil (120 calories per tablespoon) to your pasta, but cutting half a cup of pasta (100 calories) or a slice of bread (100 calories) for every tablespoon of oil you add.
"If you want more oil, what are you going to give up?" asks Lichtenstein. "The OmniHeart results assume that you don't gain weight. If you do, it could wipe out any potential benefit of the diets."
As a recent study made clear, if you're losing weight, an Atkins-like high-protein diet that's high in saturated fat doesn't raise bad cholesterol. But once your weight stabilizes, sat fat becomes risky again.
Sacks agrees that weight is crucial. "You still need calorie consciousness," he cautions. A new study is now testing the three diets on weight loss.
"We're putting 800 overweight or obese people on diets that are high in carbohydrate, protein or unsaturated fat," he explains. Results are due in mid-2008.
In the meantime, the biggest benefit of OmniHeart is clear. "The good news is that this gives people more flexibility," says Lichtenstein.
"If they have several diets that are compatible with lower risk of cardiovascular disease, they can stick to those diets longer."
The key is the healthy core of all three diets.
"As long as you follow the backbone of a DASH diet-rich in fruits, vegetables and low-fat dairy, low in saturated fat and cholesterol, and reduced in sodium--you can expect to cut blood pressure and LDL cholesterol," says Appel.
"Then if you want to maximize the reduction, you can tweak it further by replacing some carbs with protein or unsaturated fat."
The take-home message: "LDL on all three diets dropped an average of 20 points and blood pressure reductions were impressive, too," says Appel.
What's more, adds Lichtenstein, those same diets should help ward off other illnesses. "Whether it's cancer or diabetes or heart disease, everyone is recommending a similar dietary pattern."