"What's most surprising is they found substantial metabolic benefits in the absence of calorie reduction or weight loss," said Dr. JoAnn Manson, chief of preventive medicine at Harvard's Brigham and Women's Hospital.
In the study, appearing Monday in the Archives of Internal Medicine, the people who improved most were told to eat about three whole walnuts, seven or eight whole hazelnuts and seven or eight whole almonds. They didn't lose weight, on average, but more of them succeeded in reducing belly fat and improving their cholesterol and blood pressure.
Manson, who wasn't involved in the study, cautioned that adding nuts to a Western diet -- one packed with too many calories and junk food -- could lead to weight gain and more health risks. "But using nuts to replace a snack of chips or crackers is a very favorable change to make in your diet," Manson said.
The American Heart Association says 50 million Americans have metabolic syndrome, a combination of health risks, such as high blood pressure and abdominal obesity. Finding a way to reverse it with a diet people find easy and satisfying would mean huge health improvements for many Americans, Manson said.
Nuts help people feel full while also increasing the body's ability to burn fat, said lead author Dr. Jordi Salas-Salvado of the University of Rovira i Virgili in Reus, Spain.
"Nuts could have an effect on metabolic syndrome by multiple mechanisms," Salas-Salvado said in an e-mail. Nuts are rich in anti-inflammatory substances, such as fiber, and antioxidants, such as vitamin E. They are high in unsaturated fat, a healthier fat known to lower blood triglycerides and increase good cholesterol.
More than 1,200 Spaniards, ranging in age from 55 to 80, were randomly assigned to follow one of three diets. They were followed for a year. The participants had no prior history of heart disease, but some had risk factors including Type 2 diabetes, high blood pressure and abdominal obesity.
At the start, 751 people had metabolic syndrome, about 61 percent, distributed evenly among the three groups.
Metabolic syndrome was defined as having three or more of the following conditions: abdominal obesity, high triglycerides, low levels of good cholesterol (HDL), high blood sugar and high blood pressure.
The low-fat group was given basic advice about reducing all fat in their diets. Another group ate a Mediterranean diet with extra nuts. The third group ate a Mediterranean diet and was told to make sure they ate more than four tablespoons of olive oil a day.
Dietitians advised the two groups on the Mediterranean diet to use olive oil for cooking; increase fruit, vegetable and fish consumption; eat white meat instead of beef or processed meat; and prepare homemade tomato sauce with garlic, onions and herbs. Drinkers were told to stick with red wine.
After one year, all three groups had fewer people with metabolic syndrome, but the group eating nuts led the improvement, now with 52 percent having those heart risk factors. In the olive oil group, 57 percent had the syndrome. In the low-fat group, there was very little difference after a year in the percentage of people with the syndrome.
The nut-rich diet didn't do much to improve high blood sugar, but the large number of people with Type 2 diabetes -- about 46 percent of participants -- could be the reason, Salas-Salvado said. It's difficult to get diabetics' blood sugar down with lifestyle changes alone, he said.
To verify that study volunteers ate their nuts, researchers gave some of them a blood test for alpha-linolenic acid found in walnuts.
The study was funded by the Spanish Ministry of Health and the government of Valencia, Spain.
Salas-Salvado and another co-author disclosed in the publication that they are unpaid advisers to nut industry groups. Salas-Salvado said all of their research "has been conducted under standard ethical and scientific rules" and that peer-review journal editors determined the study results were not influenced by food industry ties.
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