Diets high in monounsaturated fats, such as the “Mediterranean Diet,” with high levels of monounsaturated fats such as olive oil, have received attention recently. Nuts are complex foods that contain considerable monounsaturated fat and have been a traditional part of diets in Mediterranean countries. Because of their high fat content, macadamia nuts have been avoided in many diets. Curb and associates examined variations in serum lipid levels in response to a diet high in monounsaturated fats based on macadamia nuts. Three dietary options were studied to evaluate the substitution of monounsaturated fat for saturated fat and to compare the high–monounsaturated-fat diet with the lower-fat American Heart Association (AHA) diet.
Thirty participants were evaluated in a randomized crossover trial of three 30-day diets. A wide range of ethnic groups and ages was represented in each dietary phase. Each of the participants was randomized to consume a “typical American” diet high in saturated fats, an AHA step 1 diet and a macadamia nut–based diet. Later, participants were randomized to the remaining diets so that all had equal probability of being in each of the three diet sequences. A 10-day cycle menu was designed using whole foods to match a specific nutrient profile. All three diets contained 17 percent of total energy from protein, with the percentage of energy from carbohydrate and fat depending on the diet (46 percent for the AHA step 1 and macadamia-nut diets and 53 percent for the typical American diet). The cholesterol content (300 mg) and polyunsaturated fatty acids (7 percent) were kept constant. After a 12- to 14-hour fast, each participant's lipid levels were measured on the last three consecutive days of each dietary sequence. Because the baseline free-living diets were not comparable, all comparisons were made between values at the end of the dietary periods and not between changes from baseline to the end of the diet period.
Only one participant had more than a 1.35-kg (2.97-lb) weight change during the study period, and no differences in skin fold thickness were demonstrated. Compared with the typical American diet, the mean total cholesterol level was significantly lower on the macadamia nut and AHA step 1 diets, as was the mean low-density lipoprotein cholesterol level. Mean triglyceride values were significantly higher than with the typical American diet on the AHA step 1 and significantly lower for the macadamia-nut diet. Mean high-density cholesterol levels were lower after the AHA step 1 and macadamia-nut diets. Results were the same regardless of gender or ethnic origin.
Results demonstrated that the macadamia nut–based, high-fat (37 percent) and moderately low-fat (30 percent) AHA diets had similar effects on lipid profiles. It is suggested that replacing saturated fats in the typical American diet with monounsaturated fats present in macadamia nuts has a favorable effect on lipoprotein levels. Dietary studies of the walnut and almond have provided more specific evidence of the potential cholesterol-lowering properties of nuts.
The authors stress that although nuts may be an important source of energy, if they are added to the diet rather than substituted for other fatty foods, weight gain could result. The authors conclude that the results of this study indicate that the consumption of a diet high in monounsaturated fats, a significant proportion of which were derived from macadamia nuts, appears to lower serum cholesterol levels when total energy balance and percentage of energy from fat are maintained. The effect of this diet was not statistically different from the effects of the AHA step 1 diet. These results suggest that consumption of macadamia nuts can be recommended as part of an overall diet plan.
editor's note: Although it is assumed that macadamia nuts would not be the only source of fat in a diet plan, it is recognized that some population groups consume large amounts of nuts in many varieties. Religious and health advocacy groups have purported the health benefits of nuts for many years although scientific data were lacking. Given the data in this study, it is proposed that macadamia nuts be given their just due along with the walnut and almond for lowering serum cholesterol levels.—b.a.