Scientific Updates
A Review of Recent Scientific Papers Related to Vegetarianism
By Reed Mangels, Ph.D., R.D.
Move Over Oat Bran -- Maybe it's Avocado or Walnuts
Nuts and avocados are commonly said to be high-fat foods that should be used in limited amounts. Two recent studies question this advice.The first, supported by a grant from the California Walnut Commission, studied 18 men who were either placed on a diet with 30% of calories from fat from a variety of foods, or a diet with the same amount of fat which included about three ounces of walnuts per day. Walnuts were substituted for high fat foods such as oils and margarines, and the portion size of meat was reduced, so that both groups ate similar amounts of fat and calories. The walnut-containing diet was lower in cholesterol.
The men who were placed on the 30% fat diet had a 6% drop in blood cholesterol. The men who ate walnuts had a drop in blood cholesterol of about 22 mg/dl, or about 18%.
The researchers believe that the drop in cholesterol in the walnut group was due to the walnuts' high content of polyunsaturated fatty acids and fiber, and their amino acid content. Several questions arise from this study. The subjects already had low blood choles- terol levels. Would the same effects have been seen in men with higher cholesterol levels? In women? Was the effect of the walnuts actually due to a reduction in meat intake? What would be the effect of adding nuts to a vegetarian diet? How would a 30% fat diet with walnuts compare to a 20% fat diet? Which diet would be more effective in reducing cancer risk?
Do the results of this study support eating more nuts? Not if doing so increases your calorie intake, fat intake, or weight. Remember, the results of this study were achieved by replacing foods with walnuts, not by adding nuts to an already adequate diet.
A similar study, supported by a grant from the Australian Avocado Growers' Federation, studied 15 women who either ate a diet with 37% of calories from fat including a half to 1-1/2 avocados per day, or a diet with 20% of calories from fat. The avocado diet was higher in monounsaturated fatty acids.
The women on the avocado diet had an 8% drop in blood cholesterol, which was statistically more than the 5% drop seen in the women on the low fat diet. Although this is a statistically significant decrease in cholesterol, it is certainly not as dramatic as that seen in the walnut study. The authors attribute the favorable changes to the monounsaturated fatty acid content of avocado.
We can only wonder if an even greater drop in blood cholesterol would have been seen on a lower fat diet containing avocado, or on a low-fat vegetarian diet. Should we all eat half an avocado per day? Probably not. The results of this study and of the walnut study suggest to me that neither walnuts nor avocados should be absolutely prohibited. They can be used, as part of a low-fat diet, to replace other fatty foods so that the amount of fat remains low.
For further information see: Sabate J, Fraser GE, Burke K, et al. Effects of walnuts on serum lipid levels and blood pressure in normal men. N Engl J Med 328: 603-607, 1993. Colquhoun DM, Moores D, Somerset SM, Humphries JA. Comparison of the effects on lipoproteins and apolipoproteins of a diet high in monounsaturated fatty acids, enriched with avocado, and a high-carbohydrate diet. Am J Clin Nutr 56: 671-677, 1992.
