Most people know that refined grain removes a great deal of the nutritional value of a grain. Whole grain is felt to be healthier because it has more nutritional value. A number of medical studies have been done that show a lower risk of heart disease for those people who eat more whole grain foods. One early whole grains study that is often discussed is from the Nurses' Health Study (1).
The Nurses' Health Study
An early report was done on this prospective study of over 75,000 nurses during a 10 year period. In this particular whole grains study report (1) the nurses were divided into 5 groups. They were ranked according to how much whole grain they ate. The researchers compared the number of nonfatal and fatal heart attacks in the five groups. The percentage of people experiencing heart attacks decreased as the amount of whole grain eaten increased.
The study was started at a time when dietary fat was considered the main culprit in heart disease. Other components of food were only beginning to be considered. There was little emphasis on whole grain. This was one of the early studies of grain products.
The definition of whole grain by the FDA did not come until after this study had been started. The main sources of whole grain were dark bread and breakfast cereal. In this whole grains study, if only 25% of a cereal was whole grain or bran, it qualified as a whole grain cereal. Despite this difference, most subsequent studies using other whole grain definitions have confirmed their conclusions.
Most Americans eat very little whole-grain. The average American eats only 1/2 serving a day. In this particular study the group eating the least amount of whole grain ate 1/7 serving a day and those eating the most ate 2 1/2 servings a day.
Other food groups and food macronutrients were also considered as possible influences. In addition to studying grain, the researchers also followed the vegetable fruit and red meat intake. They also kept track of the fat and protein.
Most other food groups and macronutrients were consumed in approximately the same amount by the participants. There was no difference in the amount of red meat eaten by each group. And each group ate almost the same amount of fat and protein.
Other Possible Explanations For The Reduced Risk
One big difference was the amount of bran eaten by those eating more whole grain. Since whole grain contains bran, it would be expected that they ate more bran. What was unexpected was bran's strong correlation with less cardiac risk. Among the whole grain factors examined, bran showed the strongest likelihood of being correctly correlated with less coronary heart disease.
Because bran has a very high potassium to sodium ratio this makes it likely that bran is a major, although not only, component in whole-grain that reduces cardiac risk. This study was before the contribution of low potassium intake to cardiac risk was well understood, so the potassium and sodium levels were not studied. Thus we do not know how much the potassium sodium ratio of bran contributed to the reduced risk.
The biggest difference in food group consumption among those eating more whole grain was that they also ate significantly more vegetables and fruit. Both vegetables and fruit are high potassium foods (as is bran and intact grain). So, like bran, they also provide a higher potassium sodium ratio. This may have been an additional contribution to the reduced cardiac risk in the higher whole grain groups. Other more recent studies that have measured potassium and sodium intake have shown the reduction of cardiac risk from a higher ratio.
Possible Non-food Explanations
One concern of the researchers was that the high grain eaters may have had the improved risk from leading a healthier lifestyle rather than from eating more whole grain. At the time of this whole grains study, other known confounders included smoking, alcohol drinking, and lack of physical exercise.
The researchers corrected for these factors, and then compared the lowest to the highest quintile of whole-grain intake. They found that although these factors had an effect on cardiac risk, the effect did not erase the effect from whole-grain.
One example is that smoking may have explained the risk. If smokers and non-smokers were included, the risk was reduced by 33%. However if only non-smokers were looked at, the risk for nonfatal and fatal coronary heart disease was reduced by 50%. This indicated that smoking did not fully account for the risk reduction.
Some other problems studies face today are the same as they were at the time of the study. “Whole-grain” has a different meaning to different people. See this post to find out what can be labeled whole grain. This particular study was done with a food frequency questionnaire. Food frequency questionnaires are the most common way of estimating food intake. They ask the participants to estimate the amount of different types of food they have eaten. Memory is not always accurate.
Even with an accurate memory, there are labeling problems. There are so many food labeling problems for “whole-grain” that it is difficult to determine the true amount of whole-grain products that are eaten. This is true even if you record everything you eat while you are eating.
Up to 50% of the product can be other substances, such as the marshmallows found in cereal labeled “whole grain,” as discussed here. Some people may consider these types of cereal a whole-grain food when filling out the questionnaire. However such a cereal will be unlikely to reduce their cardiac risk.
1. Whole-grain consumption and risk of coronary heart disease: results from the Nurses' Health Study. Liu S, Stampfer MJ, Hu FB, Giovannucci E, Rimm E, Manson JE, Hennekens CH, Willett WC. Am J Clin Nutr. 1999 Sep;70(3):412-9.