In Vitamin D and Cholesterol: The Importance of the Sun (2009) by David Grimes, an excellent book, I came across a 1977 study of healthy middle-aged men. The researchers measured their diet and watched them from 1966 to 1976. The question: What diets were associated with better health? There turned out to be associations with cholesterol (lower better) and systolic blood pressure (lower better), but these were less interesting than two strong dietary associations. One was between energy intake and heart disease. Men in the lowest third of energy intake had 23 cases of heart disease; men in the highest third had 7 cases. That’s probably due to exercise: the more you exercise the more you eat. We already know exercise is good.
The other association was with cereal fiber. Men in the lowest third of consumption (2-7 g/day) had 25 cases of heart disease; men in the highest third (8-34 g/day) had 5 cases. (A Wasa cracker has about 2 g cereal fiber.) You might dismiss this as healthy-person bias: healthy people do many healthy things, such as eat fiber. However, there was no association of heart disease and fiber from fruit and nuts. They’re healthy too. “The advantage of a diet high in cereal fibre cannot be explained [by us],” said the authors.
Later studies have found the same thing. For example, a 2006 review reached a similar conclusion: “There is an increasing body of evidence, including that from prospective population studies and epidemiological observational studies, suggesting a strong inverse relationship between increased consumption of wholegrain foods and reduced risk of CVD.” A study of health-conscious people — to reduce healthy-person bias — found a similar association: “Persons who habitually ate wholemeal bread had a lower mortality from cerebrovascular disease.” A 2002 review and a 2013 review provide even more evidence for the association.