Anyone who has ever stubbed a toe knows what inflammation feels like. The redness and swelling that follow an injury are part of a normal healthy response of the immune system, which is aimed at healing and protecting damaged tissues. As healing occurs, inflammation disappears. In some people, however, the immune system goes into overdrive, leading to production of compounds that promote low-level chronic inflammation. Chronic inflammation is thought to contribute to an increased risk of heart disease, diabetes, rheumatoid arthritis, and cancer.
While the causes of chronic inflammation aren’t well understood, diet appears to have an impact. Evidence suggests certain foods raise risk for inflammation while others relieve or prevent it. In particular, different types of dietary fat are thought to have especially important effects on inflammation.
Types of Polyunsaturated Fat
There are two types of polyunsaturated fat, omega-6 and omega-3 fatty acids. Corn, safflower and sunflower oil provide the essential omega-6 fatty acid linoleic acid. The main fatty acid in soy oil is also linoleic acid, but soy also contains the essential omega-3 fatty acid alpha-linolenic acid. Both of these polyunsaturated fats are essential nutrients since they are required for survival and the body can’t manufacture them. Two other omega-3 fats, DHA and EPA, are not considered essential but they are the focus of much interest because they might lower risk for heart disease. These are the fats that are found in fish oils.
Fat and Inflammation
Foods high in omega-6 fats have long been thought to promote inflammation while omega-3 fats—the kind found in soyfoods and fish oils—are thought to have anti-inflammatory effects.
Omega-6 fats were thought to influence inflammation in two ways. First, they were believed to lead to the production certain cytokines, which are hormone-like compounds, that promote inflammation. Second, omega-6 fats interfere with production of DHA and EPA in the body resulting in lower levels of these anti-inflammatory compounds. A new review published in the Journal of the Academy of Nutrition and Dietetics has challenged at least one of these assumptions, however.
Researchers Guy H. Johnson of the University of Illinois and Kevin Fritsche from the University of Missouri, analyzed the results from 15 human intervention studies (1). They found that there is virtually no evidence to suggest that omega-6 fatty acids increase inflammatory markers in healthy children or adults. In fact, it appears that some of the cytokines produced in response to omega-6 fats are actually anti-inflammatory.
Research has also shown that the body tightly controls levels of omega-6 fatty acids in tissues where cytokines are produced (2). This suggests that the level of these fats in the diet doesn’t have much effect on cytokine production. The American Heart Association, after extensively reviewing the evidence, also concluded that there is little evidence to support a pro-inflammatory role of linoleic acid (3).
All of this bodes well for people who include soyfoods in their diet. Soyfoods are unique in that they contain both omega-6 and omega-3 fatty acids. In the past, some nutrition professionals expressed concern about using soy oil or consuming too much soyfoods, since the high ratio of omega-6 to omega-3 fats in soy was thought to promote inflammation. With the new findings, there is no longer a basis for concern. In fact, a new study from China suggests that soyfoods may have anti-inflammatory effects. The investigators looked at the relationship between blood levels of cytokines and soy intake among 1,005 middle-aged Chinese women (4). They found that levels of two important measures of inflammation were between 14 and 25% lower among women in the highest soy intake category. Women in this category consumed about two servings of soyfoods per day. These findings present further evidence that soyfoods can be part of a diet that lowers risk for chronic disease.
1. J Acad Nutr Diet 112: 1029-1041, 2012
2. Nutr Metab (Lond) 8:36, 2011
3. Circulation 119: 902-907, 2009
4. J Acad Nutr Diet 112: 996-1004, 2012