Plant-based omega-3 polyunsaturated fatty acids (PUFA) may have a protective effect on bone health, according to a team of Penn State researchers who carried out the first controlled diet study of these fatty acids contained in such foods as flaxseed and walnuts.
Normally, most of the omega-3 fatty acids in the diet are plant-derived and come mainly from soybean and canola oil. Other sources are flaxseed, flaxseed oil, walnuts and walnut oil. Smaller amounts also come from marine sources, mainly fish, but also algae. Omega-3s are thought to have an anti-inflammatory effect and may play an important part in heart and bone health.
"The unique thing about this study is that we know exactly what the participants ate because we closely controlled their food," says Dr. Rebecca Corwin, associate professor of nutrition. "These people are really dedicated to spend a total of 24 weeks in the study with 18 weeks eating only what was supplied to them."
Previous studies of omega-3s on bone health used oil supplements rather than whole food sources. The researchers note in a recent issue of Nutrition Journal, that "supplement studies typically do not involve control of the background diet, and it is possible that differences across studies could be explained by failure to control for other nutrients that affect bones."
The researchers developed three diets that they fed sequentially to the 23 participants. Twenty of the subjects were men and three were postmenopausal women not on hormone replacement therapy for six months. This study was part of a larger one investigating the effects of omega-3 fatty acids on cardiovascular health. For six weeks the subjects ate either the control diet, dubbed average American diet or two other diets high in PUFA. After six weeks the group had three weeks off to resume their typical eating pattern and then for the next six weeks they ate one of the other diets. This continued for 24 weeks until all participants consumed six weeks of all the diets.
Monday through Friday the participants ate either breakfast or dinner in the diet center and packed the remaining meals, including weekend meals and snacks home. The researchers designed the diets so that individual body weight remained unchanged; participants carried out their normal activities and exercise levels. Blood tests showed that all subjects ate their supplied food and did not cheat on their regimens.
The two high PUFA diets had different amounts of linoleic acid (LA), an omega-6 fatty acid and alpha linolenic acid (ALA), an omega-3 fatty acid. Walnuts, which are high in omega-6 and omega-3 fatty acids, supplied half the total fat in both diets. They appeared in walnut granola, honey walnut butter, walnut pesto and as snacks. The ALA diet also contained flaxseed oil to increase the ALA content of the diet. Other sources of ALA, such as canola oil, were not used in this study.
Blood tests screened for two biological markers of bone health, one that indicates bone formation and one that indicates bone resorption or breakdown. Throughout life, two different types of cells - osteoblasts and osteoclasts - constantly build and break down bone. In this process they produce chemicals that researchers can measure in the blood. This process allows broken bones to heal, and bones to remain strong, but if more bone is lost than is rebuilt, osteoporosis occurs.
The biomarker for bone resorption, N-telopeptides, decreased significantly during the ALA diet and marginally during the LA diet compared to the average American diet. Levels of bone-specific alkaline phosphatases, a measure of bone building, were unaffected by the diets.
"If less bone is being resorbed and the same amount of bone is being created, then there is a positive balance for bone health," says Corwin.
Some scientists believe that the ratio of omega-6 and omega-3 fatty acids is the important factor. The ratio of these fatty acids in the average American diet was about 9.5, while in the LA and ALA diets it was 3.5 and 1.6 respectively.
The researchers caution that it is unknown if the observed effects are due to increased ALA or conversion of ALA to eicosapentaenoic acid, EPA. Fish oils in fish, are the main source of EPA in the American diet.
The researchers note that "recent epidemiologic data suggest that the effects of dietary fats on bone health may be particularly strong in men." So, while middle-aged men are often overlooked in studies of bone health, incorporating plant sources of omega-3 PUFA into the diet may not only improve cardiovascular health, but also enhance bone health.
The team included Corwin; Amy E. Griel, recent doctoral recipient and dietetic intern, Penn State dietetic internship; Penny M. Kris-Etherton, distinguished professor of nutrition at Penn Stsate; Kirsten Hilper, previous doctoral recipient, registered dietitian, Sodoexho USA; Guixiang Zhao, previous doctoral recipient, senior service fellow, Centers for Disease Control; and Sheila G. West, associate professor of biobehavioral health at Penn State.
The California Walnut Commission supported this research and partial support was provided by Penn State's General Clinical Research Center NIH grant.