A review of previously published studies suggests that vegetable and nut intake and a Mediterranean dietary pattern appear to be associated with a lower risk for heart disease, according to a report published in the April 13 issue of Archives of Internal Medicine, one of the JAMA/Archives journals. However, intake of trans-fatty acids and foods with a high glycemic index may be harmful to heart health.
"The relationship between dietary factors and coronary heart disease has been a major focus of health research for almost half a century," the authors write as background information in the article. Although "a wealth of literature" has been published on the topic, "the strength of the evidence supporting valid associations has not been evaluated systematically in a single investigation."
Andrew Mente, Ph.D., of the Population Health Research Institute, and colleagues conducted a systematic search for articles investigating dietary factors in relation to heart disease published between 1950 and June 2007. A total of 146 prospective cohort studies (looking back on the habits of a particular group of individuals) and 43 randomized controlled trials (where participants are randomly assigned to a dietary intervention or a control group) were identified and included in the systematic review.
When the researchers pooled the study results and applied a predefined algorithm, "we identified strong evidence of a causal relationship for protective factors, including intake of vegetables, nuts and monounsaturated fatty acids and Mediterranean, prudent and high-quality dietary patterns, and harmful factors, including intake of trans–fatty acids and foods with a high glycemic index or load and a western dietary pattern," they write. "Among these dietary exposures, however, only a Mediterranean dietary pattern has been studied in randomized controlled trials and significantly associated with coronary heart disease."
In addition, modest relationships were found supporting a causal relationship between intake of several other foods and vitamins and heart disease risk, including fish, omega-3 fatty acids from marine sources, folate, whole grains, alcohol, fruits, fiber and dietary vitamins E and C and beta carotene. Weak evidence also supported causal relationships between vitamin E and ascorbic acid supplements, saturated and polyunsaturated fatty acids and total fats, alpha-linoleic acid, meat, eggs and milk.
"The modest or weak evidence of these dietary exposures is mostly consistent with the findings of randomized controlled trials, although randomized controlled trials have yet to be conducted for several factors," the authors write. "Taken together, these findings support a causal relationship between only a few dietary exposures and coronary heart disease, whereas the evidence for most individual nutrients or foods is too modest to be conclusive."
"Although investigations of dietary components may help to shed light on mechanisms behind the benefits of dietary patterns, it is unlikely that modifying the intake of a few nutrients or foods would substantially influence coronary outcomes," they conclude. "Our findings support the strategy of investigating dietary patterns in cohort studies and randomized controlled trials for common and complex chronic diseases such as coronary heart disease."
Low glycemic breakfast may increase benefits of working out
The benefits of physical activity and a balanced diet are well documented and form the basis of many public health recommendations. This is because each of these factors can independently influence risks for many chronic diseases such as obesity, type 2 diabetes, and some forms of cancer. Some research also suggests that exercise and diet interact to influence health. For instance, exercising after short-term fasting (such as before breakfast) may increase the amount of fat burned. Similarly, consumption of a meal eliciting a low blood glucose response prior to exercise may also boost the use of body fat (instead of glucose). However, most of these studies have used either trained athletes or recreational exercisers, and none has looked at effects of the type of pre-exercise meal on metabolism during and after exercise. To better understand the effects of pre-exercise meal composition on fat metabolism in more typical (sedentary) individuals, a group of researchers headed by Dr. Emma Stevenson at the University of Nottingham conducted a controlled human intervention trial. The results of their study are published in the May 2009 issue of The Journal of Nutrition.
As expected, blood glucose concentrations were higher after the HGI than the LGI meals and had returned to baseline levels by the time exercise was commenced, after which they were not influenced by breakfast type. Plasma free fatty acids (FFA; a marker for adipose oxidation) fell after consumption of both HGI and LGI breakfasts, but began to rise at ~2 h post-breakfast in the LGI (but not HGI) treatment. Exercise caused a rapid increase in FFA in both groups, but this was higher in the LGI trial compared to the HGI trial (P < 0.001). Circulating concentrations of FFA were not different between treatments following lunch. Overall, fat oxidation was higher in the LGI treatment than in the HGI treatment (P < 0.05) during the post-breakfast and exercise periods. Following lunch, fullness scores were higher in the LGI trial than in the HGI trial (P < 0.05). The authors concluded that consuming a LGI breakfast increases fat oxidation during subsequent exercise and improved satiety during recovery in sedentary females. As such, individuals trying to shed fat may consider choosing LGI foods eaten prior to when they exercise.
Dietary acrylamide not associated with increased lung cancer risk in men
Dietary acrylamide was not associated with an increased risk of lung cancer, according to data from a large prospective case-cohort study in the April 28 online issue of the Journal of the National Cancer Institute.
Acrylamide is formed in some starchy foods, such as potato chips and French fries, during high-temperature cooking. Epidemiological studies have found a positive association between dietary acrylamide intake and the risk of endometrial, ovarian, renal cell, and estrogen-receptor positive breast cancers.
To investigate whether dietary acrylamide intake is associated with lung cancer risk, Janneke G. F. Hogervorst, M.Sc., of Maastricht University in the Netherlands, and colleagues conducted a case-cohort study among 58,279 men and 62,573 women in the Netherlands Cohort Study on Diet and Cancer. Intake of acrylamide was estimated based on food-frequency questionnaires completed upon enrollment in the study.
After a follow-up of 13 years, 1,600 men and 295 women were diagnosed with lung cancer. When the investigators divided participants into five groups based on dietary acrylamide intake, they found no statistically significant difference in lung cancer incidence in men who consumed the highest and lowest amounts of acrylamide-containing foods. By contrast, the researchers found that women who ate the most acrylamide-containing foods had a statistically significant lower incidence of lung cancer compared with those in the group who consumed the least acrylamide-containing foods. All analyses were adjusted for smoking.
"Acrylamide intake was not associated with lung cancer risk in men but was inversely associated in women… This finding suggests that acrylamide is involved in human carcinogenesis through pathways other than genotoxicity," the authors write. They hypothesize that acrylamide may affect hormonal balances, which might explain how it could be associated with a reduction in lung cancer risk but an increase in risk of endometrial and ovarian malignancies.
In an accompanying editorial, Lorelei A. Mucci, ScD. and Hans-Olov Adami, M.D., Ph.D., of the Harvard School of Public Health in Boston, review past studies of dietary acrylamide and cancer, as well as the initial discovery of acrylamide in foods. They note that several studies have found a positive association between dietary acrylamide and some types of cancer. The editorialists also express caution about concluding that dietary acrylamide may have a protective effect in women with respect to lung cancer risk, pointing out the potential for false-positive associations.
"In our view, speculation about the potential mechanisms of the protective effect of acrylamide on lung cancer among women should await confirmation of the association in additional studies," the editorialists conclude. "Perhaps the safer conclusion we can make from the Netherlands study is that the findings do not support a positive association between acrylamide intake from diet and risk of lung cancer."