September 7, 2017

The mystery of the 'perfect diet' deepens: New study recommends more fat, fewer carbs

Daily Briefing

    Countering global dietary guidance, a diet that incorporates more fats and fewer carbohydrates—and some fruits and vegetables—may be tied to lower mortality rates, according to three new papers in the Lancet

    Study details

    For the studies, researchers assessed data from 135,000 people participating in a 10-year Prospective Urban-Rural Epidemiology (PURE) study. The study participants resided in 18 countries: Argentina, Bangladesh, Brazil, Canada, Chile, China, Colombia, India, Iran, Malaysia, Pakistan, occupied Palestinian territory, Poland, South Africa, Sweden, Turkey, United Arab Emirates, and Zimbabwe.   

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    According to Forbes, the PURE study tracked participants' diets; long-term health outcomes, such as cardiovascular disease and stroke; and mortality rates. Overall, by the study's conclusion, nearly 5,800 people had died and nearly 4,800 had experienced a non-fatal heart event.

    The researchers published three studies on the PURE data: One paper exploring the data on fat and carbohydrates; the second exploring the data on fruits, vegetables, and legumes; and the third focused on how differing diets affected participants' lipid levels and blood pressure.

    Key findings

    For the first study, the researchers found that participants who reported the highest intake of fat as part of their daily diets (35 percent of calories per day) were 23 percent less likely to die during the study period than those who reported the lowest levels of fat intake (10 percent of daily calories). Further, according to the researchers, each type of fat assessed correlated to a lower risk of death: Monounsaturated fat was associated with a 19 percent lower risk of death, polyunsaturated fate was linked to a 20 percent lower risk of death, and saturated fat was linked with a 14 percent lower risk of death.

    Meanwhile, the researchers found that participants who reported the highest intake of carbohydrates in their diet (77 percent of daily calories) were 28 percent more likely to have died during the course of the study than those who reported the lowest daily intake of carbohydrates (46 percent of daily calories). The researchers noted that for both carbohydrate and fat intake, the difference in mortality risk stemmed from non-cardiovascular health issues.

    In the second study, the researchers found that participants who ate more fruit, seeds, and beans reported greater overall health benefits than participants who reported eating more vegetables. However, the researchers said the lackluster findings on vegetables' health benefits could stem from limitations in the data: Raw vegetables were linked to somewhat lower risks of heart disease and mortality risk when compared with cooked vegetables, the researchers explained, but it was difficult to verify the benefit because some countries in the study do not eat many raw vegetables. 

    Further, the researchers said while people who ate three or four servings of fruit and vegetables per day reported the greatest health benefits, those benefits didn't increase significantly by eating additional servings. In fact, the likelihood of death during the study period was the same for people who ate three or four servings of fruit and vegetables per day and those who ate eight servings, the researchers found.

    In the third study, the researchers found that participants who said they ate higher amounts of fat in their daily diet, or ate less carbohydrates, had lower levels of LDL, so-called bad cholesterol, and triglycerides and higher levels of HDL, so-called good cholesterol—findings that, according to STAT News, "help corroborate the main findings."

    So what is the ideal diet?

    The researchers concluded, "Our findings do not support the current recommendation to limit total fat intake to less than 30 percent of energy and saturated fat intake to less than 10 percent of energy." Instead, the findings suggest "cutting back on starch and sugar and adding more fat and more foods from plants, especially bioactive fruits and seeds, is where we should be headed," according to study author Mahshid Dehghan from McMaster University in Canada.

    Salim Yusuf, senior author on the study also from McMaster University, stressed that moderation is key. "Eat a balanced diet—a bit of meat, fish, several portions of fruit and vegetables, but you don't have to be vegan or eat an excessive amount of plants to be healthy," he said, adding, "This is good old-fashioned advice. When I showed these results to my mother, she said, 'Why did you bother doing this study? This is what our grandmothers and their grandmothers have been advocating for centuries.' And actually she is right."

    Dehghan said the researchers "are hoping that dietary guidelines are reconsidered in light of the new findings."

    However, some experts say more research is needed. In a comment accompanying the PURE studies in the Lancet, Christopher Ramsden and Anthony Domenichiello—physicians at the National Institute of Health— said the studies, while an "impressive undertaking," counter "conventional diet-disease tenets that are largely based on observational associations in European and North American populations, adding to the uncertainty about what constitutes a healthy diet." They continued, "This uncertainty is likely to prevail until well-designed randomized controlled trials are done. Until then, the best medicine for the nutrition field is a healthy dose of humility."

    Ramsden and Domenichiello hypothesized that one reason mortality rates improve with higher levels of fat could be because nutrient-dense meats addressed various nutrient deficiencies common in the standard diet of many countries included in the studies. Further, they suggested that the lower mortality rates linked to higher levels of carbohydrate consumption could stem from highly processed foods, which may include added sugars and refined grains.

    AHA voices concerns

    Separately, the American Heart Association—which last week issued an advisory recommending people minimize saturated fat intake in favor of polyunsaturated fat or carbohydrates—expressed concerns about the studies. "A nutrition study of PURE's scale and scope is extremely challenging," AHA said, recommending the findings "be interpreted with significant caution."

    For instance, AHA pointed out that the study relied on self-reported data, which AHA said was limited because people "tend to over- or underrecall food intake, in general." Further, according to AHA, "the tool may not fully account for cultural differences in food patterns and may underrepresent locally relevant foods" (Walton, Forbes, 8/30; Seaman, Reuters, 8/29; Skerrett, STAT News, 8/29; Hughes, MedScape, 8/29). 

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