By Marika Sboros
PURE is the acronym for a major new (Prospective Urban Rural Epidemiology) cohort study that is creating controversy about dietary guidelines globally. It shows that the more fat you eat, including saturated fat, the lower your risk of dying from heart disease. It also shows that the more carbohydrates you eat, the higher your risk of premature death.
PURE is the largest study ever investigating links between carbs, fats, cardiovascular disease (CVD) and death. It involves 37 researchers looking at dietary habits of 135,335 people in 18 countries over five continents with an average follow-up of 7.4 years.
The study is also the first to investigate dietary habits in low-, middle- and high-income population groups global. Regions covered include the Middle East, Africa, China, North and South America, Europe and South Asia.
Among other conclusions, the study authors say that the recommended cap on dietary saturated fat (no more than 10% of energy intake) is wrong and that the guideline needs to change.
Low-fat lethal for hearts?
Advocates of low-carb diets say that PURE proves that low-fat diets are lethal for hearts. They are, therefore, calling for a complete overhaul of conventional dietary guidelines. Others say that PURE does not exonerate saturated fat and, therefore, no change is needed to guidelines.
Click here to read: Swiss Re: Dietary guidelines a ‘failed human experiment’
The PURE authors, led by epidemiologist Dr Mashid Dehghan, say that the guidelines are based mainly on research in Europe and North America. They say that the guideline recommendation to restrict saturated fatty acids at below 10% of energy intake lacks evidence. It is also largely based on “selective emphasis on some observational and clinical data”.
They say that this is despite evidence from several randomised trials and observational studies that “do not support these conclusions”. And they say that the guidelines should focus on reducing carbohydrate, not fat, intake.
Dehghan is an investigator at Canada’s Population Health Research Institute and a senior research fellow at McMaster University Department of Medicine. In media releases on McMaster’s website, Dehghan says that the guidelines are based largely on “moderate” carbohydrate consumption in Europe and North America at around 50% to 55% of energy intake.
Carbs under scrutiny
The current focus on low-fat diets “ignores the fact that most people’s diets in low- and middle-income countries are very high in carbohydrates (upwards of 65%)”. She also says that this seems to be linked to “worse health outcomes”.
Guidelines should, therefore, refocus on reducing carbohydrate intake where it is high and not on reducing fats, Dehghan says. She says that the best diets will include “a balance of carbohydrates and fats”. By that, she means approximately 50-55% carbohydrates and around 35% total fat, including saturated and unsaturated.
PURE co-author Dr Andrew Mente is associate professor in McMasters University Department of Health Research Methods, Evidence, and Impact. In media press releases, he gives the finding from a related PURE study showing an association between a higher fruit, vegetable and legume consumption with lower mortality risk.
Click here to read: You need 5-a-day fruit, veg? No you don’t! – Harcombe
Data show that people will derive maximum benefit from three to four servings of fruit, veg and legumes a day, Mente says. Higher servings achieve no added benefit. This makes the three-to-four recommendation “more affordable” for poorer countries compared to the current recommended five servings, he says.
Nutrient density spotlighted
The study data do not show an association between low-carbohydrate intake and adverse health outcomes. However, the authors say that this “does not provide support for very low carbohydrate diets – less than 50% energy”.
They note limitations, including that they based diet measures on when the study began. They also did not look at specific types of foods from which people derived nutrients.
In a linked comment in The Lancet, Dr Christopher Ramsden and Dr Anthony Domenichiello, from the US National Institute on Aging, describe the PURE study as “impressive” but say that key questions remain.
So, will dietary guidelines change anytime soon? US specialist Dr Alice Lichtenstein thinks not.
Lichtenstein is a professor, senior scientist and director of Tufts University’s Cardiovascular Nutrition Laboratory. She was vice-chair of the US Department of Agriculture’s 2015 Dietary Guidelines Advisory Group. Lichtenstein is also primary author of the American Heart Association’s (AHA’s) Diet and Lifestyle Recommendations.
She says that guidelines are rarely changed based on a single study.
A basic message
The basic message, whether from the 2015 DGA or AHA, is “to replace SFA (saturated fatty acid) with unsaturated fat, MUFA and PUFA (monounsaturated and polyunsaturated fatty acids)”, Lichtenstein says.
The recommendation to choose low- and non-fat dairy products and lean meat “seems reasonable”, she says. That’s as long as it goes with a recommendation to replace animal fats with vegetable fats (eg soybean oil, canola oil).
Lichenstein also says that clinical trials do, in fact, support benefits of replacing SFA (saturated fatty acid) with PUFA.
“The critical factor,” she says, ” is that the clinical data do not support replacing SFA with carbohydrate, particularly, refined carbohydrate”. Many reviews “have not taken this into consideration”.
It would be preferable to have dietary fat recommendations as an unsaturated to saturated fat ratio rather than solely on saturated or total fat, Lichtenstein says.
The most important measure …
British obesity and public health researcher Dr Zoë Harcombe is muted in PURE’s praise. She emphasises the need for caution with epidemiological studies as these can only suggest association not causation.
“Total mortality is always the most important measure to review,” Harcombe says. No study should claim “that X is better for condition Y” unless researchers have also assessed total mortality.
For example, there is no point reducing heart disease if cancer increases concomitantly,” she says. Harcombe quotes Scottish GP and CVD specialist Dr Malcolm Kendrick: “I can do an RCT (randomised controlled trial) where no one in the intervention arm will die of cardiovascular disease. The intervention shall be jumping off a cliff!”
RCTs have tested dietary fat interventions and total mortality, Harcombe says. “Even better, we now have meta-analyses of these RCTs.” However, none that bears repeating found “any significant difference in dietary fat interventions and total mortality”, she says. Click here for a summary of RCT meta-analyses.
Involvement of 18 different countries in the PURE study is an “interesting caveat”. As the researchers note, most existing data are from European and North American populations, Harcombe notes. Thus, it may be that higher fat intake “causes” lower mortality when researchers study more diverse populations. But they can’t claim this yet, she says.
Harcombe has spent many years examining the evidence base for dietary fat guidelines and supports the PURE conclusion that “global dietary guidelines should be reconsidered.”
US science journalist Nina Teicholz sees different “big news” in the PURE study.
Teicholz is author of the groundbreaking Big Fat Surprise, Why Butter, Meat and Cheese Belong in a Healthy Diet. Her book is a compelling history of just how and why nutrition science lost its way. In it, she shows that ” overzealous researchers, through a combination of ego, bias, and premature institutional consensus, have allowed dangerous misrepresentations to become dietary dogma”.
Teicholz makes the point that the US dietary guidelines no longer have a formal “low-fat diet” recommendation. And that the US government has “talked back from this language”.
Fat and stroke risk
The PURE researchers do say that a low-fat diet is not healthy. However, they don’t endorse what she and other nutrition experts would call a “high-healthy-fat” diet. On total fats, PURE suggests that 30-35% is a healthy range. Teicholz and others consider this a “fairly low-fat diet” and, thus, that’s not “such big news”.
Low-carb experts would also consider the recommendation of carbs at around 50-55% as high, not “moderate” as the PURE researchers describe it.
One of the most important findings of PURE is the association between low consumption of saturated fats (below 10%) and higher mortality and increased risk of stroke, Teicholz says.
“These are worrisome findings,” she says. ” They should spur the US guidelines to reconsider their recommendation to eat below 10% of calories as saturated fat.” The American Heart Association especially should reconsider its advice for those with cardiovascular disease to lower their saturated fat intake to 5-6% of calories.
“The PURE findings suggest that the AHA guidelines are increasing risk for these patients,” Teicholz says.
Click here to read: Teicholz: How low-fat diets can kill you
Fat caps should go?
The PURE results clearly do not support continued caps on saturated fats, she says. Just as critical is that clinical trial data also support this observational data.
Thus, all the data confirm that saturated fats have no effect on cardiovascular or total mortality, Teicholz says.
The limits on saturated fat are the sole reason that the guidelines recommend “lean meat” and “low-fat” milk. That makes it clear “that the science does not support these recommendations”, Teicholz says.
Therefore, those in positions of power should “retire these caps”.
On fruits and vegetables, the researchers found that three to four servings are enough for benefits. However, the findings also show that the benefit came from fruit but not from vegetables, Teicholz says.
Thus, PURE supports moderate consumption of fruits only and veggies “not at all”. PURE therefore clearly does not support the “plant-based” diet, she says, despite claims.
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