A study presented at the European Society of Cardiology meetings in Barcelona and simultaneously published in The Lancet earlier this month caught the attention of many of my readers. Media headlines trumpeted “Huge New Study Casts Doubt On Conventional Wisdom About Fat And Carbs” and “Pure Shakes Up Nutritional Field: Finds High Fat Intake Beneficial.”
Since I’ve been casting as much doubt as possible on the conventional nutritional wisdom to cut saturated fat, they reasoned, I should be overjoyed to see such results.
What Did the PURE Study Find?
The Prospective Urban Rural Epidemiology (PURE) study, involved more than 200 investigators who collected data on more than 135000 individuals from 18 countries across five continents for over 7 years.
There were three high-income (Canada, Sweden, and United Arab Emirates), 11 middle-income (Argentina, Brazil, Chile, China, Colombia, Iran, Malaysia, occupied Palestinian territory, Poland, South Africa, and Turkey) and four low-income countries (Bangladesh, India, Pakistan, and Zimbabwe)
This was the largest prospective observational study to assess the association of nutrients (estimated by food frequency questionnaires) with cardiovascular disease and mortality in low-income and middle-income populations,
The PURE team reported that:
–Higher carbohydrate intake was associated with an increased risk of total mortality but not with CV disease or CV disease mortality.
–Higher fat intake was associated with lower risk of total mortality.
–Each type of fat (saturated, unsaturated, mono unsaturated ) was associated with about the same lower risk of total mortality.
These findings are consistent with my observations that it is becoming increasingly clear that cutting back on fat and saturated fat as the AHA and the Dietary Guidelines for Americans have been telling you to do for 30 years is not universally helpful (see here and here ).
When you process the fat out of dairy and eliminate meat from your diet although your LDL (“bad”) cholesterol drops a little your overall cholesterol (atherogenic lipid) profile doesn’t improve (see here).
Another paper from the PURE study shows this nicely and concluded:
Our data are at odds with current recommendations to reduce total fat and saturated fats. Reducing saturated fatty acid intake and replacing it with carbohydrate has an adverse effect on blood lipids. Substituting saturated fatty acids with unsaturated fats might improve some risk markers, but might worsen others. Simulations suggest that ApoB-to-ApoA1 ratio probably provides the best overall indication of the effect of saturated fatty acids on cardiovascular disease risk among the markers tested. Focusing on a single lipid marker such as LDL cholesterol alone does not capture the net clinical effects of nutrients on cardiovascular risk.
Further findings from PURE:
-Higher saturated fat intake was associated with a lower risk of stroke
-There was no association between total fat or saturated fat or unsaturated fat with risk of heart attack or dying from heart disease.
Given that most people still believe that saturated fat causes heart disease and are instructed by most national dietary guidelines to cut out animal and dairy fat this does indeed suggest that
“Global dietary guidelines should be reconsidered …”
Because the focus of dietary guidelines on reducing total and saturated fatty acid intake “is largely based on selective emphasis on some observation and clinical data despite the existence of several randomizesed trials and observational studies that do not support these conclusions.”
Pesky Confounding Factors
We cannot infer causality from PURE because like all obervational studies, the investigators do not have control over all the factors influencing outcomes. These confounding factors are legion in a study that is casting such a broad net across different countries with markedly different lifestyles and socioeconomic status.
The investigators did the best job they could taking into account household wealth and income, education, urban versus rural location and the effects of study centre on the outcomes.
In an accompanying editorial, Christopher E Ramsden and Anthony F Domenichiello, prominent NIH researchers, ask:
“Is PURE less confounded by conscientiousness than observational studies done in Europe and North American countries?
“Conscientiousness is among the best predictors of longevity. For example, in a Japanese population, highly and moderately conscientious individuals had 54% and 50% lower mortality, respectively, compared with the least conscientious tertile.”
“Conscientious individuals exhibit numerous health-related behaviours ranging from adherence to physicians’ recommendations and medication regimens, to better sleep habits, to less alcohol and substance misuse. Importantly, conscientious individuals tend to eat more recommended foods and fewer restricted foods.Since individuals in European and North American populations have, for many decades, received in influential diet recommendations, protective associations attributed to nutrients in studies of these populations are likely confounded by numerous other healthy behaviours. Because many of the populations included in PURE are less exposed to in influential diet recommendations, the present findings are perhaps less likely to be confounded by conscientiousness.”
It is this pesky conscientiousness factor (and other unmeasured confounding variables) which limit the confidence in any conclusions we can make from observational studies.
I agree wholeheartedly with the editorial’s conclusions:
Initial PURE findings challenge 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. This uncertainty is likely to prevail until well designed randomised controlled trials are done. Until then, the best medicine for the nutrition field is a healthy dose of humility.
Ah, if only the field of nutrition had been injected with a healthy dose of humility and a nagging conscience thirty years ago when its experts declared confidently that high dietary fat and cholesterol consumption was the cause of heart disease.!
Current nutritional experts and the guidelines they write will benefit from a keen awareness of the unintended consequences of recommendations which they make based on weak and insufficient evidence because such recommendations influence the food choices (and thereby the quality of life and the mechanisms of death) of hundreds of millions of people.