BARCELONA
-- An enormous prospective study of food intake in adults, reported
here, challenges several staunchly held beliefs about dietary components
and their association with health risks: finding, for example that
diets rich in fats, including saturated fats, don't increase mortality
risk, but high-carbohydrate diets do.
And the study, called PURE (Prospective Urban Rural Epidemiology),
also found that the benefits of fruits, vegetables, and legumes top out
at just three to four total servings per day.
In
sum, the results suggest that nutritional guidelines and conventional
wisdom regarding these basic dietary elements may be seriously mistaken.
PURE investigators recorded food intake using questionnaires in
135,000 people in 18 countries, including high-, medium- and low-income
nations. The latest findings from the ongoing study, with median
follow-up of 7.4 years, were outlined in two separate presentations at
the European Society of Cardiology meeting here, which were accompanied
by simultaneous publications in
The Lancet and in
Lancet: Diabetes & Endocrinology.
Carbohydrates and Fats: Unexpected Findings
One presentation and
Lancet paper
led by Mahshid Dehghan, PhD, of McMaster University in Hamilton,
Ontario, focused on the association of fats and carbohydrate intake with
cardiovascular disease and mortality.
Defying expectations, PURE found that high carbohydrate intake was
associated with a significant increase in the risk of death, while both
total fat and saturated and unsaturated fats were associated with a
decreased risk of death. However, fat consumption was not associated
with cardiovascular disease or cardiovascular mortality, though
saturated fat had an inverse association with stroke.
"Global dietary guidelines should be reconsidered in light of these findings," Dehghan concluded.
These findings may be partly explained by the
paper in Lancet: Diabetes & Endocrinology,
which looked at the effect of dietary nutrients on lipids and blood
pressure. The authors found that high intake of carbohydrates had "the
most adverse impact on cardiovascular risk factors" while
monounsaturated fats had a beneficial effect and saturated fat had a
neutral effect.
"Reducing saturated fatty acids and replacing them with carbohydrates
might have an adverse effect on cardiovascular disease risk," they
concluded. "Current recommendations to reduce total fat and saturated
fatty acids in all populations, which de facto increases carbohydrate
intake, are not supported by our data."
Fruits, Vegetables, Legumes: Benefits Limited
The second presentation and
Lancet paper,
by Andrew Mente, PhD, also of McMaster University, challenges the
widely held and nearly religious belief that more is always better when
it comes to fruits, vegetables, and legumes.
The
study did confirm that fruits and veggies (and legumes) in moderation
are good for you, but it did not show that the benefits keep growing
with increased consumption. Instead, the PURE researchers found that the
maximum benefit was achieved with three to four serving per day.
Current guidelines recommend that people consume five servings a day.
The authors note that many people in lower income countries are unable
to afford this high level of consumption.
"Optimal health benefits can be achieved with a more modest level of
consumption, an approach that is likely to be much more affordable,"
write the PURE investigators.
"For the first time, our study provides a global look at the
realities of people's diets in many countries and gives a clearer
picture of people's fat and carbohydrate intake," said Dehghan, in a
press release. "The current focus on promoting low-fat diets ignores the
fact that most people's diets in low and middle income countries are
very high in carbohydrates, which seem to be linked to worse health
outcomes. In low- and middle-income countries, where diets sometimes
consist of more than 65% of energy from carbohydrates, guidelines should
refocus their attention towards reducing carbohydrate intake, instead
of focusing on reducing fats. The best diets will include a balance of
carbohydrates and fats – approximately 50-55% carbohydrates and around
35% total fat, including both saturated and unsaturated fats. "
Interpreting the Data
In an interview, Mente stressed both the strength and limitations of
the PURE findings. As an observational study PURE is only capable of
finding associations; causation is impossible to prove, though in their
multivariate analysis the authors attempted to adjust for every known
risk factor.
The
PURE results are particularly relevant in poorer countries and in the
poorer sections of richer countries, where carbs -- largely low
quality -- comprise more than three-quarters of energy intake. "It's
that population that needs to reduce carb intake to more moderate
levels. Our data doesn't support low carb but certainly it supports a
moderate carb intake of 55%," said Mente.
Similarly, he noted that existing guidelines have traditionally
called for lower fat intake, below 30%, "but they haven't specified what
that should be." The current papers don't give details on individual
foods. Mente said that is the next stage of their research.
Mente also emphasized that the PURE results need to be put in the
proper perspective. Although the associations have very large long-term
population effects, at the individual level the effects are almost
negligible, a fact which many people fail to grasp.
"The effects are modest effects, in the neighborhood of a 20%
reduction in relative risk. So if the annual [absolute] risk of
mortality is 1%, it would be reduced to 0.8%. At the individual level,
it is tiny. And nowhere near what you find for smoking and lung cancer
-- about 200 times smaller in fact," said Mente.
"Dietary exposures in general have modest effects on clinical
outcomes. Even in the PREDIMED trial in 2013, where they evaluated an
entire dietary pattern, and presumably an additive effect of multiple
foods, they found a 30% reduction in risk. So you might imagine how
modest the effect for individual foods would be. Typically most foods or
nutrients are associated with about a 10% change in relative risk.
Small."
"Having said all that, at a
population level, if these small
effects are true and not due to confounding, they would translate into
thousands or even millions of fewer deaths annually, depending on the
size of the population, if the exposure is common which is certainly
true for diet. Therefore, from a public health perspective, which deals
with policy and impact on populations, the findings are important."
Prospective Urban Rural Epidemiology (PURE) study investigators†