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Evolving evidence about diet and health
Nutritional research initially focused almost entirely on
conditions of nutritional deficiencies (e.g. scurvy, beriberi,
pellagra). By the 1950s, with the increase in coronary heart
disease in high-income countries, attention shifted to a range
of so-called diet–heart hypotheses.
These included the putative and harmful effects of fats
(especially saturated fats) and the protective effects of the
so-called Mediterranean diet to explain why individuals in
the USA, northern Europe and the UK were more prone to
coronary heart disease, whereas those in European countries
around the Mediterranean (or Japan) seemed to have lower
risks.
Some of the initial studies were enormously influential
while undergoing limited scrutiny as to the rigor of their
methods. The lack of replication of these early claims should
have prompted caution and re-examination of whether fats
(especially saturated fats) were indeed harmful.
More recently, studies using standardised questionnaires,
careful documentation of outcomes with common
definitions, and contemporary statistical approaches to
minimise confounding have generated a substantial body
of evidence that challenges the conventional thinking that
fats are harmful. Also, some populations (such as the US
population) changed their diets from one relatively high in
fats to one with increased carbohydrate intake. This change
paralleled the increased incidence of obesity and diabetes.
The focus of nutritional research has recently shifted
to the potential harms of carbohydrates. Indeed, higher
carbohydrate intake can have more adverse effects on key
atherogenic lipoproteins (e.g. increase the apolipoprotein
B-to-apolipoprotein A1 ratio) than can any natural fats.
Additionally, in short-term trials, extreme carbohydrate
restriction led to greater short-term weight loss and lower
glucose concentrations compared with diets with higher
amounts of carbohydrate.
Robust data from observational studies support a harmful
effect of refined, high-glycaemic-load carbohydrates on
mortality. The realisation that cardiovascular disease is a
global epidemic, with most cases occurring in developing
countries, has also stimulated studies involving multiple
countries at different economic levels.
Last year, the Prospective Urban Rural Epidemiology
(PURE) study of 135 335 individuals from 18 countries in
five continents showed that a diet high in carbohydrates
(more than approximately 60% of energy) but not high in
saturated fats, was associated with higher risk of death.
However, in PURE, even the group with the highest level of
fats (i.e. quintile 5; mean total fat intake 35% of energy, and
saturated fat intake 13% of energy) was not as high as the
average in studies from Finland (37 and 20%, respectively),
Scotland (37 and 17%, respectively), or the USA (38 and
16%, respectively), done in the 1960s and 1970s.
Therefore, a marked reduction in fat intake in several
countries might have occurred over the past few decades
in several countries. It is not clear that further reductions
in dietary fat intake will lead to reductions in incidence of
disease.
continued on page 119…