Saturated fats are safe...again
Recent high profile opinion pieces, informed by systematic reviews of randomized trials1 2 and prospective cohort studies,1 3
have called for a re-evaluation of dietary guidelines for intake and a
re-appraisal of the effects of saturated fat on health; during this time
public health efforts to remove trans fats from the food supply in
several countries have intensified.
Saturated fats contribute about 10% of energy to the North American diet.4 5
The main sources of saturated fatty acids in the food supply are animal
products, such as butter, cows’ milk, meat, salmon, and egg yolks, and
some plant products such as chocolate and cocoa butter, coconut, and
palm kernel oils. Previous meta-analyses of prospective cohort studies
reported pooled relative risk estimates comparing extremes of intake of
saturated fat of 1.07 (95% confidence interval 0.96 to 1.19; P=0.22) for
coronary heart disease (CHD), 0.81 (0.62 to 1.05; P=0.11) for stroke,
and 1.00 (0.89 to 1.11; P=0.95) for cardiovascular disease (CVD).6
Intervention trials have shown modest cardiovascular benefits of
reducing intake of saturated fat while increasing intake of
polyunsaturated fat,7
but most trials lasted only up to two years and examined surrogate
outcomes. A meta-analysis of randomized trials suggested a 17% reduction
in risk of CVD in studies that reduced saturated fat intake from about
17% to about 9% of energy (0.83, 0.72 to 0.96).8
Trans fats contribute about 1-2% of energy in the North American diet9 10 11
and are produced industrially through partial hydrogenation of liquid
plant oils in the presence of a metal catalyst, vacuum, and high heat or
can occur naturally in meat and dairy products, where ruminant animals
biohydrogenate unsaturated fatty acids via bacterial enzymes. The major
industrially produced trans fatty acids in the food supply are elaidic
acid isomers, and the major ruminant derived trans fatty acid is
vaccenic acid; both share the characteristic of having at least one
double bond in the “trans-” rather than “cis-”
configuration. A prior meta-analysis reported pooled relative risk
estimates of CHD of 1.22 (95% confidence interval 1.08 to 1.38; P=0.002)
for extremes of total intake of trans fats; 1.30 (0.80 to 2.14; P=0.29)
for intake of industrially produced trans fats; and 0.93 (0.74 to 1.18;
P=0.56) for intake of ruminant derived trans fats,12
suggesting that industrially produced trans fats might increase the
risk of CHD, though this could also reflect the low levels of ruminant
derived trans fats compared with the higher doses of industrially
produced trans fats typically consumed in studies and available in the
food supply.13
Dietary
guidelines recommend that saturated fats should be limited to <10%
(5-6% for those who would benefit from lowering of LDL cholesterol), and
trans fats to <1% of energy or as low as possible,14 15 16 17 18 19
primarily to reduce risk of ischemic heart disease and stroke. To
clarify controversies surrounding guidelines for saturated and trans
fats for adults, we have extended and updated previous work to
synthesize prospective associations between these fats and all cause
mortality and type 2 diabetes (which have not been previously
synthesized), separate estimates for risks of cardiovascular morbidity
and mortality, and assess the confidence in the observational evidence
using the Grading of Recommendations Assessment, Development and
Evaluation (GRADE) approach.20 21 22