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http://www.ajcn.org/cgi/content/full/80/5/1102
Saturated fat prevents coronary artery disease? An American paradox1,2
Robert H Knopp and Barbara M Retzlaff
1 From the Northwest Lipid Research Clinic, University of Washington
School of Medicine, Seattle
2 Address reprint requests to RH Knopp, Northwest Lipid Research
Clinic, University of Washington, School of Medicine, 325 9th Avenue,
Seattle, WA 98104. E-mail:
rhknopp@u.washington.edu.
See corresponding article on page 1175.
It is an article of faith that saturated fat raises LDL cholesterol and
accelerates coronary artery disease, whereas unsaturated fatty acids
have the opposite effect (1, 2). One of the earliest and most
convincing studies of the better efficacy of unsaturated than of
saturated fat in reducing cholesterol and heart disease is the Finnish
Mental Hospital Study conducted in the 12 y between 1959 and 1971. In
this study, the usual high-saturated-fat institutional diet was
compared with an equally high-fat diet in which the saturated fat in
dairy products was replaced with soybean oil and soft margarine and
polyunsaturated fats were used in cooking. Each diet was provided for 6
y and then the alternate diet was provided for the next 6 y (3). After
a comparison of the effects of the 2 diets in both men and women, the
incidence of coronary artery disease was lower by 50% and 65% after the
consumption of polyunsaturated fat in the 2 hospitals.
In this issue of the Journal, Mozaffarian et al (4) report the opposite
association. They found that a higher saturated fat intake is
associated with less progression of coronary artery disease according
to quantitative angiography. How can this paradox be explained? In
food-frequency questionnaires, saturated fat intake is more precisely
estimated than is total fat. If saturated fat is more precisely
estimated, it will associate more strongly in statistical analyses with
the outcome variable, even though other variables-such as total fat
or carbohydrate-could be more relevant physiologically. We believe
that these possibilities deserve a closer look.
Unlike the diet used in the Finnish Mental Hospital Study, the diet
described by Mozaffarian et al was low in fat, averaging 25% of energy.
The study subjects were women with coronary artery disease: most were
hypertensive, many had diabetes (19-31%), their body mass index
(kg/m2) ranged from 29 to 30, and their lipid profile indicated
combined hyperlipidemia (triacylglycerol concentration: 200 mg/dL;
HDL-cholesterol concentration: 40-50 mg/dL; above-average LDL
concentration: 135-141 mg/dL); these characteristics are consistent
with the metabolic syndrome. In addition, two-thirds of these women
were taking sex hormones. The importance of each of these points is
addressed below.
What are the effects of a low-fat, high-carbohydrate diet in comparison
with those of a higher-fat, lower-carbohydrate diet? The response
differs by the 2 main types of hyperlipidemia: simple
hypercholesterolemia and combined hyperlipidemia. In our studies of
simple hypercholesterolemia in men, a fat intake <25% of energy and a
carbohydrate intake >60% of energy was associated with a sustained
increase in triacylglycerol of 40%, a decrease in HDL cholesterol of
3.5%, and no further decrease in LDL in comparison with higher fat
intakes (5). In contrast, a low-fat diet in persons with combined
hyperlipidemia caused no worsening of triacylglycerol or HDL, but
intakes of fat >40% of energy and of carbohydrate <45% of energy for 2
y were associated with a lower triacylglycerol concentration at a
stable weight (6). In the subjects of Mozaffarian et al, a greater
saturated fat intake paralleled a total fat intake, which ranged from
18% to 32% of energy in the first to fourth quartiles. Modest favorable
trends in triacylglycerol and HDL-cholesterol concentrations were
observed with higher fat intakes.
Triacylglycerol and HDL-cholesterol concentrations are stronger
predictors of coronary artery disease in women, whereas the
LDL-cholesterol concentration is a stronger predictor in men (7).
Because VLDL triacylglycerol secretion and removal rates in healthy
women are double those of men (8), conditions impairing lipoprotein
removal would be expected to exaggerate the hyperlipidemic response in
women as compared with that in men (9). This sex difference is seen
with the development of diabetes. The increment in lipids is greater in
women than in men and is associated with a greater increment in
coronary artery disease risk in women than in men (9). Similarly, the
development of
insulin resistance and obesity is associated with a
greater lipoprotein increment in women than in men (10). The
exaggerated decreases in HDL- and HDL2-cholesterol concentrations
observed with the consumption of a low-fat Step II diet in women but
not in men appear to be another facet of this effect (11).
The failure of female sex hormones to prevent coronary artery disease
has been a great disappointment (9). This effect might also be due to
an estrogen-induced increase in lipoprotein entry against a fixed or
impaired rate of lipoprotein removal, as might be expected in women
with the metabolic syndrome and coronary artery disease.
Would saturated fat still be bad for anyone? Not necessarily. The
effect of saturated fat and cholesterol ingestion in the form of 4
eggs/d for 1 mo in obese, insulin-resistant subjects is 33% of that
seen in lean, insulin-sensitive subjects, likely because of diminished
cholesterol absorption (12). Thus, the classic effects of saturated fat
as compared with those of unsaturated fat seen in the Finnish Mental
Hospital Study are likely blunted in the subjects of Mozaffarian et al,
whereas the effects of low fat and high carbohydrate intakes on
triacylglycerol and HDL-cholesterol concentrations appear to be
exaggerated by the interactions of female sex, exogenous sex hormones,
and the metabolic syndrome. A major effect on cardiovascular disease
risk would be the result of hypertriglyceridemia and low
HDL-cholesterol concentrations, which are attenuated by an increase in
saturated fat intake itself or in total fat intake, for which saturated
fat is a more statistically stable surrogate (4).
In conclusion, the hypothesis-generating report of Mozaffarian et al
draws attention to the different effects of diet on lipoprotein
physiology and cardiovascular disease risk. These effects include the
paradox that *********a high-fat, high-saturated fat diet is associated with
diminished coronary artery disease progression in women with the
metabolic syndrome, a condition that is epidemic in the United States.
This paradox presents a challenge to differentiate the effects of
dietary fat on lipoproteins and cardiovascular disease risk in men and
women, in the different lipid disorders, and in the metabolic
syndrome.*************
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High-Grain Diet May Increase Risk of Cardiovascular Disease
American Journal Clinical Nutrition January 2003 77: 43-50
When humans consume more carbohydrates than can be stored, the excess
carbohydrate energy is converted to fat by the liver. This process may
maintain blood sugar control and prevent diabetes in the short-term,
however it may also increase triglyceride concentrations, which may
increase the risk of cardiovascular disease.
In the last decade, researchers established that fat production by the
liver varies depending on dietary habits and health status.
The typical Western diet has a high fat content, which means that only
a limited amount of carbohydrates are available for liver fat
production, and liver fat production tends to be very low among
individuals who eat this type of diet. However, when too many
carbohydrates were consumed, both liver fat and sugar production were
increased.
A very low-fat (10 percent of energy) and very high-carbohydrate (75
percent of energy) diet also leads to increased liver fat production,
with the increase being even more pronounced when more than half of
the carbohydrate was consumed as simple sugars. This points to the
importance of carbohydrate quality, as another study using 68 percent
of energy from complex carbohydrate resulted in minimal liver fat
production.
However, it was found that obese individuals with high insulin levels
who consume a high-fat (40 percent of energy) diet had a liver fat
production rate three to four times higher than that of lean
individuals with normal insulin levels. But, both normal and high
insulin groups had lower liver fat production on the high-fat diet
than on a low-fat, high-carbohydrate diet.
Moreover, the low-fat, high-carbohydrate diet caused an increase in
triglyceride concentrations, a risk factor for coronary heart disease,
which was associated with the liver fat production in both normal and
high-insulin individuals.
Researchers concluded that the low-fat, high-carbohydrate diet might
not be ideal, as it can induce liver fat production and insulin
resistance. This is especially true when most of the carbohydrate is
in the form of simple sugars.
--
1: Am J Clin Nutr 2003 Jan;77(1):43-50
Hepatic de novo lipogenesis in normoinsulinemic and hyperinsulinemic
subjects consuming high-fat, low-carbohydrate and low-fat,
high-carbohydrate isoenergetic diets.
Schwarz JM, Linfoot P, Dare D, Aghajanian K.
Department of Nutritional Sciences and Toxicology, University of
California, Berkeley (J-MS and KA), and the Department of Medicine,
University of California, San Francisco (J-MS, PL, and DD).
BACKGROUND: Hypertriglyceridemia is associated with increased risk of
cardiovascular disease. Until recently, the importance of hepatic de
novo lipogenesis (DNL) in contributing to hypertriglyceridemia was
difficult to assess because of methodologic limitations. OBJECTIVE: We
evaluated the extent of the contribution by DNL to different
conditions associated with hypertriglyceridemia. DESIGN: After 5 d of
an isoenergetic high-fat, low-carbohydrate diet, fasting DNL was
measured in normoinsulinemic (/= 115 pmol/L) obese (n = 8) subjects.
Fasting DNL was measured after a low-fat, high-carbohydrate diet in
normoinsulinemic lean (n = 5) and hyperinsulinemic obese (n = 5)
subjects. Mass isotopomer distribution analysis was used to measure
the fraction of newly synthesized fatty acids in VLDL-triacylglycerol.
RESULTS: With the high-fat, low-carbohydrate diet, hyperinsulinemic
obese subjects had a 3.7-5.3-fold higher fractional DNL (8.5 +/- 0.7%)
than did normoinsulinemic lean (1.6 +/- 0.5%) or obese (2.3 +/- 0.3%)
subjects. With the low-fat, high-carbohydrate diet, normoinsulinemic
lean and hyperinsulinemic obese subjects had similarly high fractional
DNL (13 +/- 5.1% and 12.8 +/- 1.4%, respectively). Compared with
baseline, consumption of the high-fat, low-carbohydrate diet did not
affect triacylglycerol concentrations. However, after the low-fat,
high-carbohydrate diet, triacylglycerols increased significantly and
DNL was 5-6-fold higher than in normoinsulinemic subjects consuming a
high-fat diet. The increase in triacylglycerol after the low-fat,
high-carbohydrate diet was correlated with fractional DNL (P < 0.01),
indicating that subjects with high DNL had the greatest increase in
triacylglycerols.
>>>>>CONCLUSIONS: These results support the concept that
both hyperinsulinemia and a low-fat diet increase DNL, and that DNL
contributes to hypertriglyceridemia.*****
PMID: 12499321 [PubMed - in process]