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Evidence summaries

Dietary Lipids and Blood Cholesterol: Metabolic Ward Studies

The combined effect of changing the type, but not amount of dietary fat by replacement of 10% of dietary calories from saturates by monounsaturates (5%) and by polyunsaturates (5%) together with consuming 200 mg less dietary cholesterol would be a reduction in blood cholesterol of about 0.8 mmol/l (10 - 15%), with 80% of the reduction in low density lipoprotein cholesterol. Level of evidence: "A"

A systematic review 1 including 80 studies with a total of 15,994 subjects was abstracted in DARE. The diets were consumed in controlled conditions for at least two weeks.

Isocaloric replacement of 60% of saturated fat (British diets) by complex carbohydrates for 10% of dietary calories would reduce blood total cholesterol by 0.52 (SE 0.03) mmol/l and low density lipoprotein cholesterol by 0.36 (SE 0.05) mmol/l. In contrast, replacing an intake of complex carbohydrate with polyunsaturated fat for 5% of dietary calories would have the effect of reducing blood total cholesterol by a further 0.13 (SE 0.02) mmol/l and low-density lipoprotein cholesterol by 0.11 (SE 0.02) mmol/l. Intake of monounsaturated fat had no significant effect on total or low density lipoprotein cholesterol despite raising high density lipoprotein cholesterol by about as much as polyunsaturates.

Another systematic review 2 including 14 RCTs with a total of 439 subjects was abstracted in DARE.

The pooled effect sizes for high-mono versus high-poly diets were for total cholesterol 0.07 mmol/l (95% CI -0.08 to 0.21), LDL cholesterol -0.01 (95% CI -0.16 to 0.14), HDL cholesterol 0.03 (95% CI -0.12 to 0.17) and triglycerides 0.14 (95% CI 0.00 to 0.29). When a subset of studies with greatest monounsaturate versus polyunsaturate substitution were analysed the effect sizes were larger but still statistically insignificant.

The pooled effect sizes for high-mono versus high saturated fat were for total cholesterol -0.64 (95% CI -0.84 to -0.44), LDL cholesterol -0.66 (95% CI -0.85 to -0.46), HDL cholesterol -0.07 (95% CI -0.27 to 0.13), and triglycerides -0.06 (95% CI -0.25 to -0.14). The pooled effect sizes for the high-poly versus high saturated fat were for total cholesterol -0.68 (95% CI -0.87 to -0.47), LDL cholesterol -0.66 (95% CI -0.85 to -0.46), HDL cholesterol -0.13 (95% CI -0.32 to 0.07) and triglycerides -0.13 (95% CI -0.33 to -0.07). The authors conclude that replacing saturated with monounsaturated or polyunsaturated fat while holding total fat constant lowers total and LCL cholesterol but not HDL and triglycerides. There is no difference in effect between monounsaturated and polyunsaturated fat.

    References

    • Clarke R, Frost C, Collins R, Appleby P, Peto R. Dietary lipids and blood cholesterol: quantitative meta-analysis of metabolic ward studies. BMJ 1997 Jan 11;314(7074):112-7. [PubMed] [DARE]
    • Gardner CD, Kraemer HC. Monounsaturated versus polyunsaturated dietary fat and serum lipids. A meta-analysis. Arterioscler Thromb Vasc Biol 1995 Nov;15(11):1917-27. [PubMed][DARE]

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