The quality of evidence is downgraded by suspected publication bias (asymmetrical funnel plot).
A Cochrane review [Abstract] 1 included 48 studies. Twenty five comparisons (n=61 958) compared a reduced fat diet with usual or control diet and 15 comparisons (n=13 004) compared a modified fat diet with control or usual diet. Reducing saturated fat by reducing and/or modifying dietary fat reduced the risk of cardiovascular events by 14% (RR 0.86, 95% CI 0.77 to 0.96, I2 =50%; 23 studies, n=65 508). Subgrouping suggested that this reduction in cardiovascular events was due to studies of fat modification, or fat modification and reduction (not studies of fat reduction alone), seen in studies of at least two years duration, in studies of men (and not those of women), and in those with moderate or high cardiovascular risk at baseline (not general population groups). There were no effects of dietary fat changes on total mortality (RR 0.98, 95% CI 0.93 to 1.04; 21 studies, n=71 790) or cardiovascular mortality (RR 0.94, 95% CI 0.85 to 1.04; 16 studies, n= 65 978). This did not alter with sub-group or sensitivity analysis. Few studies compared reduced with modified fat diets, so direct comparison was not possible.
Another Cochrane review [Abstract] 2 included 15 studies and 16 comparisons with a total of 56 675 subjects. The aim was to assess the effect of reducing saturated fat intake and replacing it with carbohydrate, polyunsaturated or monounsaturated fat and/or protein. Included studies used a variety of interventions from providing all food to advice on how to reduce saturated fat, and intended duration of the dietary interventions were at least 2 years.
Reducing dietary saturated fat reduced the risk of combined cardiovascular events by 17%, but no effect on cardiovascular or all-cause mortality, or risk of myocardial infarction, stroke, or coronary heart disease was observed (table T1). Meta-regression suggested that greater reductions in saturated fat (reflected in greater reductions in serum cholesterol) resulted in greater reductions in risk of CVD events, explaining most heterogeneity between studies. The NNTB was 56 in primary prevention studies (56 people need to reduce their saturated fat intake for ~4 years for 1 person to avoid experiencing a CVD event). In secondary prevention studies, the NNTB was 53. Subgrouping did not suggest significant differences between replacement of saturated fat calories with polyunsaturated fat or carbohydrate, and data on replacement with monounsaturated fat and protein was very limited.There was little or no effect on cancer mortality, cancer diagnoses, diabetes diagnosis, HDL cholesterol, serum triglycerides or blood pressure, and small reductions in weight, serum total cholesterol, LDL cholesterol and BMI. There was no evidence of harmful effects of reducing saturated fat intakes.
| Outcome | Follow-up mean duration | Relative effect(95% CI) | Participants (studies) | ||
|---|---|---|---|---|---|
| 1) I2 = 67%; 2) I2 =62% | |||||
| All-cause mortality | 56 months | RR 0.96 (0.90 to 1.03) | 55 858(12 studies) | ||
| Cardiovascular mortality | 53 months | RR 0.94 (0.78 to 1.13) | 53 421(11 studies) | ||
| Combined cardiovascular events1 | 52 months | RR 0.83 (0.70 to 0.98) | 53 758(13 studies) | ||
| Myocardial infarctions | 55 months | RR 0.90 (0.80 to 1.01) | 53 167(11 studies) | ||
| Non-fatal MI | 55 months | RR 0.97 (0.87 to 1.07) | 52 834(8 studies) | ||
| Stroke | 59 months | RR 0.92 (0.68 to 1.25 | 50 952(7 studies) | ||
| CHD mortality | 65 months | RR 0.97 (0.82 to 1.16) | 53 159(9 studies) | ||
| CHD events2 | 59 months | RR 0.83 (0.68 to 1.01) | 53 199(11 studies) | ||
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