Comment: The quality of evidence is downgraded by inconsistency (unexplained variability in results).
A Cochrane review 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 included 15 studies and 17 comparisons with a total of about 59 000 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.
Rreducing dietary saturated fat reduced the risk of combined cardiovascular events, but no effect on cardiovascular or all-cause mortality, or risk of myocardial infarction, stroke, or coronary heart disease was observed (table ). Subgrouping suggested that the reduction in cardiovascular events was seen in studies that primarily replaced saturated fat calories with polyunsaturated fat, and no effects were seen in studies replacing saturated fat with carbohydrate, protein, or monounsaturated fats. Subgrouping and meta-regression suggested that the degree of reduction in cardiovascular events was related to the degree of reduction of serum total cholesterol, and there were suggestions of greater protection with greater saturated fat reduction or greater increase in polyunsaturated and monounsaturated fats. There was no evidence of harmful effects of reducing saturated fat intakes on cancer mortality, cancer diagnoses or blood pressure, while there was some evidence of improvements in weight and BMI.
| Outcome | Follow-up mean duration | Relative effect (95% CI) | Absolute effects (per 10 000) | Participants (studies) | |
|---|---|---|---|---|---|
| 1) I2 = 65%; 2) I2 =66% | |||||
| All-cause mortality | 56 months | RR 0.97 (0.90 to 1.05) | 17 fewer (from 57 fewer to 29 more) | 55 858 (12 studies) | |
| Cardiovascular mortality | 53 months | RR 0.95 (0.80 to 1.12) | 10 fewer (from 39 fewer to 23 more) | 53 421 (12 studies) | |
| Combined cardiovascular events1 | 52 months | RR 0.83 (0.72 to 0.96) | 138 fewer (from 33 fewer to 228 fewer) | 53 300 (13 studies) | |
| Myocardial infarctions | 55 months | RR 0.90 (0.80 to 1.01) | 32 fewer (from 63 fewer to 3 more) | 53 167 (11 studies) | |
| Non-fatal MI | 55 months | RR 0.95 (0.80 to 1.13) | 13 fewer (from 51 fewer to 33 more) | 52 834 (9 studies) | |
| Stroke | 59 months | RR 1.00 (0.89 to 1.12 | 0 fewer (from 25 fewer to 25 more) | 50 952 (8 studies) | |
| CHD mortality | 65 months | RR 0.98 (0.84 to 1.15) | 3 fewer (from 25 fewer to 23 more) | 53 159 (10 studies) | |
| CHD events2 | 59 months | RR 0.87 (0.74 to 1.03) | 80 fewer (from 160 fewer to 19 more) | 53 199 (12 studies) | |
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