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A newly published Cochrane Review paper shows a small reduction in cardiovascular events with reduced saturated fat intake. As we have noted in our guide on saturated fat, few topics in nutrition are as contentious as saturated fat, with science and emotions running high on both sides of the argument.
What makes this analysis different? For one, it did not rely on poor-quality observational nutritional epidemiology studies. Instead, it provided a meta-analysis of 15 randomized controlled trials. This is the highest quality evidence based on our review of scientific studies.
What’s not new? The studies. The included RCTs were published from 1966 to 2006, meaning the analysis did not include any data from the past 14 years.
As usual, the study unfortunately ignores the fact that saturated fat containing foods are not all the same. Did the saturated fats come from meat or dairy? Or did they come from cakes, cookies and other baked goods which combine saturated fats, trans fats and sugar? This analysis is unable to tell us that information, which practically speaking is a very important point.
Despite that, this paper was a statistical juggernaut with numerous different evaluations of the included studies. In brief, they found lower saturated fat intake compared to higher intake was associated with:
- A small decrease in combined cardiovascular events
- No difference in cardiovascular death
- No difference in all-cause death
Interestingly, looking at the trials included in the analysis of reduced cardiovascular events, nine of the 12 trials were negative trials, meaning there was no difference in cardiac events between the low-saturated-fat and high-saturated-fat groups. That means there were only three trials showing a difference.
Yet the combination of all 12 trials showed a small statistical difference. This is essentially the whole reason for meta-analysis: to detect changes that are so small and that require a much larger selection group.
However, I find it interesting to look at the positive trials in more detail. The STARS trial enrolled only 24 people in each arm of their study and showed a difference in arterial narrowing of 0.2mm. and a reduction in cardiac events.
The Oslo diet-heart study used an intervention diet of 45% carbs, 8.5% saturated fat and 20% polyunsaturated fat, but did not list the control diet, so we have no idea what the subjects ate. It showed a reduction in heart attacks but not overall mortality.
The third was a study whose primary outcome was diabetic retinopathy, with cardiovascular events as an unspecified secondary outcome. The control diet was 50% carbs and 35% saturated fats, whereas the intervention diet was 45% carbs with 40% total fat, one third of which was the omega-6 fat linoleic acid.
All of the other nine studies showed no significant difference between the groups.
But here is what I find most confusing about the Cochrane report. When they did a separate analysis of the studies at low risk of summary bias, there was no significant difference in cardiovascular events. So, in essence, when they controlled for the quality of the studies, the benefit was wiped out.
This is troubling as it shows that even RCTs can have problems with bias in trial design and reporting.
The other interesting aspect is that the benefit appears to be most associated with a difference in cholesterol between the groups. This means higher saturated fat intake in the absence of cholesterol changes likely did not show a meaningful difference.
So, once again, as we get deeper into the data, the nuance becomes increasingly important.
Here are the main conclusions as I see them:
- Comparing a high-carb diet low in saturated fats to a high-carb diet higher in saturated fats shows:
- A small reduction in cardiac events that disappears when controlling for study quality
- No difference in cardiovascular or all-cause death
- With the caveat that we have no idea where the saturated fats came from
- Since the difference seemed related to cholesterol, we can conclude:
- A high-carb, high-saturated-fat diet without an increase in cholesterol is likely not associated with a small increased risk.
- All of the diets were combined high-fat and high-carb diets. These diets have been shown to increase small, dense LDL particles as well as oxidized LDL particles. Would the same results hold true for low-carb diets higher in saturated fats? That is unknown, as the Cochrane data did not include any such data.
Looking at the headline of the study, the action item is clear: reduce saturated fat.
However, looking at the details of the study makes it much more confusing. If someone is following a whole-foods, low-carb diet that is relatively high in saturated fat, does the same headline apply?
That is a much murkier question. Especially if eating foods higher in saturated fats helps someone NOT eat highly processed foods or high sugar foods, or helps someone reduce their overall calorie intake and improve their metabolic health. Now all of sudden the health benefit-to-risk ratio seems less certain.
So, although this was a thorough review of RCTs with robust statistical analysis, the question we have to ask is, what does this mean for you?
Thanks for reading,
Bret Scher, MD FACC
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