Last week I wrote about the latest American Heart Association (AHA) guidelines and their warning that a recent “health food” fad, coconut oil, is actually not good for your health. There has been a lot of reaction to this report, however, reigniting the public controversy over the true health risks of fat vs sugar.

The traditional camp, the one represented by the AHA, is that diets high in fat, particularly saturated fat, contribute to atherosclerosis and heart disease. The alternate view, perhaps epitomized by journalist Gary Taubes, is that fat in food is fine; it is sugar that is the real health risk. I think the public controversy is out of proportion to the real underlying scientific debate, but there is room for debate here because there is evidence on both sides and that evidence is ambiguous.

What we do know

I often like to begin discussions of disagreement with what we can consider common ground. In this case there isn’t much common ground, because even the most basic pillars of understanding cardiovascular disease (CVD) are challenged in some circles. But there is a strong consensus that vascular disease is largely caused by high cholesterol in the blood which leads to build up of fat in the inner lining of blood vessels (atherosclerosis). This fat buildup can restrict the flow of blood through vessels. Also, the resulting plaques can rupture, forming a crater that then becomes a location where platelets can build up, forming a blood clot that acutely blocks the artery (causing a stroke or heart attack, depending on which artery is affected).

This, of course, is not the entire picture. High blood pressure is also a risk factor for CVD and stroke, as is diabetes (correlating with high blood sugar and high insulin levels). Inflammation is also playing a role. Further, not everyone is the same. Genetics plays a huge role in vascular risk and the response to other risk factors.

I also think there is broad agreement that maintaining a lean body mass and regular exercise reduce the risk of vascular disease.

The role of diet

Once we start getting into diet, however, the controversy begins. Over the years the AHA recommendations have shifted, as new evidence came in. The classic view was that high fat and high cholesterol diets contributed to high serum cholesterol and therefore vascular risk. In the 1980s, if you are old enough to remember, there was a low-cholesterol craze. However, later research showed the dietary cholesterol was not really an independent risk factor for vascular disease. While it did increase total cholesterol somewhat, it increases both LDL (bad fat) and HDL (good fat) in the blood without changing the ratio, which appears to be the main factor in terms of risk.

There isn’t as much discussion about cholesterol anymore. The debate now focuses on fat in the diet. It’s clear that the old recommendation of an overall low-fat diet were too simplistic. There are different kinds of fat, as I mentioned above. LDL transports fat from the liver to the blood vessel walls, while HDL transports fat from the blood vessels to the liver. Therefore, LDL bad, and HDL good.

So the question became – which kinds of dietary fats increase LDL and/or lower HDL? The current consensus, represented by the AHA, is that saturated fats, trans-fats, and hydrogenated fats all increase LDL. Polyunsaturated fats increase HDL. Monounsaturated fats may increase HDL a bit, but not as much as polyunsaturated, and perhaps not enough to have a health impact.

There is ongoing debate about the relative impact of LDL vs HDL and their ratio. Coconut oil, for example, is high in saturated fat, and increases both LDL and HDL. The AHA recommendation to avoid coconut oil comes from evidence that raising LDL has more of an impact of vascular risk than raising HDL.

There is not that much controversy over the relationship between saturated and polyunsaturated fats and LDL/HDL profiles. The real controversy is over the impact this has on vascular risk, and that was the focus of much of the backlash against the AHA report. Critics argue that the evidence for a net health benefit (longer survival, for example) from lowering dietary saturated fat is slim.

Here is a 2015 systematic review. The authors concluded:

The findings of this updated review are suggestive of a small but potentially important reduction in cardiovascular risk on reduction of saturated fat intake. Replacing the energy from saturated fat with polyunsaturated fat appears to be a useful strategy, and replacement with carbohydrate appears less useful, but effects of replacement with monounsaturated fat were unclear due to inclusion of only one small trial.

They further reported in the results:

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.

These conclusions match the more recent AHA review of the evidence. However, those in the camp that believe sugar is the culprit and fat is not a real risk factor point to this part of the review:

The included long-term trials suggested that reducing dietary saturated fat reduced the risk of cardiovascular events by 17% (risk ratio (RR) 0.83; 95% confidence interval (CI) 0.72 to 0.96, 13 comparisons, 53,300 participants of whom 8% had a cardiovascular event, I² 65%, GRADE moderate quality of evidence), but effects on all-cause mortality (RR 0.97; 95% CI 0.90 to 1.05; 12 trials, 55,858 participants) and cardiovascular mortality (RR 0.95; 95% CI 0.80 to 1.12, 12 trials, 53,421 participants) were less clear (both GRADE moderate quality of evidence).

They characterize this evidence as slim. This, of course, is a subjective judgement. That is the whole point of having panels of experts review the evidence – we want their subjective judgment. I can also give you mine, from an SBM perspective. In the case of saturated fat, there is a highly plausible mechanism, and there is a clear and consistent signal in the evidence that saturated fat raises LDL, and increases the risk for heart disease. Further, there is often uncertainty with these kinds of studies because we are trying to track uncommon events. This requires very large studies over long periods of time, often just to obtain barely statistical results. There also seems to be a dose-response effect in the evidence, which further supports a true causal connection.

So I would say that the conclusion that saturated fat is a significant risk factor for cardiovascular disease is plausible and probable. Further, there seems to be little risk in exchanging saturated fat for polyunsaturated fat. Therefore, a risk-benefit analysis favors a diet that replaces saturated fat with polyunsaturated fat to a significant degree.

The anti-sugar crowd, however, differs from this consensus opinion. They focus on the uncertainty in this data. They blame the AHA’s conclusions on their historic bias against fat, but that judgement does not seem justified. There are numerous reviews by independent experts all coming to the same conclusion. Part of the reason they want to exonerate saturated fat is because they advocate a very low carbohydrate diet, which means you need to have more calories from fat and protein. It would be simpler to advocate for such a diet if dietary fat were not a real risk factor.

There is some mainstream dissent from the AHA, however. Here is a 2016 review of the relative role of sugar and fat in the diet:

This paper reviews the evidence linking saturated fats and sugars to CHD, and concludes that the latter is more of a problem than the former. Dietary guidelines should shift focus away from reducing saturated fat, and from replacing saturated fat with carbohydrates, specifically when these carbohydrates are refined.

This seems to be a bit of a strawman, however, since the reviews and guidelines show that replacing saturated fat with sugar has no health advantage. They recommend replacing saturated fat with polyunsaturated fat. Limiting refined and simple sugars in the diet is also part of the standard guidelines.

In both cases, fat and sugar, the guidelines have moved away from worrying about the total amount of fat vs sugar in the diet to focusing on the type of fat and type of sugar. There is further nuance, as the 2016 review does state, in that different sources of saturated fat have different effects on LDL and HDL and affect people differently based on their genetics. We are drilling down pretty far into the details. You also have to consider the effect of dietary changes on body mass index (BMI) and the secondary effects from that.

The bottom line is that there are trade-offs and complexities when it comes to biology. A high carbohydrate diet is a risk factor for many diseases, and this is highly genetically variable. Types of sugar matter, weight matters, exercise matters, and genetics matter. The same is true of fat.

All of this can be overwhelming and leave the average consumer confused and without clear direction on how best to optimize their diet for health. This often leads to latching onto a simplistic narrative (such as fat bad, or sugar bad) that can be counterproductive. So while understanding the nuance is important and affects our recommendations, we also have to be careful about how we translate this nuance into lifestyle advice. Giving people too much complexity can lead to bad decision-making.

Most people will be best served from the basic advice to eat a varied diet, eat plenty of fruits and vegetables, exercise regularly, and maintain a healthy weight. As long as you avoid extreme diets, that basic advice will get you most of the way to an optimal diet. I also think the overall evidence supports avoiding too many saturated fats, and using oils that are higher in polyunsaturated fats. And avoid too many refined and simple sugars. Part of the problem is that some people will start using coconut oil, thinking it is a superfood, instead of making the more basic adjustments to their diet and lifestyle. There are no superfoods. As one of my med school lecturers put it – you can’t sprinkle fish oil on your steak and think it’s healthy. Take care of the basics first.

I do not think the evidence supports any extreme diet, such as a low carbohydrate or low fat diet. Balanced macronutrients from a varied diet is likely best.


Posted by Steven Novella

Founder and currently Executive Editor of Science-Based Medicine Steven Novella, MD is an academic clinical neurologist at the Yale University School of Medicine. He is also the host and producer of the popular weekly science podcast, The Skeptics’ Guide to the Universe, and the author of the NeuroLogicaBlog, a daily blog that covers news and issues in neuroscience, but also general science, scientific skepticism, philosophy of science, critical thinking, and the intersection of science with the media and society. Dr. Novella also has produced two courses with The Great Courses, and published a book on critical thinking - also called The Skeptics Guide to the Universe.