How bad are ‘cheat days’ for our health?

keto cheat day

Let’s face it. Nobody’s perfect. Nor should we try to be. Slipping up and making mistakes is part of human nature. Yet a recent study brought up the question of whether following a low-carb, high-fat diet makes us more prone to vascular damage during our “cheat days.”

First, you can find a detailed discussion on “cheating” in our Guide to low-carb & keto diet cheating. The main point is understanding why we are cheating. Is it because a unique opportunity came up? Or is it because we want to fit in, or we failed to prepare properly, or we are struggling to manage our cravings? Each reason has different motivations and different potential solutions that deserve attention.

Aside from the “Why?” behind cheating, a new study left many wondering if a chronic avoidance of carbohydrates makes us more susceptible to danger when sudden carbohydrate exposure leads to dramatic blood glucose spikes. Could this lead to vascular damage? The study, published in the journal Nutrients, suggests that the answer is “yes,” and the popular press is suggesting that keto eaters need to skip cheat days or pay the consequence.

Everyday Health: A cheat day on the keto diet may damage your blood vessels

The theory is that by avoiding carbs, we cause our bodies to be ill-prepared to handle them when, on occasion, we consume them. Somehow, chronically eating carbs may make our bodies more able to tolerate them. Thus arises the first question that we need to ask. Even if there is a spike in potential vascular damage from a “cheat day,” is that better or worse than chronically eating carbs and the resulting sustained negative metabolic effect on our system? The recent study did not address this question, but the studies that have shown LCHF diets reverse diabetes, improve blood pressure, and reduce the 10-year cardiovascular risk score would all suggest the clinical benefits are outweighed by any potential concerns.

The study in question was a very small study, with just nine healthy male volunteers with an average age of 21. The subjects were all new to a high-fat diet, as anyone who had tried it before was excluded from the experiment. The authors measured flow mediated dilatation (a marker of endothelial function or general vascular health) after a 75 gram glucose load while following a standard diet. The same subjects then switched to a LCHF diet for seven days and they remeasured the flow mediated dilatation before and after the 75 gram glucose load.

Investigators found that the glucose load with the standard diet and the low-carb, high-fat diet both triggered the same amount of endothelial dysfunction. Of note, in both cases, it was a tiny change — less than 1% change from baseline (0.58% change) — but it was statistically significant. There was no compounding of the effect with the intervention as the authors expected.

If the study ended here it would have been a null study. There was no significant difference in vascular function after a glucose load when they followed a standard diet compared to a low-carb, high-fat diet.

However, when the subjects were following a low-carb, high-fat diet, they did have a tiny decrease (0.71% change) in endothelial function while fasting. I suspect this has to do with the study’s short time frame and lack of adequate time for keto adaptation. The authors acknowledge that longer-term studies are inconsistent about whether low-carb, high-fat diets cause endothelial dysfunction. Likewise, I suspect adaptation plays a big role in that inconsistency.

The study did not end there. Investigators also measured something called endothelial microparticles. If this is the first time you are hearing about endothelial microparticles, don’t worry. You are in good company. I had never heard of them either, nor had a dozen of my colleagues I asked about them. Suffice it to say that they are a research tool meant to measure an increase in vascular inflammation or stress, but that they have little known clinical utility in humans.

The study did show a greater release of these microparticles in the subjects eating the glucose load after a week on a low-carb, high-fat diet compared to when they were on the control diet. Thus, this is the source of the conclusion that glucose loads are more dangerous when following a low-carb, high-fat diet. Is that conclusion warranted?

Before we get carried away, let’s put these findings into perspective:

  1. There were only nine subjects in the study, and looking at the data, one was an extreme outlier in his microparticle response while on an LCHF diet. With a larger sample size, individual effects are muted. But with such a small sample size, one outlier dramatically affects the results as in this case.
  2. The abnormal result in response to the glucose challenge was in an esoteric biomarker, and there was no significant difference in the more clinically useful measurement of the physiologic response of the blood vessel. Thus the most clinically useful endpoint was null.
  3. The subjects were only on an LCHF diet for seven days. We know there is an adaptation period to LCHF diets that takes weeks to months to complete. The very slight decrease in endothelial function on a low-carb, high-fat diet while fasting is difficult to interpret given the short time frame.
  4. The real question to ask is if LCHF with occasional “cheat days” is better or worse than a low-fat, high-carb diet in the long run. For those with diabetes and metabolic syndrome, the data certainly suggests the answer is “yes,” although quantifying the number and extent of cheating is problematic.

In the end, this study is intriguing, but far too preliminary to cause concern. More important is understanding the long-term benefits we see from LCHF eating and understanding why we might need “cheat days” and what we can do about it (see our Guide to low-carb & keto diet cheating). I look forward to seeing more data along these lines in the future, but for now, let’s file this study away as weak evidence that needs further investigation.

Thanks for reading,
Bret Scher, MD FACC


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