Nutrition Notes

Do Low-Carb Diets Increase Risk for AFib?

Here we go again. Another round of sensationalist headlines seeking to instill fear regarding low-carb and ketogenic diets. These days it seems you can’t watch the news or scroll through social media without being bombarded by articles purporting to discredit ketogenic diets or finger them as the cause of everything from stubbed toes to flight cancellations. First there was “keto crotch,” and now we’re being told that ketogenic diets cause atrial fibrillation (AFib).  Fortunately, both of these have been skillfully debunked by medical doctors writing for the number one site in the world for low-carb and ketogenic diets. (See here for these rebuttals to the “keto crotch” myth and the purported link between keto and AFib.)

Let’s look a little more closely at the AFib story, though. Ketogenic diets are very low in carbohydrate and high in fat. While it’s entirely possible to follow a ketogenic diet that emphasizes monounsaturated and polyunsaturated fats, many keto dieters consume liberal amounts of saturated fat, so there could be some concern regarding saturated fat intake and cardiovascular health. However, research indicates there’s no need to reduce saturated fat intake on low carb diets, and even outside the context of carbohydrate restriction, saturated fat intake is not associated with increased risk for cardiovascular or coronary heart disease. So what, really, is the issue here? Why would low-carb diets cause AFib?

The most important thing to know about this study is that it didn’t evaluate low-carb diets at all. Study participants were asked to report their daily intake of 66 different food items, so right off the bat, the data may not be reliable. Self-reported food intake is notoriously untrustworthy and may be shaky ground upon which to draw solid conclusions.

But even if we are to draw conclusions from this information, according to a press release from the American College of Cardiology, in this study, on average, carbohydrates comprised about half of calories consumed, yet it’s also stated that “People getting a low proportion of their daily calories from carbohydrates such as grains, fruits and starchy vegetables are significantly more likely to develop atrial fibrillation…” But these subjects weren’t getting a “low” proportion of their daily calories from carbohydrates. At about 50% of total calories, how can this be considered a low intake?

Researchers divided the subjects into tertiles of carbohydrate intake, low (<44.8% of total daily calories), medium 44.8-52.4% of total daily calories), and high (>52.4% of total daily calories). Note that there’s less than an 8% difference between the lowest and highest tertiles. In the lowest tertile, self-reported carbohydrate intake appears to account for over 40% of total daily calories—not an amount most low-carb and ketogenic-oriented researchers would consider “low.” Some of the world’s leading researchers in this area define ketogenic diets as having less than 10 percent of total calories coming from carbohydrates (≤50 g carbs/day), and low carbohydrate diets as having less than 26% of total energy intake coming from carbohydrate (<130 g/day). Consuming nearly 45% of total energy intake, it’s unlikely most of the subjects in the lowest tertile were consuming a truly low-carb diet.  

According to the press release, “Participants reporting low carbohydrate intake were the most likely to develop AFib.” Taking a closer look at that, again it was participants reporting a low carbohydrate intake—the truth is, we don’t know for sure what the participants consumed, so it may be somewhat misleading and irresponsible to make the leap that low carb diets are “tied to” AFib, as the title of the press release suggests. Never mind that the study contradicts itself: it’s stated clearly that the lowest observed risk for AFib was associated with a carbohydrate intake proportion of 39%-61%. So at least some participants with the lowest risk for AFib fell into the lowest carbohydrate intake tertile, at 39% of total calories.

If you read more closely, the details come through a bit more clearly. Lead author Xiaodong Zhuang, MD, PhD, acknowledged that “while the research shows an association, it cannot prove cause and effect. A randomized controlled trial would be needed to confirm the relationship between carbohydrate intake and AFib and assess the effect in a more ethnically diverse population.”

It’s odd that ketogenic diets would be associated with a heart arrhythmia when in most respects, keto has been shown to beneficial for cardiovascular health. The triglyceride-to-HDL ratio is a powerful predictor of cardiovascular disease, coronary heart disease and all-cause mortality, and carbohydrate restriction reliably improves this ratio while also favorably affecting other cardiovascular risk factors.

A study from March of this year found that patients with heart failure and reduced ejection fraction (HFrEF) show increased myocardial use of beta-hydroxybutyrate (BOHB), one of the ketone bodies generated on a ketogenic diet. (It’s also available now as a supplement, typically in powdered form that can be added to water.) Infusion of BOHB in these patients resulted in increased cardiac output and left ventricular ejection fraction, leading the study authors to conclude, “3-OHB may potentially constitute a novel treatment principle in HFrEF patients.”

This shouldn’t have been a big surprise, as it’s known that cardiomyocytes are one of the largest consumers of ketone bodies, along with skeletal muscle cells and neurons. In fact, the myocardium is the highest ketone body consumer per unit mass. Low-carbohydrate diets may have cardioprotective effects through multiple mechanisms, potentially including an increased number of myocardial mitochondria, decreased oxidative stress, and maintaining ubiquinone in the oxidized state. Additionally, ketone body oxidation is even more energetically efficient than fatty acid oxidation, with fatty acids being the main fuel for the heart.

Dr. Zhuang speculated that low-carb diets increase oxidative stress, which is associated with AFib. Other researchers have shown that ketogenic diets—the lowest of low-carb diets—reduce oxidative stress. Zhuang also speculated that people following low-carb diets may be consuming fewer fruits, vegetables and whole grains, which could result in inflammation, another factor associated with AFib. However, ketogenic diets need not be low in vegetables and some fruits, and moreover, ketogenic diets have been shown to be anti-inflammatory.  

It may be premature to completely dismiss a potential link between lower carbohydrate intakes and increased risk for AFib, but this study was based entirely on food intake questionnaires—actual food intake was never measured, so we have no way of knowing how many participants—if any—were truly following a “low-carb” diet. A controlled clinical trial would be needed to replicate these findings and perhaps to elucidate a mechanism by which reducing carbohydrate intake could increase AFib risk, rather than vague speculation about inflammation and oxidative stress, particularly considering that carbohydrate restriction has beneficial impacts on atherogenic dyslipidemia and other cardiovascular risk markers