Dear Mark, A friend of mine just found out that I’ve been eating a keto diet for the past few months, and they told me I should stop right away and get my selenium levels checked. They said I could be at risk of a heart attack because of keto. Now I’m freaking out a bit. Help?
Don’t freak out. Let’s look at the evidence.
First Off, What is Selenium, and Why Do We Need It?
Selenium is an essential trace element that we get from our diets. Enzymes called selenoproteins play a variety of important roles throughout the body. Notably, selenoproteins in the thyroid gland facilitate the conversion of T4 to T3.1 Selenoproteins require adequate selenium intake.
Selenium deficiencies can be very serious. Selenoproteins act as antioxidants. Without enough selenium—or really, selenoproteins—to offer protection, heart muscle cells can sustain free radical damage.2 This is the case with Keshan disease, a potentially fatal cardiomyopathy (disease of the heart muscle). Keshan is a region in China where the soil is depleted of selenium. As a result, residents were suffering high rates of heart disease before a supplementation program was introduced. Selenium deficiencies can also lead to male infertility because a selenoprotein known as GPx4 protects spermatozoa from oxidative stress.3
However, your friend might want to know that aside from severe deficiencies, the jury is still out on the role of selenium in cardiovascular disease. Some, but not all, observational studies have found that low selenium is a risk factor for cardiovascular disease.45 Others have found that high selenium may also be a problem.6 Selenium supplementation doesn’t appear to prevent heart disease, but clinical trials have mostly been carried out in adult males who already get enough selenium from their diets.7
In any case, selenium deficiencies are rare except in certain parts of the world where the soil is significantly depleted. Most adults in the U.S. get at least twice the recommended daily intake.8 So already, your friend’s basic premise seems shaky, but let’s do our due diligence here and ask whether following a keto diet puts you at greater risk.
Is There Any Evidence that Keto Causes Selenium Deficiency?
Yes, specifically among children who were prescribed a therapeutic ketogenic diet to treat intractable epilepsy.
As of 2020, there were at least 66 documented cases of selenium deficiency among children on a therapeutic ketogenic diet.9 Three deaths were attributed to cardiomyopathies associated with low selenium.1011 A fourth child died after experiencing QT prolongation (abnormal heartbeat, essentially). However, QT prolongation can be triggered by acidosis,12 so selenium might not have been the culprit here.
It’s unclear exactly how prevalent selenium deficiency is among pediatric epilepsy patients on keto. One study of 110 kids found that nearly half of them had low selenium. None of them showed evidence of cardiomyopathies as a result.13 Another study followed 91 children who were following a variant of the keto diet and receiving vitamin and mineral supplementation, including selenium, for 12 months.14 Selenium levels decreased over time, with some kids falling under the recommended range by the end.
So it’s clear that selenium deficiency is a risk for these kids. However, the current medical opinion is that nutrient deficiencies can be managed with proper monitoring and supplementation and, moreover, the risk of serious adverse events is small.
What about in Healthy Adults?
I haven’t seen any evidence that selenium deficiency is a concern among healthy adults following ketogenic diets, much less heart problems as a result.
Here’s the deal: it’s incredibly easy to get selenium in your diet, keto or otherwise. The RDA for selenium is 55 µg per day. Here is just a sampling of the selenium content of common foods:
One 5-ounce can of tuna: 103 µg
4 ounces (113 grams) of coho salmon: 43 µg
4 ounces of skinless chicken breast: 31 µg
4 ounces of 80% lean ground beef: 23 µg
2 ounces of beef liver: 20 µg
1 large egg: 15.4 µg
You can see that most keto dieters will exceed the RDA without even trying. If you’re concerned, eat a Brazil nut. Yep, just one. A single Brazil nut packs 90.6 µg of selenium.
So Why Are Kids with Epilepsy at Risk?
It boils down to the specific type of keto diet they are prescribed. All the kids in the aforementioned studies were following a therapeutic ketogenic diet designed for intractable epilepsy. This diet is wildly different from a Primal keto diet, and this is the crux of the matter.
Therapeutic keto diets usually follow a 4:1 ratio, meaning that for every four grams of fat, the child gets 1 gram of carbohydrate and protein combined. In other words, they have to restrict carbs and protein enough so that together they only comprise 20 percent of their food intake. In fact, the goal is to eat the bare minimum of protein necessary, in order to drive ketone levels as high as possible. As you can imagine, this makes the diet difficult to follow. It also increases the risk of nutrient deficiencies and associated health problems. (Hence the increasing interest in “modified Atkins” diets, which might be just as effective using 2:1 or even 1:1 ratio that allows for more protein.)
The keto diet an average person follows for health, weight-loss, or longevity purposes is probably nowhere near as strict a therapeutic 4:1 diet, nor should it be. The version of keto I recommend in The Keto Reset Diet and Keto for Life contains plenty of protein and embraces a colorful variety of plant foods to cover your nutritional bases.
All in all, this isn’t something I’m worried about. Any version of a Primal keto diet that includes meat will contain plenty of selenium. To ease your mind, you can always ask your doc for a selenium test, but that seems like overkill in my book.
One last note: as with any vitamin or mineral, it’s possible to get too much selenium. Don’t go eating a cup of Brazil nuts per day because your friend has you worried. Just eat your regular, balanced diet, and you should be good to go.
Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.