For every person who talks up the ketogenic diet, there’s another warning about the ill effects on your heart. The worry is that following a very high-fat, moderate-protein, and very low-carbohydrate diet will create a subsequent spike in cholesterol levels, which could, in turn, raise your risk for heart disease.

The worry is certainly valid. On the keto diet, each day you’re eating up to 80 percent of your calories from fat and sticking with 20 to 50 grams of net carbs, a term used in popular diets, including keto and Atkins but one that is not officially recognized in the medical community. (To calculate net carbs, subtract fiber and sugar alcohols from the total carb amount, per Atkins.) This all means you’ll be eating a lot of fat per day, and in the name of meeting that quota, that may include unhealthy choices such as butter, coconut oil, and animal fat. These are all sources of saturated fat, which has been linked to poor heart health, according to the American Heart Association (AHA). (The point of keto is to shift your body from a carb-burning state into a fat-burning one, which is called ketosis.)

But the effect of keto on heart health isn’t as straightforward as it sounds. Indeed, the keto diet may not have as detrimental an effect on your cholesterol as previously thought.

“When you review the data, a couple things are clear. Looking at population studies and clinical trials, the impact the keto diet has on lipids is modest,” says Daniel Soffer, MD, an internist and lipidologist at Penn Medicine in Philadelphia. Dr. Soffer is a member of the National Lipid Association, an organization dedicated to managing lipid problems. In October 2019, the National Lipid Association released a position statement on low-carb and very-low-carb, ketogenic diets and their relationship to lipids, and published it in the Journal of Clinical Lipidology (PDF). Most commonly, he says, research shows keto produces modest reductions in triglycerides, modest changes in heart-protective high-density lipoprotein (HDL, or "good"), and minimal changes in low-density lipoprotein (LDL, or "bad") levels. Some studies, the review notes, show an increase in LDL on low- or very low-carb diets. (Long-term studies, however, are lacking.)

Whether these numbers — particularly HDL — go up or down largely depends on the quality of your keto diet. “One problem is that there’s not a single ketogenic diet. There are different ways to follow it, and some people are careful about the types of fats they eat, while others are not,” says Soffer.

To his point, one past article noted that in human studies on people of normal weight and with obesity, keto diets are generally associated with a reduction in total cholesterol, an increase in HDL, a decrease in triglycerides, and reductions in LDL. At the same time, other research, the review points out, shows something different: a rise in LDL or no swing either way. In trials where LDL decreased, people ate a low-carb diet high in healthy unsaturated fat and limited in unhealthy saturated fat, the authors point out.

One past study included in the above article compared a low-calorie and a very low-carb, high-fat diet (what would constitute a keto diet) in more than 360 overweight and obese participants. Some of the participants had diabetes, others did not. After about a year, those in the keto group saw their total cholesterol, triglycerides, and LDL decrease, while HDL rose.

Predicting How Keto May Affect Your Cholesterol Levels

To have an idea if the keto diet may have a negative effect on your cholesterol, consider your starting triglyceride number. According to Mayo Clinic, triglycerides are another type of lipid in your blood that your body uses for energy; high triglyceride and LDL levels can lead to fatty deposits that gum up arteries, notes the AHA. Normal levels are less than 150 milligrams per deciliter (mg/dL). “About a quarter of the population has triglyceride levels above 150 mg/dL, which is the threshold to consider elevated triglycerides. This is a large percentage of the population, so it’s not uncommon,” says Soffer.

If you have normal levels, keto is likely safe to try. “For people with normal or perfect triglyceride levels, the impact from keto is minimal or none,” says Soffer. If, on the other hand, you have insulin resistance, type 2 diabetes, and/or abdominal obesity, and your triglycerides are elevated — it may seem like you should shy away from keto, but the opposite may be true, Soffer argues, explaining that the low-carb nature of keto has the potential to decrease insulin resistance and improve triglycerides.

Research suggests this effect as well. A small randomized, controlled study published in August 2020 in Nutrition & Metabolism looked at 34 older adults with obesity over eight weeks. Those who were on a very low-carb diet lost 3 times the visceral fat compared with the low-fat group; low-carb dieters also had more improvements in insulin sensitivity, triglyceride levels, and HDL cholesterol.

Speaking of which, the relationship between triglycerides and HDL cholesterol adds another layer. When triglycerides are high, HDL is usually low, notes the AHA. Flip that script, and “anything that reduces triglycerides will also [tend to] increase HDL,” says Soffer. Remember, HDL is a type of “good” cholesterol that ferries some LDL out of the bloodstream and into the liver where it can be metabolized and discarded, per the AHA — so increasing HDL is ultimately good for your heart.

Still, though, the aforementioned Nutrition & Metabolism study involved just 34 people — so more research (particularly larger, longer-term studies) is needed before scientists can fully understand the true effects of keto on insulin resistance and triglycerides.

A larger study pool of 567 people with diabetes, culled from over 13 studies in a meta-analysis in November 2020 in Nutrition & Diabetes, found that following a ketogenic diet decreased fasting blood glucose and — in eight studies — also lowered HbA1c levels (a measure of average blood sugar over time). On average, HbA1c fell by about 1.5 percent; the authors indicate that research suggests a 1 percent reduction in HbA1c could lower the risk of a heart attack by 14 percent.

But overall, when it comes to using keto for fat or weight loss, the research is mixed. One major caveat: There isn’t enough data to show whether keto can produce long-term results. As one past review pointed out, weight loss on keto peaks at five months, followed by a slow weight regain. And a meta-analysis of 38 studies in the December 2020 issue of Nutrients that compared low-carb with low-fat diets concluded that they each exert different effects on the body, but only in the short-term. Low-carb diets were associated with greater weight loss (about 3 pounds) and better HDL and triglyceride numbers compared with low-fat. On the other hand, low-fat diets more effectively lowered LDL and total cholesterol. However, these results were only applicable for the short-term, and the small number of studies that looked at outcomes after two years found the results were no longer significant, possibly because people had trouble sticking with it long-term.

How Keto May Change Your LDL Cholesterol

The impact of high-fat, very low-carb eating on your LDL isn’t as clear. Again, as the AHA says, LDL is the type of cholesterol that’s linked to atherosclerosis, which can increase your risk for heart disease and stroke. It may go up, go down, or stay relatively the same. Hypothetically speaking, if you have a normal triglyceride level and are doing keto to lose weight, your LDL may remain stable, says Soffer. Similarly, if you have high triglycerides to start and that number decreases on keto, your LDL may still stay put.

The bottom line is that keto alone may not contribute to the cholesterol spike that some people report when starting the diet. But if you start eating more saturated fat because you’re on keto, then your LDL will likely rise. An increase in saturated fat intake “has been shown over and over again” to increase LDL, Soffer says.

Keto May Harm People With a Genetic Mutation That Affects LDL Regulation

Where things get possibly dangerous is if you’re one of the people who has an inherited genetic mutation that affects the way LDL particles are regulated. “If these individuals follow a keto diet, their LDL levels can skyrocket,” says Soffer. This result doesn’t happen often enough to skew overall results in population studies, but what matters most is the effect the diet has on your individual health. “It’s a long-recognized phenomenon that hasn’t been published well,” he says, adding that the genetics that drive this response aren’t completely understood. (The APOE gene may be one, but it’s likely not the only one, he says.)

You may not be aware that you have a preexisting genetic mutation, also called familial hypercholesterolemia (FH). In fact, as the AHA notes, only 10 percent of people with FH know they have it. Being aware of your family history, including members who have had a heart attack at a young age, is important and can help point your provider in the right direction. In some instances, research shows that a low-carb diet may actually be beneficial for those with FH who also have insulin resistance, suggests a past analysis. This is not something to tease out on your own. Instead, it’s just another reason why you should talk to your doctor about a dietary change, particularly if you have other health conditions, like high cholesterol.

The average person likely won’t notice anything amiss about their cholesterol, but “for someone with this genetic predisposition, [the keto diet] can be a hazardous diet,” says Soffer. In the short term, a small, six-month spike in cholesterol isn’t harmful — it takes a decade or more of high LDL to cause damage, he says — but if this is a rest-of-your-life way of eating, it can be dangerous.

If You Have High Cholesterol, Should You Avoid Keto?

Having high cholesterol doesn’t automatically disqualify you from keto. “We’re seeing really good cardiovascular results from a keto diet,” says Susan Ryskamp, RDN, a clinical dietitian at Michigan Medicine’s Frankel Cardiovascular Center in Ann Arbor.

Doctors will want to look at the overall picture with the goal of improving biomarkers like triglycerides, A1C, blood pressure, and body mass index (BMI). “On keto, if patients lose weight, get their BMI closer to, if not under 30 (under the threshold for obesity), and these biomarkers improve, we feel they’re at less of a cardiovascular risk,” says Ryskamp.

If a patient has severely elevated triglyceride levels — upwards of 1,000 mg/dL (remember under 150 is normal) — then Ryskamp would likely suggest another diet first. But a triglyceride level of 300 may still be a go, with close monitoring and frequent lipid checks. “Most of the patients I see can safely go on keto,” she says.

The fact is, says Ryskamp, people start down the road to heart disease in different ways (genetics, hardening of arteries), and cardiovascular patients respond uniquely to treatment. Not everyone will want to go on keto, but it may be recommended for some, she says. As a study published in November 2019 in Clinical and Scientific Debates on Atherosclerosis points out, keto is not the only option. “Other diets are as effective, more sustainable and safer,” the authors write. The AHA still recommends a plant-based diet high in fruits, vegetables, legumes, whole grains, and lean vegetable or animal protein as a good heart-healthy diet.

How to Follow Keto in a Heart-Healthy Way

One of the draws of a keto diet is the idea that you can eat any and all fat that you want. That’s not a healthy mindset to step into. “I recommend patients choose cardioprotective [unsaturated] fats, like nuts, nut butter, seeds, avocado, and extra virgin oils,” such as olive oil, says Ryskamp. Similarly, avoid foods that are high in saturated fat (like butter) as your primary fat sources.

It’s not just fats that are important, but maximizing the small amount of carbohydrates you’re allowed. Getting enough fiber can be a challenge, and you’ll want to make sure you’re eating plenty of low GI-index veggies (nonstarchy choices such as spinach and broccoli). Ultimately, the benefits come from replacing foods, like processed carbs, that experts believe are detrimental to overall wellness, with those that are heart-healthy, like unsaturated fats, she says.

It’s been repeated over and over: If you’re drastically changing your diet — which includes going keto — you need to talk to your doctor first and make sure it’s safe for you and your individual health concerns. You also want to get your lipids checked before and during keto, so your doctor can watch for a cholesterol spike and determine if this is a healthy diet for you.

One of the biggest concerns, says Soffer, is that once you come off keto, you continue eating a high-saturated-fat diet in combination with a typical American diet — a high-calorie diet that’s rich in refined grains and proteins, but low in fruits and vegetables, per the U.S Department of Agriculture (PDF). Because keto is highly restrictive, most people don’t stay on it for the long haul, so watch your diet and plan appropriately when you go off the diet.

The Bottom Line on Trying the Keto Diet to Lower Cholesterol

If you have high cholesterol or triglycerides, speak with your medical team first before embarking on a keto diet. If you get the green light, continue to seek their help. “There are a lot of popular books out on how to follow a keto diet, but I think patients need additional support and guidance and ongoing conversations about if it’s working for you,” says Ryskamp. “It’s a tool that someone can use to improve their health, but it may not be the right tool for them, or the right tool the way they’re doing it,” she says.