Quick: Think of two foods that could fuel your next workout. We're guessing you didn't picture beef or cheddar cheese. But those are staples of the popular ketogenic, or "keto", diet, which has a reputation for getting people lean, fast. "The eating plan can help people reduce body fat while maintaining muscle mass", says Catherine Saenz, PhD, RD, a certified strength and conditioning specialist and assistant professor of kinesiology at Jacksonville University in Florida. It sounds almost too good to be true. Is it?
"The eating plan can help people reduce body fat while maintaining muscle mass".
Catherine Saenz, PhD, RD, a certified strength and conditioning specialist and assistant professor of kinesiology at Jacksonville University in Florida
"The goal of the keto diet is to get your body to turn to its natural fat stores for your primary source of energy", a state called ketosis, says Saenz. To do that, followers eat a seriously strict macro breakdown. The US National Institutes of Health considers that breakdown to be 55 to 60 percent of daily calories coming from fat, 30 to 35 percent from protein and 5 to 10 percent from carbs, but many keto dieters lean even more heavily on fat.
When you're eating primarily fat, then burning your own for energy, you're typically fuller and better able to manage your hunger, says Saenz. This can lead to some appealing short-term benefits: Research has shown that people on keto see improvements in their weight, insulin resistance, blood sugar and blood pressure, even though the diet was actually created as a medical therapy to treat epilepsy. But after about a year, the effects level out to be comparable to other weight-loss diets.
Can Fat Fuel a Workout?
When it comes to training on keto, the reviews are … mixed. One small 2020 study on women found that those on an eight-week keto diet increased their strength for a back squat but not a bench press, while women on a higher-carb plan actually saw improvements in both. In another small study published in "Metabolism: Clinical and Experimental", endurance athletes who adapted to a keto diet and, consequently, lost fat over 12 weeks sprinted with more power and maintained more endurance than those who ate more carbs. It's a logical result: When you lower your body fat so that a higher percentage of your weight is muscle, you can become more powerful, stronger and faster.
In a separate study published in "The Journal of Physiology", elite endurance athletes on keto improved their VO2 max, a measure of how much oxygen a person can use during intense exercise. But they also needed more oxygen to perform, which, in effect, negated those benefits, says study author Louise Burke, PhD, the chief of nutrition strategy at the Australian Institute of Sport. That's because it takes more oxygen to turn fat, versus carbs, into energy, she says. And another study in the "Journal of the American College of Sports Medicine" found that when keto runners tried to dial up their intensity, they actually ran 5 percent slower, likely because fat isn't as helpful as carbs are in supporting muscle contractions.
That doesn't mean you're totally screwed if you love cardio and want to try keto. Burke notes that most research shows that for low- or moderate-intensity exercise, being on the diet shouldn't negatively affect performance, though it shouldn't positively affect it either. It's when you go long or hard that your training might suffer. That's why for athletes who want the best of both worlds, Burke suggests experimenting with a slightly higher carb count a few times a week before higher-intensity sessions—think having berries with your eggs and avocado. But she recommends doing so only once you've figured out whether you'll stay in ketosis (everyone differs, and you can tell with ketone strips).
Things to Consider When Considering Keto
If you want to give keto a try, it's good to know exactly what you're getting yourself into, because it's not all bacon and butter-flies. As with most restrictive eating plans, the diet is seriously difficult to sustain in the long run, so it probably isn't for you if you're looking for a simple lifestyle change. Then there's the physical, uh, fun: As your body adjusts to the lack of carbs, its former source of energy, you might feel sluggish. So sluggish, in fact, that your workouts feel harder and you feel sorer from them the next day, says Saenz. At the same time, your heart rate may speed up, you could feel dizzy or have trouble sleeping, and you might experience stomach cramping and bloating.
Your body's response to the diet could be wildly different from someone else's. Some people feel amazing relatively quickly, while others feel sloth-like and foggy for weeks, says Burke. These side effects may often be signs that you need to tweak something within your diet, especially if they persist, says Saenz. Once the initial adaptation period is over (usually within the first few weeks), people report feeling steadier energy levels during workouts, recovering faster, sleeping better and having more focus all around, she says.
So, are the possible downsides and definite restrictions worth the potential gains? That's your call. If the diet sounds right for you and you've got the green light from your doc (a strong rec from experts, as some research suggests keto could mess with cholesterol levels), feel free to shun carbs and favour fat for a little while. Just keep in mind that, whether you go keto or not, focusing on whole, nutrient-dense foods will help you perform at your best—and there's no science contesting that.