The low-carb, high-fat ketogenic diet is majorly trending right now. In fact, you probably know a few people who have tried it. Just like any other eating style, it works well for some people, while others find it impossible to stick with. And if you’re spending a lot of time in the gym, it’s natural to wonder: will it help or hurt your workout efforts?
If you choose to try the eating style (or are considering it), you should know about how the keto diet changes your body’s source of energy. When that happens, your workout performance and how you feel during exercise can change, too.
Here, nutrition pros explain how going keto can affect your workouts:
The keto diet involves eating a very small amount of carbs and very high amounts of fat. “While those following the keto diet may cut out processed sweets and starches, they are also eliminating or severely restricting nutrient-dense simple carbohydrates found in fruit and low-fat dairy products as well as whole grains and legumes,” explains Mindy Haar, PhD, RDN, a registered dietitian and associate dean of undergraduate affairs at NYIT School of Health Professions. For this reason, among others, Haar does not recommend the keto diet for anyone.
For those who are active in particular, missing out on high-quality carbohydrates can have some less-than-ideal repercussions. “As carbohydrates are the best fuel for the body, following a ketogenic diet can result in low energy levels,” Haar says. That might be less energy during your workouts, and possibly even less motivation to get to the gym in the first place.
“Starting a ketogenic diet is a major adjustment for your body, especially if you are used to eating a moderate to high carbohydrate diet,” notes Chelsea Amengual, MS, RD Manager of Fitness Programming & Nutrition at Virtual Health Partners.
“If you are following this low-carb, high-fat diet properly, then in the first few weeks, your body will be switching over from using glucose as the primary source of fuel to using ketones produced from the breakdown of fats,” she explains. This switch has a major impact on your power and endurance.
“Be sure not to try any new or very intense workouts during this time,” Amengual suggests. Stick what you’re used to, and possibly consider dialing back the intensity a bit. “For example, if you normally take four indoor cycling classes a week, maybe try two or three classes instead, or go a little easier on those sprints during class.”
Another downside of keto in terms of exercise is that you need to be very on top of how much water you’re drinking. “Excreting the waste products from this diet requires abundant fluids, putting people at a higher risk for dehydration,” Haar explains. This is one of the reasons she doesn’t recommend the diet in general.
Dehydration is also part of the reason people experience what’s known as “keto flu” when they first start the ketogenic diet. So if you do choose to go keto, be sure to drink plenty of water — especially around your workouts.
For those who are concerned about personal bests or are training for a race, it’s important to note that a true ketogenic diet is not usually associated with better athletic performance. Some studies have suggested that endurance athletes may perform well on the keto diet, but not all sports nutrition pros are convinced, especially since other research has found the opposite. When it comes to power sports like weightlifting or sprinting, most research has found that the diet does not benefit performance in any way.
In addition to lacking sufficient carbs, strict ketogenic diets are low to moderate on protein. “Protein is required for muscle strength and recovery, along with a multitude of other tissue and enzyme functions in the body,” Amengual explains. When strength gains and recovery are impaired, fitness gains will happen at a slower rate.
On a positive note, weight and body composition may improve when on the ketogenic diet, Amengual says, which is why some competitive athletes choose to follow keto in the off-season.
Here’s the good news: “After a few weeks on a strict ketogenic diet (75% fat, less than 5% carbs), you will have likely transitioned into ketosis, meaning your brain and body are burning ketones (from fat) as fuel,” Amengual explains. This means you’ll probably be better adjusted to having fewer carbs available for fuel, and therefore will likely have improved energy levels, mood and concentration compared to the first few weeks on the diet.
“That said, you still want to focus on workouts that are less dependent on carbohydrate, like yoga, lower-intensity walking/jogging or light weightlifting,” she adds. “You’ll want to avoid endurance exercise (over 1 hour) or very intense workouts like HIIT, CrossFit and boxing unless you are supplementing with additional carbs before and after these types of workouts.” In other words, if you’re a bootcamp pro, keto is probably a no-go.