Working Out on a Low-Carb Diet: Benefits and Risks

High-protein, low-carbohydrate diets have gained popularity as a strategy for weight loss and managing certain health conditions. These diets involve increasing the intake of protein-rich foods while restricting carbohydrates found in grains and starchy vegetables. While many individuals adopt this approach for its potential benefits, it's crucial to understand both the advantages and potential risks associated with combining exercise and a low-carb diet.

Understanding Low-Carb, High-Protein Diets

Low-carbohydrate, high-protein diets prioritize protein intake, typically recommending that dieters obtain 30% to 50% of their total calories from protein. Popular diets like Paleo, South Beach, and keto fall into this category. These diets grew in popularity during the 1970s, thanks to The Atkins Diet, which was widely promoted as an effective weight loss plan. By comparison, the American Heart Association recommends getting 10%-35% of your daily calories from protein. This is about 46 grams of protein for adult women and 56 grams of protein for adult men.

When carbohydrate intake is limited, the body turns to fat as its primary energy source, entering a state called ketosis. In ketosis, the body breaks down fat into ketones, which then become the primary fuel. This metabolic shift can lead to weight loss, increased muscle mass, and a potentially faster metabolism.

Benefits of Working Out on a Low-Carb Diet

Many people are drawn to high-protein, low-carb diets because they help shift the body’s fuel source from carbs and sugars to fats and proteins. High-protein, low-carb diets can offer several benefits, including:

• Weight Loss: High-protein diets can promote satiety and reduce cravings, contributing to weight loss. When your fat stores become a primary energy source, you may lose weight. However, it's important to note that some initial weight loss may be due to water loss from reduced carbohydrate intake.
• Blood Sugar Control: Low-carb diets can help regulate blood sugar levels, making them beneficial for managing diabetes and prediabetes.
• Maintaining and Building Muscle: Increasing protein intake while on a low-carb diet can help preserve and build muscle mass during weight loss.

Potential Risks and Considerations

Despite the potential benefits, high-protein, low-carb diets also carry some risks:

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• High Cholesterol: Some protein sources, particularly fatty cuts of meat and whole dairy products, can elevate cholesterol levels, increasing the risk of heart disease.
• Nutrient Gaps: Restricting carbohydrates can lead to deficiencies in essential nutrients found in fruits, vegetables, and whole grains. Because carbohydrates include many food groups (like fruits and vegetables), you face a higher risk of missing out on certain nutrients when you cut carbs, Derocha says.
• Kidney Problems: High protein intake can strain the kidneys, potentially worsening existing kidney issues. If you have any kidney problems, eating too much protein puts added strain on your kidneys. This could worsen kidney function.
• Osteoporosis and Kidney Stones: Some experts believe that high-protein diets may increase calcium excretion, potentially raising the risk of osteoporosis and kidney stones. When you’re on a high-protein diet, you may urinate more calcium than normal. There are conflicting reports, but some experts think this could make osteoporosis and kidney stones more likely.
• Constipation Concerns: A low-carb, high-protein diet can lower your fiber intake and lead to constipation.

Certain groups should avoid high-protein, low-carb diets due to related health risks, Derocha says. She suggests avoiding a high-protein, low-carb diet if you have certain conditions and health issues, such as:

• Kidney disease or kidney issues, as too much protein strains your kidneys
• History of disordered eating or eating disorders, as low-carb diets promote restrictive eating
• Pregnancy or breastfeeding, as nutrient-dense diets are critical during these stages

The Impact on Athletic Performance

The effect of a low-carbohydrate diet (LCD) on athletes’ aerobic, anaerobic, and upper and lower body strength performance. An LCD may aid in maintaining or increasing upper and lower body strength. However, this type of diet provides no consistent benefit and may even negatively impact some measures of aerobic and anaerobic performance. Taken together, no strong evidence indicates the benefit of an LCD on athletic performance.

Aerobic Exercise

Studies of aerobic exercise performance tests for participants on ketogenic diets of varying lengths are summarized in Table 1. In a randomized controlled trial with 16 semi-professional soccer players that followed a 30-day LCD, the Yo-Yo intermittent recovery test (of maximum aerobic fitness) indicated significantly increased aerobic performance on day 30 compared to the pre-dietary intervention baseline [20]. However, a randomized sequence crossover trial with 14 recreational male athletes showed no change in their exercise efficiency or economy after a 14-day LCD [22]. Furthermore, there were no significant changes in 10-km race times for 21 professional male walkers after a 21-day LCD clinical trial [23]. However, a repeat of this clinical trial with 26 professional male and female race walkers showed a significant decrease in 10-km race times after a 5.5-week LCD [24]. In a two-arm randomized controlled trial, seven competitive male runners that followed a six-week ketogenic diet had no significant change in their 5-km time trial at days 4 and 42, nor did they have a difference in time to exhaustion after six weeks of consuming an LCD [25]. In another randomized cross-over trial, 14 recreational male athletes had no significant change in work output on a 90-minute cycle ergometry test after two weeks on a low-carbohydrate, high-fat diet [26]. In a randomized, repeated measures crossover study, eight trained male endurance athletes were tested in a run-to-exhaustion time trial following a 31-day ketogenic diet [27]. When the respiratory exchange rate (RER) was >1.0, indicative of anaerobic conditions and a greater dependence on carbohydrate metabolism, there was a statistically significant decrease in the run time to exhaustion. However, for an RER <1.0, there was no statistically significant change. In addition, the athletes who completed the ketogenic diet had impaired exercise efficiency, which was more pronounced at >70% VO2max. In a randomized crossover trial, eight recreational male swimmers had no significant change in swimming economy at 50%, 60%, and 70% VO2max after a three-day LCD [28]. In a randomized controlled trial, six well-trained competitive cyclists found a significant decrease in time-to-exhaustion cycling trials at 10°C and 30°C after a three-day LCD [29]. In a comparative study, 22 professional male soccer players significantly decreased the total distance covered in a 90-minute soccer game after a four-day LCD [30].

Anaerobic Exercise

Results of studies of ketogenic diets of varying lengths on anaerobic exercise performance tests are summarized in Table 2. In a randomized sequence crossover trial of a four-day ketogenic diet with exercise-trained participants, there was a statistically significant increase in anaerobic capacity as measured by the Wingate test [19]. However, in studies where the ketogenic diet was longer, anaerobic performance either stayed the same or decreased [31,32]. In a clinical trial with a four-week LCD, there was a notable drop in anaerobic performance in basketball players, with significantly decreased performance on the Wingate test [31]. This contrasts with a randomized clinical trial using a 10-week LCD with trained college-aged men, in which researchers found that subjects’ Wingate performance test results did not significantly change [32].

Upper and Lower Body Strength

Studies of upper and lower body strength after a ketogenic diet are summarized in Table 3. A one-repetition maximum weighted bench press was used to measure strength in the upper body. In contrast, a one-repetition maximum weighted squat or counter-movement jump (CMJ) test was utilized to measure lower body strength [32,33]. To assess upper body strength, a randomized, parallel arm, controlled prospective study tested the one-repetition bench press maximum load of 21 strength-trained women before and after an eight-week LCD; no significant change was observed [33]. In another two-arm randomized clinical trial, 20 experienced resistance-trained men underwent an eight-week ketogenic diet; again, no substantial change in their one-repetition bench press maximum load [31]. However, a different two-arm randomized clinical trial found a statistically significant increase in bench press after a 10-week ketogenic diet in 25 resistance-trained college-aged men [32]. Another randomized controlled parallel study, which utilized an eight-week ketogenic diet and included 19 male athletes, also observed that the participants significantly increased their bench press [21]. For lower body strength, a randomized, parallel arm, controlled prospective study tested the one-repetition maximum squat load of 21 strength-trained women after an eight-week LCD; a significant increase was found in both one-repetition maximum squat load and CMJ [33]. Similarly, a two-arm randomized clinical trial found a statistically significant increase in one repetition maximum squat load after a 10-week ketogenic diet in 25 trained college-aged men [32]. Another randomized controlled parallel study had similar results, in which 19 male athletes significantly increased their one-repetition maximum squat load after an eight-week ketogenic diet [21].

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To assess the effects of a six-week ketogenic diet on total upper and lower body strength, a single-arm within-subjects clinical trial examined the effects on total workout repetitions per week and maximum volume load, the multiplication of weight used by the set and number of repetitions, in 13 resistance-trained individuals. There were statistically significant increases of 11.4% in weekly repetitions and 7.6% in volume load [35]. In a randomized crossover study, 14 trained powerlifters self-selected a lift of choice and underwent a three-month ketogenic diet. There was no significant change in their self-selected lift’s one-repetition maximum load before and after a three-month ketogenic diet [36].

Considerations for Exercise

• High-Intensity Exercise: The majority of high-intensity exercise - activity that requires short, intense bursts of energy - is powered by the carbohydrates you eat. The human body stores carbohydrates in the muscle cells as muscle glycogen. That muscle glycogen becomes an immediate fuel source for your body when you do any sort of activity that periodically requires intense movement, including strength training, sprinting, and other sports.
• Fat Burning: Keto may boost fat burn in people who like to do steady state aerobic exercise, like distance running or cycling at a steady pace.
• Energy Levels: As your body becomes “keto-adapted,” it begins to train itself to burn fat for energy instead of carbohydrates. Initially, that process may leave you feeling less energized than usual when it comes to working out.
• Muscle Mass: Keto Is Better for Maintaining Muscle Mass Than Adding Muscle.

Practical Tips for Starting a High-Protein, Low-Carb Diet

If you want to start a high-protein, low-carb diet, you should first talk to your doctor and go over any health concerns.

According to Grace Derocha, RDN, CDCES, Spokesperson for the Academy of Nutrition and Dietetics, here are some ways to get started:

• Stock your house with high-protein, low-carb foods such as fish, lean meat, red meat, eggs, cheese, broccoli, kale, and spinach.
• Get high-protein, low-carb snacks, such as string cheese, hard-boiled eggs, and Greek yogurt, as many go-to snack foods aren’t high in protein.
• Stay hydrated, especially because removing fruit from your diet can lower your water intake, as many fruits contain water.
• Look at menus before eating out and research takeout options.
• Setting realistic and sustainable goals that you can stick with.

“Make small adjustments that don’t take over your whole life to set yourself up for success,” Derocha says. “Remember that food is more than fuel for your body and find ways you can still enjoy it and incorporate it into your traditions, celebrations, and social gatherings.”

Some examples of high-protein, low-carb foods include:

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• Meats, including poultry, fish, and red meat (heart-healthy options such as salmon, mackerel, anchovies, sardines, and herring, Derocha says).
• Eggs
• Healthy fats and oils such as avocados and olive oil
• Low-carb vegetables such as broccoli, kale, spinach, and cucumbers
• Cheese
• Nuts
• Cauliflower rice
• Beans, legumes, and lentils
• Greek yogurt
• Tofu and tempeh
• Lower-carb fruits such as raspberries and blackberries

Here are a few meal ideas for high-protein, low-carb diets:

• Breakfast. Omelet with bell peppers, spinach, and cheese; tofu scramble with kale and broccoli; Greek yogurt with blackberries
• Lunch. Tuna salad with egg, feta, and avocado; chopped salad with shrimp, egg, and avocado; cobb salad with grilled chicken
• Dinner. Chicken and fried cauliflower rice; lettuce tacos with beef, veggies, and guacamole; spicy salmon nori wraps; salmon and steamed broccoli

Sample Recipe: Grilled Tuna with Fresh Avocado Salsa

Here’s a recipe option you can try for lunch or dinner:

Ingredients

• One cup of grape or cherry tomatoes, quartered
• One-quarter English cucumber, cut into small pieces
• Two scallions, chopped
• One tablespoon grated fresh ginger
• Three tablespoons chopped fresh cilantro
• One tablespoon fresh lime juice
• One tablespoon plus one teaspoon extra-virgin olive oil
• One-quarter teaspoon kosher salt
• One avocado, diced
• Two fresh wild tuna steaks (8 ounces each), 1-inch thick (like yellowfin or albacore)
• Half a teaspoon of freshly ground black pepper

Directions

1. Combine tomatoes, cucumber, scallions, ginger, cilantro, lime juice, olive oil, and salt.
2. Add avocado after salsa is mixed together.
3. Rub oil on both sides of the tuna and season with black pepper.
4. Grill over medium-high heat for three to four minutes per side for a medium-rare steak, or a little longer for medium to well-done.
5. Divide the tuna into four servings. Serve with salsa.

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