What is a ketogenic diet?
A ketogenic diet (KD) consists of very low amounts of carbohydrates, adequate protein, and high amounts of fat. Typically, your body relies on carbohydrates, or glucose, for energy production. When your body has limited carbohydrates available—as in a KD, a fasted state, or with prolonged exercise—it can use fat as fuel. So, the goal of the KD is to shift your body’s primary energy source from carbohydrates to fat. Ketone bodies then replace glucose as the primary source of energy.
The ketogenic diet was originally recognized in the early 1900s as a way to improve childhood epilepsy symptoms. However, as epilepsy medications continued to develop, the ketogenic diet was nearly forgotten.. That was until the 1970s, when low-carb diets made a resurgence and were popularized for their weight-loss potential. Beginning with low-carb diets like the Atkins and South Beach Diets, the ketogenic diet has become the low-carb diet of choice in recent years. Today, the diet’s popularity continues to grow. In addition to helping control childhood epileptic seizures, the KD has been proposed to improve body composition, health, and athletic performance. Let’s review the up-to-date nutrition science to better understand the KD and its effects on health and performance.
What’s the nutritional breakdown of a ketogenic diet?
The “classic” definition of the KD consists of one gram of protein per kilogram of body weight per day, 10–15 grams of carbohydrates per day, and the remaining calories from fat. Yet, definitions of KDs vary, but typically break down to 5–10% carbohydrates, 20–25% protein, and 70–75% fat. The circle graphs below compare the typical macronutrient distribution for KD and the standard American Diet (AD), followed by a breakdown in grams of each macronutrient based on a 2,000-calories-a-day diet.
Typical Keto Diet (KD) Macronutrient Distribution vs. American Diet (AD)
KD (2,000 kcal diet): Fat 155–180 g; Protein 75–100 g; Carbs <25 g
AD (2,000 kcal diet): Fat <70 g; Protein 50–100 g; Carbs 250–300 g
The KD leads to a state of “nutritional ketosis,” which is generally considered safe and shouldn’t be confused with ketoacidosis, which is a life-threatening condition characterized by high amounts of ketone bodies and dangerous changes to blood pH levels. Under typical dietary conditions, ketones can be found in the blood at concentrations around 0.2 millimolar (mM) or less. A state of ketosis is generally considered blood concentrations of ketones greater than 0.5mM. Yet, with ketoacidosis, blood concentrations of ketones can exceed 12–15mM.
The amount of carbohydrates allowed in the diet to remain in ketosis will vary based on the individual, but broad guidelines suggest that carbohydrate intake should remain below 50 grams per day. Because KDs are hard to sustain, ketone supplements have been introduced to the marketplace. To learn more, you can read the Operation Supplement Safety article about ketone supplements.
Can a ketogenic diet help with weight loss?
The KD is possibly most famous for its effects on body weight. There’s ample evidence that it can be an effective weight-loss strategy in the short term. Research often shows that low-carbohydrate diets (like the KD) beat out other calorie-restricted diets when studies lasted 6 months or less. However, once researchers started following people for at least 12 months, the differences in weight loss between the KD and other low-calorie diets diminished. When comparing the KD + exercise with three other dietary patterns (calorie restriction + exercise, 5/2 intermittent fasting + exercise, and time-restricted feeding + exercise), a combination of caloric restriction and exercise showed the best overall effects on weight loss, body fat percentage reduction, and muscle maintenance, and the KD was the least effective. Still, one of the biggest arguments surrounding the KD is that it’s very hard to sustain for long periods of time. This is one reason why studies that compare the KD with other diets don’t see major differences at 12 months or longer.
Can a ketogenic diet help with mental health and performance?
Since the KD has been used to treat pediatric epilepsy, many wonder if it can impact other aspects of mental health or cognitive well-being. The brain, although small, requires a constant supply of energy to support its varying functions. In addition, the brain is well-known for its ability to use ketone bodies for energy production. Since ketone bodies have been identified to contribute to the brain’s energy needs and influence neural health and function, research using KD to influence brain health and performance is justified.
High-quality studies on the effects of the diet on working memory, attention, processing speed, and other human cognitive measures are limited. Of those that have been published, some showed improvements following the KD, while others found no changes. Similarly, research using medium chain triglyceride (MCT) supplementation shows some promise. MCT supplementation can promote nutritional ketosis for a few hours, and may even be better for long-term compliance compared to a KD. However, although evidence indicates the potential for cognitive improvement, there’s not enough knowledge on this topic to make definitive conclusions. More research is needed to determine if a KD or MCT supplementation can improve brain health and performance in young and healthy populations. Similarly, more research is needed to explore the effects of KD or MCT supplementation on anxiety, depression, and brain injury before definitive conclusions can be made.
Can a ketogenic diet improve physical health and performance?
In the 1980s, the U.S. military examined the KD as a possible approach for optimizing performance. The hypothesis was that relying more on fat than carbs for energy might improve performance during the low-intensity and long-duration demands of many military missions, while saving carbohydrate stores for high-intensity activities. This shift could be beneficial by tapping into the most dense energy source: fat stores. At that time, nothing really came of the KD because the results were insufficiently conclusive and opportunities for more research became limited. Fortunately, more evidence is available to explore today.
Even though KD increases the ability to burn fat, the scientific literature hasn’t consistently demonstrated it as superior to current sports nutrition guidelines for improving performance. Up to this point, only 16 high-quality studies have explored the effects of KD on endurance performance in young healthy adults. Overall, KD tends to impair athletic performance in elite athletes, and have either no or a negative effect on recreational athletes. Similarly, only 10 high-quality studies have explored the effects of KD on strength and weight lifting performance in young, healthy adults. All but one study found no significant differences in strength-training performance between KD and other diet groups. Even when someone is “keto-adapted,” performance in real-world scenarios is mostly impaired. When looking at the effects of the KD on physical performance, it’s important to consider that most of the studies didn’t exceed 12 weeks, and most didn’t include females. So, the effects of KD on females, or after long-term use, are unknown at this time.
What are the potential side effects of following KD?
Short-term side effects of KD can include “keto flu” symptoms like nausea, thirst, headache, dizziness, fatigue, and increased urination. These side effects should subside once your body has “adapted,” but it isn’t clear how long that takes (usually 1–4 weeks). Also, any increase in carbohydrate intake will disrupt ketosis, and side effects can recur if you don’t strictly adhere to your KD. In other words, there are no “cheat meals” when you’re on a ketogenic diet. Long-term effects are still being studied. However, some other potential side effects have been noted.
- Hypoglycemia: Low blood sugar which might cause symptoms like dizziness, confusion, shaking, or sweating.
- Dyslipidemia: Abnormal levels of lipids in the blood (LDL cholesterol) may occur, increasing cardiovascular risk.
- Nutrient deficiencies: Several nutrients such as folate and fiber often decrease below recommended levels on a KD.
- Changes in gut health: Changes to microbial communities following the KD may be harmful to gut health.
- Kidney stones: KD is a risk factor for kidney stones.
- Muscle loss: KD often leads to the loss of both fat and muscle mass.
Furthermore, low-carb diets haven’t been adequately researched in adolescents and youth, and some recent evidence suggests that it may impair skeletal development and increase injury risk during growth and maturation. However, KD may still be considered as a treatment option, especially if the child or adolescent has epilepsy or other neurological disorder. Therefore, extreme caution and medical oversight should be used with younger populations who consider KD.
Challenges understanding KD outcomes on health and performance
As research continues to grow and evolve, the true effects of KD on human health become more transparent. However, there are still many challenges and limitations in the research, making it hard to identify clear and definitive conclusions. For example, some of the research making beneficial health claims is done on animals (such as mice or rats) or in “clinical” populations (such as those with type 2 diabetes or polycystic ovary syndrome). So, even if the KD had positive effects on the populations studied, you can’t assume that the same effects will be seen in a “healthy human” population.
In addition, there’s no clear agreement on the definition of a KD or “low-carb” diet. The terms KD and low-carb diet are often used interchangeably, but can be different. For example, some low-carb diets limit carbohydrates, but not to the point of nutritional ketosis. Some may say a diet is KD or “low carb” if it contains less than 10% carbohydrates, others may suggest it’s less than 50 grams of carbohydrates, and there are others who might say it’s 30–40 grams of carbohydrates per kilogram. Without consistent definitions and testing parameters, it’s challenging to draw definitive conclusions.
Furthermore, some studies have combined a KD with other “treatments,” such as calorie restriction, omega-3 supplementation, or a mix between the KD and Mediterranean diet, making it hard to determine whether the effects were caused by KD or the combination of other treatments.
Lastly, there’s very limited research exploring the effects of KD on females. The biological differences in metabolism between males and females may lead to differences in the effects of the KD on body composition, health, and performance. Even among the studies that demonstrate body-fat loss in both male and female participants following a KD, males seem to experience larger benefits in weight loss. More research in females is needed to determine the full effects of KD.
Ketogenic diet considerations for Service Members and tactical athletes
When considering a KD as a tactical athlete, look at all factors, such as access to “keto-friendly” foods downrange, ease of use when carrying “keto-friendly” foods in pockets, bags, or stow spaces, and the physiological risk you’re willing to assume on any mission. For example, if you are part of a ground unit or aircrew and begin to experience symptoms of hypoglycemia, will you still be able to perform the requirements needed for mission success and team safety? In some circumstances, physical performance can separate life from death, and the effects of diet on physical performance shouldn’t be overlooked. With this in mind, remember to think critically before changing your diet. Consider your goals and priorities when making dietary choices. And remember, nutrition science is always evolving, so sometimes checking in with credible and reliable resources can help you adapt and optimize your nutrition strategies over time.
Summary
The ketogenic diet appears effective for reducing body fat in the short term, but does not outperform other weight-loss strategies in the long term. In addition, KD seems to have null or negative effects on muscle mass, physical endurance, and strength performance. And while some research is promising, not enough evidence is available to draw conclusions of its effects on cognitive performance. Therefore, the research to support KD for enhancing the performance or health of any military community is insufficient. Service Members should leverage the expertise of their unit dietitians to develop individualized nutritional strategies and engage with their human performance team to optimize all aspects of performance. The military will continue to follow the nutrition regulations published by DoD: “Army Regulation 40–25, OPNAVINST 10110.1/MCO 10110.49, and AFI 44–141: Nutrition and Menu Standards for Human Performance Optimization.” The regulation is based on the most up-to-date science and forms the foundational nutritional strategies for Service Members. Ultimately, a Service Member’s specific health goals and successful performance of mission-essential tasks should drive their diet choices.
Tools and resources
If you’re a provider, share this handout with Service Members to help them understand how KD affects performance. Also, check out the Performance Nutrition section for helpful worksheets, meal planners, and more.
