The keto diet has hit the dieting world by storm with top athletes and fitness celebrities starting to explore its potential benefits. But is a keto diet right for athletes that have long relied on carbs as their primary source of fuel?
Traditional sports nutrition has taught us one thing for years: carbs are the single best source of energy for workouts and everything else is inferior, especially fat. But is this really the case for all types of exercise?
To test out the possibility of tapping into a new source of energy for training, I decided to do some anecdotal research - putting myself on a strict keto diet for one month and engaging in multiple types of exercise. Combined with the existing research on keto for athletes and sports nutrition in general. Here's my complete breakdown on ketosis for fitness and some tips for whether or not you should consider going keto for fitness purposes.
First, What are Ketones?
What makes a ketogenic diet unique compared to any other weight loss trend that we've seen is a measurable shift in your metabolism - switching from carbs to fat as your primary source of fuel. And this switch is not just a re-prioritization of macros, as much as it is a complete change in how you get energy from food and supply it to your organs and tissues - using ketones.
Ketone bodies are acidic molecules produced by your liver when carbohydrates are not readily available for energy. The primary building block of ketones or ketone bodies is fatty acids - from either stored or dietary fat. And there are three main types of ketones you produce.
- Acetoacetate - the main ketone produced. It is either used for energy, converted into Beta-hydroxybutyrate, or excreted through urine (you can test for acetoacetate with keto test strips!).
- Beta-hydroxybutyrate (BHB) - used as fuel by the body and brain. Reaching higher BHB levels is often a key desired result of ketosis.
- Acetone - produced in small quantities and not readily used by the body. A majority is excreted through breath, resulting in a fruity, keto breath.
It is thought that when you start producing enough ketones that they become your primary source of fuel for your muscles and organs that you enter a state known as "nutritional ketosis".
Getting into ketosis is not an easy feat for everyone and can take multiple weeks depending on your training, glycogen stores, and your body's ability to use fat efficiently.
I used urine test strips to monitor my levels and it took me approximately 12 days to reach small to moderate ketone levels, while consuming less than 20g of net carbs and training daily.
Aerobic vs. Anaerobic Training
Exercise can be classified in two different ways: aerobic or anaerobic. Aerobic means with oxygen and anaerobic means without oxygen.
Aerobic training is typically low to moderate, with adequate oxygen available. Anaerobic training occurs when your output increases close to maximum output, and your body's consumption of oxygen increases to a level where aerobic metabolism alone cannot support your needs. Almost all anaerobic exercise will also incorporate aerobic metabolism - it's just also using the anaerobic metabolism to supplement - so anaerobic activity is technically "anaerobic and aerobic".
|Aerobic Exercise||Anaerobic Exercise|
|Aerobics Class||Explosive movements like quick turns, jumps, hitting/pitching a baseball, all out finishes, etc.|
|Moderate Weight Lifting|
Because oxygen is a key component in many nutrition pathways, the type of training you do can significantly impact the way your body utilizes nutrients and whether or not a keto diet is right for you.
All energy is supplied in the form of adenosine triphosphate, or ATP, regardless of whether it comes from protein, fat or carbs. While the process of generating ATP and how it works on a molecular level can be complex, in basic terms, ATP is the gas that fuels your engine. Everything you eat, regardless of what type of macro it is, has the potential to become ATP and provide energy to your body.
The real differences lie in how you generate ATP. Energy metabolism is extremely complex and involves numerous pathways. But to break it down in basic terms, there are three main ways you get fuel (aka ATP) during exercise:
- ATP-PC: Consists of adenosine triphosphate (ATP) and phosphocreatine (PC). Muscle stores of ATP are broken down and used for immediate fuel - leaving adenosine diphosphate (ADP) and a single phosphate (Pi) as by-products. Your stored supply of ATP is very limited and can only supply energy for a few seconds at a time. But phosphocreatine can also be broken down quickly to produce creatine and Pi, which releases enough energy for the ADP to Pi to rejoin and from additional ATP for use. Together these two pathways can support ~10 to 15 seconds of explosive movement and are quickly replenished during rest. Used for short bursts with maximum output like max lifts, full out sprints, pitching a baseball, etc.
- Glycolysis: The quick break down of available glucose into usable ATP to support high-intensity bursts for 30 to 40 seconds at a time. Used for high-intensity output like HIIT training, heavy lifting or sprinting. Glycolysis can be used in aerobic or anaerobic conditions.
- Oxidative Phosphorylation and Lipid Metabolism: Stored fat or carbs are used to supply long lasting to supply a moderate or low output. Used for long runs, endurance, and continued exercise.
Both the ATP-PC and glycolytic pathways are anaerobic and do not require oxygen, making them an ideal source of energy when your breathing is heavy or labored. Getting energy from these pathways is quick and efficient, but not long lasting.
Oxidative energy is aerobic, requiring adequate oxygen availability to supply fuel. Getting energy from this pathway is slower, requiring more steps, but provides more long-lasting fuel to help you go for more than a few minutes at a time. Both fat and carbs are commonly used to fuel this pathway regardless of what type of diet you are on.
You can experience all three energy pathways by going from a full out sprint, to a fast run, to a jog or walk. The full out sprint cannot be held forever, and your overall speed decreases as stored ATP and quick acting glucose are depleted.
Ketones vs. Glucose for Fuel
Your training may take a major hit when first starting keto or any low carb diet, as you deplete glycogen stores and force your body to find a new primary source of energy. And how long this lasts can differ from one person to the next. The real question is, once you reach ketosis, does the increase in ketone production create a new abundant source of energy and improve your performance?
Increasing ketone levels supports oxidative energy. Instead of relying on stored carbs or glycogen, your body is able to more efficiently use fat. And because fat is energy abundant (provides twice as many calories as carbs per gram) and you are able to store more fat than carbs overall, ketones could theoretically supply more long-lasting energy to athletes. In one study ketone bodies supplied 31% more energy than pyruvate from glucose (1).
But oxidative pathways are only efficient when enough oxygen is available. As intensity increases and oxygen consumption increases, your body needs energy more quickly and relies on the other two pathways to supplement.
Interestingly, ketosis is thought to increase the rate of fat oxidation overall, meaning you may be able to use fat for energy at higher outputs through keto-adaption (2,3). But this increase still may not be enough to fuel all of your high-intensity training (4,5,6).
If you are looking to continue training at a higher level, your body requires glucose. In other words, ketones are not a desirable source of fuel when oxygen is not readily available and cannot replace glucose as a source of fuel during anaerobic training.
But this doesn't mean you cannot use glucose for fuel on a keto diet. The terms glucose and carbs are often used interchangeable - mainly because glucose is technically a carb, but carbs are not the only way for your body to get glucose. Confusing I know! Glucose can come from dietary sources (like carbs), but it can also be created from protein and fat through a process called gluconeogenesis.
It is also important to note that going into ketosis does not eliminate glucose needs entirely. Ketones cannot supply 100% of your energy needs and some glucose is still needed to support certain organs and balance blood sugar levels. So gluconeogenesis is a desirable process for your survival.
What is Glycogen?
Glycogen is the storage form of glucose and plays a key role in performance - especially during anaerobic conditions. Glycogen is most easily built up from eating plenty of carbs, but you can also make glycogen from stored protein (amino acids) and potentially fat (glycerol) - glucose is produced by the liver using amino acids or glycerol and then delivered to muscles, where it’s stored as glycogen.
However, this process of storing glycogen is not as efficient as getting glucose from food and also seems counter-intuitive to ketosis, in which depleting glycogen stores is a primary goal. In addition, the research seems somewhat divided with some studies suggesting keto diets deplete glycogen stores and other studies indicating the impact is not as drastic (7,8,9). Using more fat in aerobic conditions is glycogen sparing, so if you are keto-adapted you might not be using a lot of glycogen stores unless you are training more anaerobically. It could also be that continually relying on glycogen for training could be a competing interest against getting into ketosis since you will prioritize gluconeogenesis - although there isn't any research to support this theory.
I was training five to six days a week, with three of the days containing some sort of high intensity/interval training and at least two other days with some heavy lifting. I found I hit a wall very quickly (within a few days) when working to deplete my glycogen stores. And even though I was likely depleted and taking ketone supplements, it took longer than i thought to increase my ketone levels. Could it be that my glycogen needs for training were interfering with my ability to get into ketosis?
Let's take a closer look at how ketosis can potentially support or impact different types of fitness.
Endurance Training on Keto
Endurance training and a ketogenic diet may be a match made in heaven. After all, these types of athletes have been trying to increase fat utilization for years. Relying on fat or ketones means you can store more energy and may not need to work as hard to get it - helping you go for longer. It's no surprise that cyclists, runners, and other endurance athletes have found a keto diet to be extremely useful towards their performance (10,11,12).
I also found this to be the case during my keto diet. My output during moderate paced running and lifting was not impacted while in ketosis or during the time period before ketone levels started to increase.
However, a keto diet does not eliminate the need for nutritional support during longer training. Electrolyte balance, hydration, and overall calorie needs are still an essential part of endurance and stored fuel will only get you so far. You will still need to support and replenish when exercising for more than 60 to 90 minutes at a time.
In addition, for competitive athletes, decreased glycogen stores may inhibit your ability to all-out sprint to the finish or go for that final push.
Weight Training on Keto
Moderate weight training on keto is likely not inhibited significantly because ketones or fat would be an adequate source of fuel under aerobic conditions. In addition, any heavy lifts can be supported by available ATP-PC, as long as it is kept to only a couple reps with at least 30 seconds of recovery time. Repeated heavy lifting or CrossFit style training may be more difficult, as more quick energy would be needed.
Under normal circumstances, 20 to 50g of carbohydrate intake is sufficient to supply your body's minimum glucose needs. For active individuals who require higher glucose for training, many fear that muscle will be broken down to supply this increased need. But as long as protein intake is adequate, research suggests that lean body mass can be fairly well maintained even in a calorie deficit promoting fat loss and weight loss overall (13,14,15,16). In other words, you shouldn't lose a lot of your muscle mass when going keto for fat loss if you are using your muscles regularly and eating enough protein.
However, a majority of the studies on maintaining muscle mass and strength are not long-term, use small sample sizes and do not always control for calories and protein. If you find that you are losing strength and power on a keto diet after reaching ketosis, it might not be a good fit for you.
What about the effect on muscle gain? Can ketosis be used to gain weight?
Based on what we know about muscle building, using a keto diet to gain mass is likely difficult.
A ketogenic diet is designed to deplete glycogen (stored carbs), to force your body to rely on stored fat instead. Glycogen is not just important for providing energy to muscles, it also plays a role in protecting muscles from breaking down and promoting protein synthesis, aka building more muscle mass (17,18). And carbs play a particular role in raising insulin levels - which is needed to build muscle. Yes, protein can also help increase insulin for muscle building, but the research is much stronger in support of carbs for building muscle.
Not to mention, a keto diet is typically used to promote fat loss and weight loss overall.
Weight training on keto did not feel difficult at first, but after one week of carb depletion coupled with HIIT training on other days, I felt my strength go. Once ketosis hit, I regained some of it, but harder training days still felt like a challenge at weeks 3 and 4.
High Intensity on Keto
Using fat for energy is typically a slow process that requires adequate oxygen supply. While this type of fuel may be a meaningful choice for endurance athletes and those participating in lower intensity training, it may not cut it for anything that requires explosive movements or anaerobic exercise.
The ATP-PC can be replenished and support short burst activity on any diet, including keto. So if you are doing interval training for 15 seconds or less, with short rest time in between, you may not notice any effects on a low carb diet. But ketone bodies cannot replace glucose for high-intensity training lasting more than 15 seconds at a time. You need glycogen to do this.
Where it starts to get really complicated is finding stored glycogen while in ketosis. Since ketones may not be able to replace glucose as a source of quick energy, is gluconeogenesis using amino acids and glycerol efficient enough to keep glycogen stores adequate?
This question remains unanswered with the current research fairly limited and divided. Most studies are short with small sample sizes and don't consider all of the necessary variables. In some studies, high-intensity training was negatively impacted by low carb intake (19,20,21). In others it was not inhibited - however, glycogen stores were not measured, which is a key component to this type of training (22,23).
Some argue that with adequate time to fully adapt to keto (1 to 3 months to reach full keto-adaption), you will start to see drastic improvements during HIIT - but we don't have any solid research to support this claim.
Bottom line, if you are having difficulty performing at a higher intensity, you have two choices: stick it out and hope it gets better or
HIIT training felt extremely difficult for me while trying to get into ketosis and only mildly improved once my ketone levels increased. Adding a mere 10 to 20g of carbs around high intensity days was life changing. My energy skyrocketed and performance significantly improved. And surprisingly, this didn't significantly effect my ketone production overall.
Workout Recovery on Keto
Another area of fitness where glycogen and carbs have long been celebrated is in recovery. Any wear and tear on your muscles cause them to breakdown and this process can continue long after you've stopped working out - causing you to lose strength, mass and increase overall soreness and inflammation. Glycogen plays a role in decreasing muscle breakdown and supports muscle tissue repair instead (24,25). And carbs help increase insulin which can support delivering and storing more nutrients when you need them to recover.
Your muscle will start to repair themselves and generate some glycogen without carbs, but again this process is much more efficient when carbs are eaten post-exercise (26).
Electrolyte balance is also a concern on keto. Storing carbs requires water (meaning you store more body water) and on the flip side, high protein diets require more water to break down the protein. When cutting carbs out of your diet, you may also lose water weight and mess with your overall hydration, which will definitely mess with recovery. And many electrolytes come from carb foods. Increasing your intake of sodium, potassium, and magnesium in supplement form or through food can help you keep your electrolytes in check and may help reduce cramping and dehydration.
Hydration was an issue for me. Rapid rehydration electrolytes and sugar free sports drinks made me feel a lot better when going keto.
In addition, eating a high-fat meal after a workout actually slows the absorption of key nutrients into your muscles further exacerbating the problem. Instead, in the absence of carbs, recover with plenty of anti-inflammatory foods ( like nutrient dense low carb veggies) and lean proteins over high fat.
Regardless, your recovery is dependent on the type of training you are doing and the research on ketosis for post-exercise benefits is severely lacking, so it's hard to say. You might find yourself feeling a lot sorer when starting a low carb or keto diet and this may or may not improve over time. It could also be the buildup of acid from ketones contributes to feelings of soreness. This is also a commonly reported symptom of "keto flu".
I noticed my post workout soreness was really intense when starting keto. It did improve somewhat over time, but I never felt fully replenished. Feeling more sore than normal also had an impact on future workouts.
One common method worth considering is the use of exogenous ketones. These are ketones available in a supplement form that can be used for energy without requiring you to go on a keto diet. Some research even suggests taking ketones alone can promote ketosis (27,28).
In addition, ketone supplements are thought to increase the rate at which ketosis occurs on a keto diet and promote better energy control for workouts while going through the process.
I used exogenous ketones on my keto diet and found them to be somewhat helpful, especially on moderate intensity days. They seemed to give me an energy boost. However, they did not help my HIIT, were pricey and I am not a fan of relying on any supplement to be able perform at my standard level.
The jury is still out on this one! And it likely depends on the person. Based on the existing research, carbs remain king for certain types of fitness, but that doesn't mean there aren't some benefits to low carb eating for athletes or working out in general.
The potential benefits include:
- Using more fat for fuel, in general, may help improve body composition and help you lose more body fat.
- Fat supplies more abundant energy than carbs alone and tapping into this fuel source can be of great benefit to endurance athletes.
- Becoming more efficient at using fat during training may increase the level at which you are able to train aerobically, supplying more long-lasting energy at higher outputs. Aka you can perform better at a higher intensity, just not max intensity.
The downsides of keto for performance include:
- Low carb diets may impair your ability to build and repair muscle mass.
- You might lose strength and power without adequate glycogen stores
- Your recovery may suffer
It could also be that there is a sweet spot for some the uses carb cutting and fat adaption to increase certain areas of performance, promote a better body composition or weight loss when needed, and still be able to maintain muscle mass, strength, and output.
To reach the happy middle:
- Become more fat adapted without going full ketosis all the time.
- Increase carbs just enough to fuel your higher training needs.
- Time carbs around your training and consider going low carb on non-training days.
- Keep protein intake high to protect muscle mass and support recovery.
How to Become a Fat Adapted Athlete
The process of getting into ketosis is different for everyone and also may not be the right fit for every person. But there are still some notable benefits to becoming more fat adapted as it relates to your training. And it is possible to go in and out of ketosis. It may also be possible that cutting carbs could potentially promote fat utilization without relying on ketosis.
To kick your body into a fat burning mode, try the following:
- Train fasted
- Cut carb intake as low as you can tolerate on aerobic training days (less than 100 grams) or eat just enough carbs to support your high-intensity training days
- Time carbs around your harder workouts
- Increase healthy fat intake on low carb days (within your calorie limit, replace carb calories with fat calories)
- Keep protein intake at least 1g per pound of lean body mass
- Increase fluids and electrolytes
- Supplement with exogenous ketones on low to moderate training days
- Consider going into ketosis and practice go in and out of this state for a few months
Just increasing your overall performance alone can also help you use fat more efficiently. Endurance trained athletes deliver nutrients and oxygen to their muscles more quickly than non-trained individuals, meaning the more highly trained you are, the better you may be able to use fat for fuel - even at a higher output (29).
Starting a Low Carb High Fat Diet
To get started on any carb cutting plan, it might help to ease into it or try carb cycling initially. Working with a sports dietitian or nutritionist can also help. And while macros and calories play a big role, don't forget about nutrition. Vitamins, minerals and healthy food, in general, all play a key role in supporting your fitness.