Carb cycling on a low-carb or keto diet: What you need to know
What exactly is carb cycling? How do you do it? Who should, or should not, consider carb cycling? Read on to learn what the science and key experts in the field have to say, and make sure you read to the end to learn practical tips about how it might work for you.
It’s important to note that not much evidence exists about the effects of carb cycling while following a baseline low-carb diet. This guide attempts to combine information from existing data with expert clinical experience to provide the best, most evidence-informed resource on the topic.
What is carb cycling?
Carb cycling is any planned temporary or cyclical increase or decrease in carbohydrate intake. While carb cycling can apply to any type of diet, even high-carb diets, this guide focuses on carb cycling for people following low-carb or keto diets.
At first, it may seem counterintuitive. If someone adheres to a low-carb diet to improve some aspect of their health, why would they purposefully increase carbs?
Perhaps not everyone should. Some individuals may find carb cycling causes more harm than benefit.
However, others may see potential benefits from carb cycling, such as improved athletic performance by increasing carbs before a competition.
Others may achieve better long-term low-carb compliance by cycling carbs one or two days per week or a couple of weeks every few months. And still, others may want to cycle carb intake seasonally to mimic presumed ancestral evolutionary patterns.
Who can benefit from carb cycling?
Athletes are the individuals most likely to benefit from carb cycling. But as we will discuss later in this article, the science regarding this issue is all over the map.
Another type of person who may benefit is someone who sees a lifetime of limiting fruit, bread, pizza, and other favorite high-carb foods as depressing and a significant barrier to committing to a low-carb diet. Knowing they can temporarily return to their higher-carb favorites for a week every couple of months might increase their long-term chances of success.
Yet, people who don’t do moderation well may find the opposite to be true, so it is essential to know how you will respond. Will relaxing your carb restriction trigger unchecked cravings or binges? We address this further in our section about who should not try carb cycling.
Lastly, some may want to mimic our ancestors’ dietary patterns as closely as possible.1 As Seattle physician, Dr. Ted Naiman says: “I think that with the exception of the past few hundred human generations, our species – particularly those living at higher latitudes – has always been on a cyclical ketogenic diet and I think it makes a lot of sense to cycle carbs if looking at diet through an evolutionary lens.”2
Although data about what our higher-latitude ancestors ate are imperfect, it makes sense that many would have eaten a very low-carb diet consisting mostly of game and meat during the winter months when vegetation was less plentiful. During spring and summer, they would have eaten a higher-carb diet with the seasonal availability of berries, tubers, and other plant species.3
Today’s society has erased any need to eat seasonally. You can get berries shipped to Alaska from South America in the dead of winter, and hydroponic gardens can even redefine the definition of “eating locally.”
Some believe that eating very low carb or being in ketosis year-round may not be healthy. While there are no data to prove if this is the case or not, if you believe we should resort to evolutionarily consistent eating patterns, you may want to consider increasing carb intake during the late spring, summer, and early fall.
Who should not try carb cycling?
If you are just getting started on a low-carb diet, you probably shouldn’t experiment with carb cycling just yet.
As San Diego-based physician Dr. Brian Lenzkes said, “I do not recommend carb cycling for those starting out on a low-carb journey. Many have true carb addiction and cycling can lead them down the wrong path.”4
If there are any signs of intense carbohydrate cravings, or difficulty controlling cravings, then introducing carbs can be a slippery slope from which it is difficult to recover.
Some may have trouble maintaining the quality of carbs they eat, immediately being drawn to high-sugar, highly processed carbs. And others may have a hard time weaning off the carbs after the specified period, regardless of the carbs’ quality.
Diet Doctor Medical Review Board member Dr. Michael Mindrum agrees. He states, “Often when patients want to take a break from a low-carb diet, they aren’t wanting to indulge in lentils and beans but are missing potato chips, ice cream, and other triggering foods. If these foods are made part of one’s routine under the cloak of ‘carb cycling,’ it can lead to further challenges in adherence.”
Addressing your risk of cravings takes top priority before considering carb cycling.
Also, some people feel much worse when adding carbs back into their diet. It is not uncommon for people to feel an immediate return of many of the symptoms that low carb helped them eliminate.5
Feelings of hunger, mental fog, low energy, bloating, and more may occur in some people after they reintroduce carbs. For these individuals, any theoretical benefits from carbs are unlikely to outweigh the physical side effects.6
Lastly, it is always helpful to assess the reasons for going low carb. Was it to lose weight? Was it to treat type 2 diabetes, PCOS, metabolic syndrome, or other health conditions? If so, monitoring markers for your underlying health condition is an essential aspect of whether you should do carb cycling.
For instance, if you got your blood sugar readings under good control with a low-carb diet, make sure they stay under reasonable control when experimenting with carb cycling. You won’t be doing yourself any favors if you find your blood sugar readings are back in the diabetes range, undoing all the gains you achieved by following a more stringent low-carb diet.
Can carb cycling impact my health?
Not many quality studies address the health benefits or concerns with carb-cycling, However, we can try to pull lessons from the data that do exist.
With respect to the impact of suddenly increasing dietary carbohydrate, one study found an increase in an esoteric marker of blood vessel damage when low-carb eaters drank a 75-gram glucose solution.7
As we detailed in a prior post, this study has serious weaknesses. The most relevant criticism of trying to extrapolate these findings to carb cycling, however, is that a 75-gram glucose solution is likely quite different from what a person cycling carbs would actually eat.
In other words, a non-physiologic stimulus may not be the best way to predict the health effects of a short-term increase in higher-carbohydrate foods.
In addition, it has long been recognized that eating less than 150 grams of carbohydrate per day in the days to weeks preceding an oral glucose tolerance test will often result in higher blood sugars after the 75-gram glucose load. As such, experts in the field have long recommended against using the test under these conditions.8
Moving on to other health effects, some have concerns that chronic carbohydrate reduction can adversely affect thyroid function, especially for women. Therefore, we should address the question of whether a temporary increase in carb intake is necessary for thyroid health.
One study found a diet low in calories and carbohydrates caused a more rapid reduction — of greater magnitude — in T3 hormone production than a higher-carb diet also low in calories.9
Another study found that subjects eating 100% fat showed a large drop in T3 levels.10
Yet another found significant thyroid hormone changes in kids being treated with ketogenic diets for epilepsy.11
However, none of these studies documented clinically symptomatic hypothyroidism. Therefore, in the absence of symptoms and the absence of clinically relevant abnormalities of other thyroid function tests, it is not clear if the decrease in T3 is at all meaningful. Further, these studies did not examine carb cycling, specifically, so their findings are not really extrapolatable to this subject.
The takeaway: we do not believe that carb cycling is necessary to maintain adequate thyroid function.
Can carb cycling have positive health benefits? Perhaps. However, no scientific studies have been conducted to answer this question.
Carb cycling for athletic performance
Carb cycling for athletic performance may look different depending on whether an athlete’s goals are competing on an elite level, competing with friends on the weekend, or just striving for a personal best.
Carb cycling may also be different if you are most concerned about body composition, endurance sports (e.g., long-distance running or cycling), or sports with short bursts of activities (e.g., tennis or martial arts).
Before we jump into talking about carb cycling, the first question we need to ask is: do athletes need to eat more carbs?
There is a fair amount of evidence showing that athletes on a low-carb diet can efficiently burn fat for fuel (i.e. they become “fat adapted”) and improve both their body composition and exercise performance without carbs.12
Other studies show that in the short term, low-carb diets can hurt athletic performance and bodybuilding results.13
A still greater number of studies have shown no effect of a low-carb diet on performance at all.14
Although many studies – even the negative ones – have shown changes in certain metabolic parameters like increased fat oxidation, these apparent advantages to a low-carb diet have not always translated into better performance.15
With respect to carb cycling for athletic performance, there is mechanistic evidence for how this strategy might be beneficial.
Consuming a low-carb, high-fat diet (LCHF) during training increases the availability of fat to use for energy, in the form of increased intramuscular triglyceride stores and plasma free fatty acids. At the same time, muscle glycogen stores are depleted due to the lack of significant dietary carbohydrate.
Because there is so much more fat available than carbohydrate, the body increasingly relies on fat oxidation (instead of carb oxidation) for energy production during submaximal exercise. This is what is meant by “becoming fat-adapted.”
In an effort to prolong the time to muscle fatigue during competitive exercise, loading up with carbohydrates prior to the event is thought to replenish muscle glycogen stores, while not significantly changing the body’s adaptation to increased fat oxidation. Therefore, a fat-adapted athlete should be able to efficiently burn fat during exercise, tapping into muscle glycogen stores only when needed (likely during the most intense intervals of exercise).16
Although the mechanistic data are sound, clinical trials of carb loading before exercise have shown mixed results.17
And, finally, the length of the period of fat-adaptation is also important. While some studies suggest it takes around 4 weeks to adapt, some athletes report that it can take up to 6 months to properly adapt for optimal performance.18
Nonetheless, some people do find a performance benefit with carb cycling, which has led to the “train low, compete high” model. Simply put, that means eating minimal carbohydrates while training and increasing carb intake prior to competition.19
How do we reconcile all the conflicting information? Here’s a summary:
- Becoming fat-adapted may be likely to show benefits in endurance sports more so than shorter, more intense sports.
- Full performance adaptation to fat-burning may take more than six months.
- Carb cycling may help some perform better, especially when they train with few carbs and then increase carb consumption for competitions.
- Carbs may help athletes who require shorter, more intense bursts of energy.
If your livelihood depends on a few seconds of improvement, you may very well need carbs before competing.
But suppose your livelihood doesn’t depend on your athletic performance.
In that case, after a period of proper fat adaptation, it’s unlikely that a low-carb diet will have a dramatic negative effect on your subjective sense of performance during exercise. If it does, consider adding in carbohydrates before or during exercise, experimenting with the timing and amount until you find what works for you.
As long as intense cravings and food binging are not problems for you, don’t be afraid to experiment with adding or subtracting carbs to see how you physically perform and how your blood sugar responds.
How to test carb tolerance
There is no clear definition of what it means to be “carb tolerant,” so there is no best way to test for it. However, we can define some parameters that may suggest someone can tolerate carbs, and we can then describe ways to measure that response.
Blood sugar and insulin response to eating carbs are two potential metrics. You can test your glucose tolerance with an oral glucose tolerance test (OGTT).20
Even better is the Kraft test, which is an OGTT measuring both glucose and insulin levels. As Dr. Kraft showed, abnormally high insulin responses often occur years before abnormally high glucose responses, thereby allowing us to identify those at risk much sooner than waiting for blood sugar to climb too high.21 In general, testing insulin levels is more expensive than a simple blood sugar test and is not done as frequently.
However, as we previously mentioned, eating less than 150 grams of carbohydrate per day in the days to weeks preceding an OGTT will often result in higher blood sugars after the 75-gram glucose load. As such, experts in the field recommend against using this test if you are eating a low-carb diet.22
For many, a better measurement may be a “real-world” tolerance test. You likely aren’t going to be drinking an overly concentrated, synthetic glucose drink in your daily life, so why should you test your response to it?
Instead, you can test your tolerance to sushi, sweet potato, or even margaritas and cookies (as mentioned by Dr. Peter Attia in the Diet Doctor Podcast). While you can’t measure insulin levels at home, you can measure your glucose response to real-world foods.
The best way to do this is first to measure your blood sugar before you eat. This is time zero. Then eat the food you want to test. Check your blood sugar at one and then two hours after eating. If, after two hours, your blood sugar hasn’t returned to your pre-meal baseline, continue checking at hourly intervals until it does.
Of course, the more frequently you test, the more information you will get. You can measure your blood sugar every 15 minutes if you would like and see the shape of its rise and fall in more detail. (But that is a lot of finger sticks!)
This is where a continuous glucose meter, or CGM, comes in. With a CGM, every food and every meal is its own tolerance test. All you have to do is pay attention to the readings before and after the meal.
Interpreting your glucose response
The general guidelines for interpreting your glucose responses are:
- The lower the maximal rise and the shorter the total duration of the rise are both signs of a healthier response.
- A “normal” 1-2 hour blood sugar response to food is defined as being below 140 mg/dL (7.8 mmol/L); prediabetes is between 140 and 200 mg/dL (7.8 to 11 mmol/L), and type 2 diabetes is a blood glucose above 200mg/dL (11.1 mmol/L).23
- Shooting for lower blood glucose is always reasonable. Just because 140 mg/dL (7.8mmol/L) is defined as normal doesn’t mean you should be happy with it. As Dr. Casey Means discussed in the Diet Doctor Podcast, it is likely best to aim for even lower values, closer to 120mg/dL (6.6mmol/L).24
- Research suggests that in the absence of diabetes, the highest post-meal glucose peak should be within one hour, but in those with type 2 diabetes, it usually peaks at two hours.25
Different protocols for carb cycling and best carbs to use when cycling
Cycling carbs does not mean eating any kind of carbs you want. The quality of the carbs still matters, and we highly recommend focusing on less processed, more complex carbs.
Examples include vegetables (both above-ground and root veggies), nuts and seeds, lower glycemic fruit, beans, lentils and other legumes. If you want to add grains, we recommend whole grains and so-called “ancient grains,” such as millet, amaranth, or barley, as these are less likely to be highly processed and refined.
There is no best protocol for cycling carbs, as the schedule depends on your goals.
Seasonal cycling is likely the most straightforward protocol to understand. It can be as simple as this:26
|October to March:||20 grams of carbs per day|
|April to September:||100 grams of carbs per day|
This pattern may more closely match the seasonal availability of game, berries, and vegetation that our ancestors may have encountered.
Another popular protocol is adjusting carb intake according to your athletic or training schedule. An example could be something like the following:
|Exercise type||Amount of carbs|
|Monday:||High volume, high-intensity training||150 grams|
|Tuesday:||Moderate intensity training||100 grams|
|Wednesday:||Rest day||20 grams|
|Thursday:||Moderate intensity training||100 grams|
|Friday:||Rest day||20 grams|
|Saturday:||High volume, high-intensity training||150 grams|
|Sunday:||Rest day||20 grams|
Or, for those who are physically active but not training or competing, it could look like the following:
|Exercise type||Amount of carbs|
|Monday:||Long walk in the morning||50 grams, consumed mostly in the morning27|
|Tuesday:||Moderate walk day||20 grams|
|Wednesday:||Resistance training in the afternoon||50 grams, consumed mostly in the afternoon|
|Thursday:||Rest day||20 grams|
|Friday:||Long walk in the morning||50 grams, consumed mostly in the morning|
|Saturday:||Fun day with family and friends||20 grams|
|Sunday:||Resistance training in morning||50 grams, consumed mostly in the morning|
To help guide you even further, here is an example of a 100-gram carb day exercising in the morning:
Or if you prefer to not snack or have dessert, you can add any of the following to your meals:
|1 cup Edamame:||15 grams of carbs|
|1 cup sweet potato:||27 grams of carbs|
|1 cup parsnip:||24 grams of carbs|
|1 cup lentils:||40 grams of carbs|
And an example of a 50-gram carb day.
|Breakfast:||Low carb banana blueberry pancakes||18 grams of carbs|
|Lunch:||Butternut pasta with tomato sauce and garlic mushrooms||19 grams of carbs|
|Dinner:||Low carb shepherds pie||15 grams of carbs|
Feel free to explore our more than 1,000 recipes to mix and match and find the meals that work for you!
Just remember, if you have consistently followed a ketogenic diet and you plan to test your ability to carb cycle, your results may change over time as you eat more carbs. In other words, your blood sugar response may be higher the first day you add in carbs and may improve over three or more days as your body adapts to responding to the carbs.
Also, if you plan to test with an oral glucose tolerance test (OGTT), this test is not reflective of what someone on a low-carb diet is going to eat in real life and has not been validated in a low-carb population. That’s why we recommend testing with real-world carbs as you would normally eat them. If blood sugars are rising significantly, then maybe carb cycling isn’t right for you.28
Is carb cycling right for you? That depends on your goals.
It may help some with endurance athletic performance, and it may help others with long-term compliance with a healthier diet.
Others who do better with consistency or who are more concerned with controlling their blood sugar may not benefit as much.
The most important place to start is defining your goals — and consider experimenting to see what works best for you.
Just remember, the quality of the carbs you eat still matters, so continue to focus on minimally processed, whole-food carb sources.
The science of carb cycling while following a baseline low-carb diet is still in its infancy. The key is to understand your goals and monitor your progress./ Dr. Bret Scher
Carb cycling on a low-carb or keto diet: What you need to know - the evidence
The guide contains scientific references. You can find these in the notes throughout the text, and click the links to read the peer-reviewed scientific papers. When appropriate we include a grading of the strength of the evidence, with a link to our policy on this. Our evidence-based guides are updated at least once per year to reflect and reference the latest science on the topic.
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Some experts believe that the explosion of diseases like type 2 diabetes and cardiovascular disease are largely related to recent changes in the human diet, brought about by modern agriculture and animal husbandry. The argument goes that we are not genetically adapted to eat so much grain, milk, and domesticated meat. Here is an article describing this position.
As mentioned earlier, the science on evolutionary nutritional patterns is flawed. Here is a talk by Amber O’Hearn addressing some of these studies and concepts: Learning about ketosis through evolutionary studies
This is based on consistent clinical experience of low-carb practitioners. [weak evidence]
The topic of food addiction, and specifically carbohydrate addiction, is an evolving field that suffers from the lack of a uniform definition. The following is a review of the topic.
Nutrients 2019: Short-term low-carbohydrate high-fat diet in healthy young males renders the endothelium susceptible to hyperglycemia-induced damage, an exploratory analysis
[nonrandomized study, weak evidence] ↩
Subjects in that study eating 50% carbs and 50% fat showed a drop in T3 that did not reach statistical significance.
Journal of Pediatric Endocrinology and Metabolism 2017: Changes of thyroid hormonal status in patients receiving ketogenic diet due to intractable epilepsy [nonrandomized study, weak evidence] ↩
The following RCT reported 10 days of LCHF diet prior to carb-loading improved fat oxidation and exercise performance.
International Journal of Sport Nutrition and Exercise Metabolism 2001: High-fat diet versus habitual diet prior to carbohydrate loading: effects of exercise metabolism and cycling performance[randomized trial; moderate evidence]
The following study showed that 12 weeks of keto-adaptation helped exercise performance and body composition.
And the following three-month pilot study of CrossFit athletes found no significant change in performance when on a keto diet vs a normal diet.
Sports 2018: The three-month effects of a ketogenic diet on body composition, blood parameters, and performance metrics in CrossFit trainees: A pilot study [randomized trial; moderate evidence] ↩
Two studies from Dr. Burke and colleagues showed reduced 30-kilometer race walking performance after 12 weeks on a keto diet.
Journal of Physiology 2017: Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walker [randomized trial; moderate evidence]
PLOS One 2020:Crisis of confidence averted: Impairment of exercise economy and performance in elite race walkers by ketogenic low carbohydrate, high fat (LCHF) diet is reproducible [randomized trial; moderate evidence]
Journal of Sports Medicine and Physical Fitness 2019:
Low-carbohydrate, ketogenic diet impairs anaerobic exercise performance in exercise-trained women and men: a randomized-sequence crossover trial[randomized trial; moderate evidence]
Journal of the International Society of Sports Nutrition 2017: Ketogenic diet benefits body composition and well-being but not performance in a pilot case study of New Zealand endurance athletes[non-controlled study; weak evidence]
This RCT of bodybuilders found that keto was helpful for “leaning out” (losing fat mass), but the standard Western diet was better for building muscle:
Nutrients 2021: Effects of two months of very low carbohydrate ketogenic diet on body composition, muscle strength, muscle area, and blood parameters in competitive natural body builders[randomized trial; moderate evidence]
And this meta-analysis found that, while keto diets plus resistance training were beneficial for reducing weight and fat mass, the training was not able to offset the tendency of keto diets to result in muscle mass loss, as well:
Critical Reviews in Food Science and Nutrition 2021: Effects of resistance training combined with a ketogenic diet on body composition: a systematic review and meta-analysis [systematic review of randomized trials; strong evidence]
There are too many neutral individual studies to list here, but here are two systematic reviews of the literature.
A 2021 meta-analysis of RCTs found that LCHF diets had essentially no meaningful effect on measures of aerobic capacity and exercise performance in endurance athletes.
Nutrients 2021: The effect of a ketogenic low-carbohydrate, high-fat diet on aerobic capacity and exercise performance in endurance athletes: a systematic review and meta-analysis [systematic review of randomized trials; strong evidence]
Another 2021 systematic review of randomized and non-randomized trials found that keto diets are unlikely to have major positive or negative effects on performance.
One proposed explanation is the decreased oxygen efficiency of burning fat compared to burning carbohydrate. In other words, carbohydrate yields more energy per liter of oxygen compared with fat. So, fat-adapted athletes on a low-carb diet may be better at using fatty acids to power their muscles, but this benefit may be offset by the need to consume more oxygen to generate this power. This may result in a net negative or neutral effect on performance, compared to their higher-carb peers.
The following paper reviews how MCT oil plus carbs slows depletion of glycogen in fat-adapted individuals (those whose bodies have learned to preferentially burn fat for fuel over carbs.
And the following mechanistic study shows that fat-burning muscles can maintain that physiology for a few days even after eating more carbs.
As discussed earlier, one reason for the mixed results may be that it is more metabolically “expensive” to oxidize fat than carbs during exercise, because it requires more oxygen per unit of fat. The type and intensity of exercise also likely play a role, given that carbohydrate availability seems to be even more important for shorter, intense bursts of activity.
PLOS One 2020:Crisis of confidence averted: Impairment of exercise economy and performance in elite race walkers by ketogenic low carbohydrate, high fat (LCHF) diet is reproducible [randomized trial; moderate evidence]
Interntaional Journal of Sport Nutrition and Exercise Metabolism 2001: High-fat diet versus habitual diet prior to carbohydrate loading: effects of exercise metabolism and cycling performance[randomized trial; moderate evidence] ↩
An oral glucose tolerance test, or OGTT, involves a fasting blood sugar test. You then drink a 75-gram carbohydrate drink and repeat glucose testing at one and two hours to see how high your blood sugar climbs. ↩
The following studies reported healthy adults spent less than 1% of their time at levels above 140 mg/dL (7.8 mmol/L) and most of their time below 120 mg/dL (6.7 mmol/L).
We are not aware of any scientific studies examining seasonal cycling, and the concept is based on assumptions and hypothesis.
The idea is to focus on seasonally available foods and spend six months in ketosis and six months out of ketosis. You can see our visual guide showing what 20 grams and 50 grams of carbs look like on your plate. ↩
It may be better to eat your carbs shortly before or after your physical exertion since that is when your metabolism is most active and you are more likely to use the carbs for fuel rather than storing them for later use. ↩