Definitive Guide to Carb Timing and Carb Cycling

June 2, 2020

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carb timing carb refeed The Primal Blueprint is generally considered a low-carb way of eating, especially in contrast to the Standard American Diet and the like. We’re not anti-carb. My Big-Ass Salad is a huge bowl of carbs from vegetables, after all. We’re selective about the sources of our carbs and generally mindful about how many we take in.

Given that, readers always want to know the “right” way to incorporate carbs. Which carb sources? How many? When? How often?

The Primal Blueprint Food Pyramid and Carb Curve provide answers to the first two questions. The latter two… well, those are more complicated.

I’ve written about these topics many times, but the questions keep on coming. Today I’m going to try to condense the main points into one post. I’ll touch on some issues you’ve raised in the comments of recent posts, too.

In truth, I keep getting questions because there are so few definitive answers about the optimal way to incorporate carbs in your diet. Underlying hormonal and metabolic health, activity level, and lifestyle variables to make it impossible to make across-the-board recommendations. Few studies address these issues, and those that do always use standard high-carb diets in their manipulations.

The best I can do is explain the logic behind different strategies and encourage you to experiment. As with so many things, it might take time to discover which strategies work best for you.

Carb Timing, Carb Cycling, and Carb Refeeds, Oh My!

Let’s get some terminology out of the way.

Carb timing refers to when you eat your carbs. Usually this means when during the day, morning versus night, but it can also mean relative to exercise or other activities.
The term carb cycling encompasses various strategies in which you rotate periods of higher- and lower-carb eating. A common carb cycling schedule is eating low-carb for five or six days, then higher-carb for a day or two. Some people eat low-carb for a period of, say, six to eight weeks, then higher-carb for a week or two. Women may also vary their carb intake around their menstrual cycles.
Carb refeeds can be synonymous with carb cycling—the higher-carb days are called “refeed” or “carb-up” days. Carb refeeds can also be less systematic than carb cycling. For example, you might throw in a couple high-carb days because your weight loss has plateaued, or you’re doing some particularly strenuous exercise.

It should go without saying that when I say “carbs,” I mean nutrient-dense, whole-food sources of carbohydrate. I’m talking about sweet potatoes and other root vegetables, in-season fruit, nuts, high-fat dairy, perhaps wild rice and occasional legumes if they work for you. There’s obviously no situation in which I’d tell you to throw back a couple donuts with a soda chaser and call it a refeed.

Why Should You Carb Cycle or Refeed?

The main reason to periodically increase your carbs is to boost your leptin levels. Leptin is an important metabolic hormone that is secreted by adipose cells. Leptin also rises after eating, especially carbohydrates but also protein and maybe fat to a lesser degree.

Leptin’s main job is to signal how much energy is available. When leptin levels fall, the brain understands that we are low on energy. This leads to hunger and energy conservation. Chronically low leptin can interfere with fertility, thyroid and adrenal function, skeletal integrity, and cardiovascular health.

Carb refeeds can “reset” leptin levels. Among other benefits, these bursts of leptin improve insulin sensitivity and may help with weight loss. Sustained caloric deficits lead to decreased metabolic rate—part of the energy conservation adaptation. Refeeds will boost metabolic rate, especially if you also eat more calories along with carbs, and reduce hunger. Plus, occasionally eating higher-carb meals offers a welcome break from constant restriction.

Some signs you might benefit carb cycling/refeeds are:

Excessive hunger
Irregular menstrual cycles
Mood disturbances or depression
Low body fat
Sleep disturbances
Weight-loss plateau
Better adherence to dietary goals with occasional “breaks”

In each of these cases, adding carbs is only one of several dietary strategies you might try. Also consider whether you are eating enough calories, and protein, to meet your needs, and whether you are minding your Ps and Qs when it comes to sleep practices and stress reduction.

People who might not need carb refeeds:

Carry significant body fat or are insulin resistant
Find it easier to adhere to dietary goals when they abstain from higher-carb foods/meals
Use low-carb eating regimens therapeutically (e.g., under medical guidance for epilepsy, cognitive decline, or type 1 diabetes)

This should be obvious, but you also don’t need refeeds if you’re not restricting carbohydrates. What does that mean, though? Anyone who is eating a ketogenic diet is clearly restricting carbs. Beyond that, there is a lot of gray area. An extreme endurance athlete eating 150 or even 200 grams of carb per day could be considered low-carb, and hence they might benefit from cycling in more carbs occasionally. Your best bet is to rely on subjective markers of how you feel.

How to Incorporate Carb Cycling or Refeeds

This depends on your goal. If you’re feeling good without carb refeeds, you probably don’t need them. In that case, you might throw in high-carb meals intuitively or when a special occasion offers the opportunity.

Otherwise, you can be more systematic about it. I’d start small and increase as needed. One meal every other week could suffice. You might find you do better with one high-carb day per week, or one meal every three or four days. It’s not uncommon for women to feel better with a slightly higher carb intake than men, but it’s still highly individual.

For weight-loss stalls, consider eating at maintenance calories for a week or two along with including more carbs than normal. This signals to your body that you are no longer in an energy shortage, so it is safe to reverse some of those energy conserving adaptations.

Premenopausal women can try increasing carbs four to five days post-ovulation (around days 19 and 20 of their cycle) and on the first day or two of their period. Women’s bodies are especially attuned to energy shortages, so these periodic boosts in leptin, timed to coincide with greater demand, can be beneficial.

There are no hard rules about how much to increase your carbs. A good place to start is doubling your normal carb intake and adjusting from there. I’d also recommend dialing back your fat intake with higher-carb meals. Triglycerides prevent leptin from crossing the blood-brain barrier.¹ You don’t have to do zero fat, just don’t pile it on. Cooking with fat shouldn’t be a problem.

Carb Timing

I’ve said before that I think carb timing is relatively low on the hierarchy of things to care about. It’s not as important as what you’re eating or how much. I think it’s also less important than your macros—getting sufficient protein and experimenting with different levels of carb intake.

That said, if you want to experiment with carb timing, go for it. You won’t get a lot of guidance about how to do it, though, at least not from empirical research. I can’t find any studies that systematically vary carb intake, morning versus evening, among people eating anything like a Primal or paleo diet.

The good news, though, is that there isn’t an obviously wrong way to do this. Carb timing is unlikely to be the factor that makes or breaks your health, fitness, or longevity goals. Still, it might move the needle, so let’s get into it.

Rationales for Eating Carbs in the Morning

Insulin Sensitivity is Higher in the Morning

In my estimation, the best argument in favor of eating most of your carbs in the morning is that that’s when you’re most insulin sensitive. It makes sense to eat your carbs at the time your body is best equipped to handle them.

Eating a greater proportion of your carbs in the morning also seems to promote insulin sensitivity.² This effect might be especially pronounced in people who have poor glucose control to start with. In one small study, carb timing didn’t matter for participants who were metabolically healthy. For those with impaired glucose tolerance, eating carbs at night led to unfavorable changes on several makers of glucose tolerance compared to eating their carbs in the morning.³

For what it’s worth, this is also why proponents of chrononutrition advocate for eating more of your total calories in the morning. Doing so, they argue, takes advantage of the natural peak in insulin sensitivity and acts as a zeitgeber to entrain your circadian rhythm.

And yes, I usually skip breakfast myself. I also eat a fairly low-carb, and therefore low-insulin-producing, diet. I’m metabolically healthy. My sleep is top notch. I’m not worried about my glucose tolerance nor my circadian rhythm. Both are in tiptop shape. For me, skipping breakfast feels natural, and I like extending my overnight fast. Since it seems to have no ill effects, I’m sticking with that schedule for now, but I’m open to change.

To “Sleep Low” for Fitness Gains

This one is really about avoiding carbs in the evening more than eating them in the morning per se. Here’s how it works: In the afternoon or evening, do a high-intensity workout to deplete glycogen stores. Do not eat carbohydrates after. In the morning, do a low-intensity session, such as a light jog, then eat breakfast with a balance of carbs, fat, and protein.

The purpose of sleeping low is to force your body to upregulate fat metabolism. Researchers have studied this protocol among elite male triathletes. They compared men who ate carbohydrates spread across three meals to men who ate all their carbs at breakfast and lunch and then “slept low.” Both groups ate the same total amount of carbs and did the same workouts.

In one report, after three weeks of this training, the group that slept low scored significantly better on a test of muscular efficiency. They also performed better on a surpamaximal test—basically pedal until you (almost) puke—and a 10k run in simulated race conditions.⁴ The sleep low group also lost fat but not lean mass. Using the same protocol, researchers showed that after only a week of sleeping low, the men improved their performance on a 20k cycling time trial, whereas the control group showed no improvement.⁵

Reasons to Eat Most of Your Carbs in the Evening

Carbs Affect Sleep

Carbohydrates increase tryptophan production. Tryptophan is a precursor of serotonin, which in turn converts to melatonin. Still with me?

Thus, the theory goes, eating carbs at night will boost melatonin production and, hence, promote sleep. It makes sense, and you can certainly try it, but there’s no concrete evidence it actually works. According to the one tiny study that has examined this effect, your best bet is to eat some high-glycemic carbs four hours before bedtime.⁶ I’d say if you want to try this, you probably don’t need a lot of carbs—maybe half a small sweet potato with dinner, a piece of fruit, or a tablespoon of honey in a mug of herbal tea.

Cortisol Levels Respond to Carb Intake

Here’s the argument: Cortisol naturally rises in the morning as part of the sleep-wake cycle. One of cortisol’s effects is to mobilize stored energy from fat. Insulin opposes the action of cortisol and inhibits the release of fat from adipose tissue. If you eat carbs in the morning, you increase insulin. Therefore, you counteract the desirable high cortisol levels that characterize a healthy circadian rhythm. You also interfere with fat burning.

It makes sense, but the data doesn’t clearly support it. Studies in healthy men⁷ and college students who were or were not stressed before eating⁸ fail to show a marked decrease in cortisol levels following carbohydrate consumption. In fact, when the men in that first study consumed carbs, protein, and fat in isolation, cortisol levels were highest in the carb-only condition. Likewise, when researchers in another study fed women high-protein or high-carb meals, the high-carb meals resulted in higher, not lower, cortisol levels among women with abdominal obesity (the kind linked to metabolic syndrome), but not peripheral obesity.⁹

What does this mean? It is true that if you’re hoping to extend an overnight fast and promote fat burning, then eating a high-carb breakfast that raises insulin will be counter to that goal. If you’re specifically worried that it will tank your cortisol, though, it may not be the case.

However, there is also tremendous variability in individuals’ cortisol responses. Certain people may indeed do better avoiding carbs in the morning. Some practitioners advise individuals with adrenal issues and cortisol dysregulation to eat most of their carbs in the evening instead of the morning.

For Weight Loss?

A lot of people tout this benefit, but there is no real evidence to back it up. There are a couple poorly done studies, and one that showed that participants who ate carbs at dinner instead of lunch lost more weight than those who did the opposite. However, that was because they lost lean tissue in addition to fat.¹⁰

There may be a benefit to eating more of your total calories in the morning, but that’s not about carbs per se.

Morning Carbs May Cause Cravings

This is anecdotal, but I’ve had many people tell me that if they start their day with a high-carb breakfast, even complex carbs, they feel hungrier and more snacky all day. If this is you, by all means listen to your body and avoid carbs in the morning.

Reasons to Consume Carbs around Exercise

There are a couple good reasons to target your carbs around exercise. One is their ergogenic effect—that is, their ability to enhance performance. Now, you know I’m a staunch advocate for becoming a fat-burning beast. I think it should be every athlete’s goal to burn as much fat as possible at all levels of intensity. Heck, I even have a book, Primal Endurance, that is all about helping endurance athletes minimize their reliance on carbs.

Still, there’s no denying that carbs can help you tap into top-end speed and power. I’ve always made room for the strategic use of carbs in training, and especially in racing. Train low, race high is a viable strategy for athletes at all levels. For athletes who are engaged in prolonged high-intensity efforts (not my preference), carbs may well be necessary to deliver their desired performance. Hardcore CrossFitters, for example, usually do better when they use carbs around exercise.

Taking in some carbs before or during intense exercise isn’t the same as eating 200 grams of carbs and sitting on the couch. During exercise, those carbs are utilized quickly for energy. Exercise actually increases the ability of cells to take up glucose from the bloodstream, thanks to a glucose transporter in muscle cells called GLUT4 that doesn’t rely on insulin.¹¹

After exercise, when glycogen stores have been depleted, muscle cells become more insulin sensitive.¹² The carbs you eat then will preferentially go to topping off glycogen.¹³ This only applies if you’ve actually depleted your muscle glycogen, though. A nice long walk, 30-minute bike ride at an aerobic heart rate, or microworkout won’t do it.

What If You’re Trying to Build Muscle?

As a pre-workout, consuming a small amount of carbohydrate, 25 to 30 grams, may be beneficial. You don’t need to worry about having full glycogen tanks, though.¹⁴ And contrary to what that swole dude at the gym might have told you, you don’t need to throw down post-workout carbs to build lean muscle. You should worry more about getting adequate protein in your diet than about how many carbs you’re eating.¹⁵

Should You Eat Carbs Separately from Fat?

This is another one of those beliefs that makes sense on paper but doesn’t quite bear out in practice. The idea here is that when you eat carbs and fat together, the carbs raise insulin, which unlocks fat cells, which allows the fat you just ate to be easily shoved inside. In other words, carbs + fat = weight gain.

It can work like that, but it doesn’t have to. This is a much bigger concern if you are eating an excess of calories. If you’re consuming more energy than your body actually needs, and you’re potentiating the fat storage process, then yeah, you’ll end up storing body fat.

If you’re not consuming more energy than you need, you don’t need to worry. For example, in one study, two groups of patients ate a hypocaloric diet where carbs and fat were eaten either separately or together for six weeks. Both groups lost similar amounts of body fat and showed comparable reductions in plasma glucose and triglycerides. ¹⁶

On the other hand, proponents of eating carbs and fat together will tell you that eating fat alongside carbs is desirable because fat blunts the glycemic response. However, the empirical data here are inconsistent and, frankly, confusing. I’m not hanging my hat on this effect.

What’s clearly true is that carbs + fat = delicious. It’s easier to overeat the combo of the two than to overeat either one by itself. In that sense, you might want to watch your consumption of mixed meals if you’re trying to lose weight. At least be mindful of the total caloric load.

Conclusions

When it comes to both carb cycling and carb timing, there is no one-size-fits-most strategy. As with most things we talk about here, you’re going to have to experiment to see what works for you. Hopefully this post has given you some ideas. To summarize the main points:

I think there is fairly decent evidence that carb cycling and/or carb refeeds may benefit you if you typically eat a low-carb diet, especially if it’s also calorie (energy) restricted. You don’t have to, though, especially if you’re feeling good.

Personally, I’m a bigger fan of listening to my body and allowing my carb intake to vary according to my circumstances and intuition rather than adhering to a particular schedule. Your mileage may vary. Premenopausal women in particular may do well to consider being more intentional about it. If you’re going to try incorporating carb cycling, the exact strategy you should try depends on what you hope to accomplish.

As for carb timing, morning versus night, I’m not convinced that it matters a whole lot for most people. If you have chronically low cortisol, or your daily cortisol rhythm is out of alignment, you might do well to consume most of your carbs later in the day. If you’re struggling with insulin resistance, try eating breakfast and including some carbs. For sleep issues, experiment with adding some carbs in the few hours before bedtime.

If you’re using carb cycling or carb timing strategies in the service of a longer term goal—losing weight, sleeping better, improving glucose tolerance—pick a strategy and stick to it for at least a few weeks if not months unless it’s clearly not working for you. If your first experiment doesn’t work, you can always try tweaking the timing, types, and/or amount of carbs you’re eating. Be patient.

For some people, carb cycling or carb timing turns out to be the key to resolving a persistent health issue. For others, they have more of a fine-tuning effect. Still for others, they make no obvious difference.

Are they worth trying? Absolutely, if you want. I’m definitely sold on targeting your carbs around exercise; that’s a clear yes in my book. Even then, though, I like to mix up my fueling to challenge my body. Sometimes I eat right after a workout, sometimes I wait. Sometimes I go out fasted, other times I eat beforehand. That’s how I roll.

How do you roll? Are you gung-ho about a particular carb strategy? Has changing up the timing of your carb intake led to any profound shifts in your health? If yes, I want to hear about it in the comments.