Fasting versus Carb Restriction: Which Works Better for What Scenarios

Both fasting and carb-restriction appear to operate along similar physiological pathways. Both lower carbs. Both increase fat-adaptation. Both have the potential to get you into ketosis. Both lower insulin and blood sugar.

But is one better than the other? Are there certain scenarios in which an intermittent fasting protocol works better than a low-carb diet, and vice versa?

Let’s find out if the distinction matters.

And what scenarios are most impacted by any difference.

Fat Loss

Ketones, shmetones. Autophagy, shmautophagy. Cognitive decline, shmognitive shmecline. (Shall I keep going?) The number one reason anyone attempts either a carb-restricted diet or intermittent fasting is to lose body fat. We all know it’s true.

Carb restriction works well. That’s been well-documented. Sure, the results get a little fuzzy if you use “low-carb” diets with 35-40% of calories from carbs or enforce calorie-matched control diets, but legitimate ad-libitum low-carb diet studies where people are free to eat what they want find that subjects spontaneously reduce calories and lose body fat faster than with other diets.

Intermittent fasting has also been shown to work. In non-obese patients, alternate day fasting increased fat oxidation and weight loss. In obese patients, alternate day fasting was an effective way to lose weight; dietary adherence remained high throughout. In young overweight women, alternate day fasting was just as effective as caloric restriction at causing weight loss, and adherence to the former was easier than to the latter.

Intermittent fasting and carb-restriction are pathways to easy calorie restriction. Fasting removes the possibility of eating entirely. Carb restriction removes the least satiating macronutrient and increases the most satiating macronutrients. Both diets increase fat burning and, provided you eat adequate protein and lift some heavy things, preserve lean mass.

The trick is sustainability: If fasting makes you unfathomably hungry, it’s probably not going to help you lose weight. Anecdotally, I find that basic carb restriction helps the most people and is the best-tolerated.

Type 2 Diabetes

You just got back from the doctor and you have Type 2 diabetes. Or maybe you have “pre-diabetes.” Perhaps you haven’t been to the doctor yet, but tracking your blood sugar at home reveals some high postprandial numbers. Or maybe you have a strong family history of diabetes, and you’re looking to avoid it manifesting in you. Whatever the reason, you know that you need to make a dietary change.

First and foremost, type 2 diabetes is a type of “carb intolerance.”

Seven subjects with untreated type 2 diabetes either fasted for 3 days or went zero-carb for 3 days. What happened on day 3?

• Overnight fasting glucose went from 196 to 160 (on zero carb) and 127 (fasting).
• 24 hour glucose dropped by 35% (zero carb) and 49% (fasting).
• 24 hour insulin dropped by 48% (zero carb) and 69% (fasting).

Both approaches worked. Fasting worked better, but you can’t just keep fasting indefinitely. At some point, you have to eat something.

A very recent study just came out on the effect of time restricted feeding (a type of IF) on prediabetes. This is also known as a compressed eating window. The compressed eating window in this study was six hours long, and it was an early one—from morning to the mid afternoon. They ate breakfast, skipped dinner. What happened?

The IFers improved insulin sensitivity, lowered fasting insulin, increased pancreatic beta cell function, and reported feeling less hunger at night. They had better blood pressure and lower oxidative stress. What’s most remarkable is they achieved all this despite not losing much weight. In previous IF studies, most of which paid no attention to the time of feeding, the benefits to people with diabetes or prediabetes were almost always dependent on weight loss.

The time of the day the fasting occurs is quite relevant. Skipping breakfast may not have the same effect as skipping dinner. If you’re using IF to treat high blood sugar, prediabetes, or full-blown type 2 diabetes, make sure you track your results and are willing to try fasting during different parts of the day.

Seizures

As far back as Hippocrates, fasting has been used to treat seizures. Ketogenic diets hit the seizure scene back in the early 1900s. Both approaches produce ketones, which appears to be the important factor. Other methods of increasing ketones, like taking supplementary ketones or eating medium chain triglycerides that convert to ketones, also reduce seizures. So, are both IF and low-carb/keto interchangeable when it comes to seizure reduction? A recent study suggests an answer:

Mice were separated into three diet groups. One group ate a ketogenic diet. Another group ate a regular lab diet. The final group combined the regular lab diet with intermittent fasting. After a couple weeks, researchers induced seizures by dosing the mice with a seizure-inducing agent or subjecting them to seizure-inducing electric shocks. Both the ketogenic diet group and the lab diet/IF group experienced relief from seizures in different ways. The keto group resisted the electric shock seizures but was vulnerable to the seizure agent. The lab/IF group resisted the seizure agent but fell prey to the electric shock.

If these results play out in humans, the best approach to combat seizures would be to do both: carb-restriction with intermittent fasting.

However, many seizure patients are children who still have a lot of growing to do. While ketogenic diets have been tested and shown to be safe and beneficial in these populations, regular fasting could have negative effects on growth and development. Best to stick with what’s known and safe. Adults who’ve got all their physical growing out of the way? Have at it.

Athletic Performance

Endurance athletes who aim to maximize their aerobic output and improve glycogen retention should do carb restriction and increase carbs for competitive events. This is known as “train low (carb), race high (carb),” and it’s a great way to teach your body to utilize its own stored body fat for energy for as long as possible during events and hold off on burning lots of glycogen until the last portion of the race. Done correctly, this method allows an athlete to have plenty of gas left in the tank when the rest of the pack is running on fumes.

Higher-intensity athletes who need/want to eat more carbs to replenish the glycogen stores they’re always emptying can’t do that on a carb-restricted diet—by definition. They may opt for a more carb-agnostic form of intermittent fasting. While intermittent fasting may not directly improve athleticism, it can certainly co-exist with it. One popular method of intermittent fasting is the Leangains approach:

• Eat low-carb, higher-fat on rest days. You won’t be burning any glycogen, so there’s no need to eat carbs.
• Eat higher-carb, lower-fat on training days. You’ll be burning through your glycogen, so it’s the perfect time to eat carbs because they’ll go directly to your muscles.
• Fast for 16 hours a day with an 8 hour eating window. Try to put your training right around the time you break your fast.

Low-carbers can always modify their diets to include more carbs with training—sort of a cyclical ketogenic approach—but that ceases to be “strict low-carb.”

Cognitive Performance

One little-known effect of not eating is that it can improve our cognitive function thanks to ghrelin. Most people know ghrelin as a hunger hormone. It makes you want to eat. But ghrelin has other cool effects:

It’s neurotrophic, improving learning and memory.

It increases the dopamine response, potentially increasing the reward of goal achievement.

This makes sense when you think about the environment under which our ghrelin system evolved. Today, hunger means plodding over to the fridge for a snack. It means ordering a vat of chicken tikka masala from the comfort of your smartphone to be delivered to your door. Ghrelin doesn’t have to do much but make us hungry. For most of human history, hunger meant you had to creep through the wilderness, spear or bow or atlatl at the ready, taking care not to step on any twigs or make any sudden movements, following the tracks of your prey. You needed to be cunning, alert, on point, and prepared for anything and everything. Of course the hormone that makes us want to eat also makes us better at thinking and acting.

Low-carb doesn’t have the same effect. For one, you’re eating. The biggest ghrelin response will come from not eating. Two, low-carb meals are bigger reducers of ghrelin than high-carb meals. This probably explains by low-carb is such an effective way to reduce hunger. This doesn’t make carb restriction bad for cognitive function. Becoming a better fat-burner, generating ketone bodies, and not having to snack every two hours or else lose cognitive steam are all great ways to improve output and productivity. It just means you won’t see the same acute effects of a spike in ghrelin that you’d see fasting.

So, which is it?

If you want to lose body fat, control dysfunctional blood sugar responses, get more mental energy during the day, be better at burning fat and saving glycogen during workouts, and/or reduce treatment-resistant seizure activity, you’d be hard pressed to find a better pair of options than low-carb/keto and intermittent fasting.

Start with a baseline of carb restriction—whereby you restrict unnecessary carbohydrates, only consuming the ones you’ll use to fuel high-octane physical pursuits like CrossFit, lactation, and fetus construction—and try skipping a meal or two when you feel up to it. Maybe you never feel up to it. That’s fine.

Maybe you even go the opposite way. You can’t hack restricting carbohydrates, but you have no problem skipping meals on a regular basis.

The key thing is that you achieve extended periods of fat-burning and low insulin. Both IF and carb restriction achieve that.

That’s it for today, folks. Thanks for reading, take care, and leave your thoughts down below!

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