Is it just a case of overlapping interests? Is it because when you stumble upon one big lie perpetrated by the experts—that cutting carbs will give you heart disease and leave your brain starving for energy/you must eat 6-8 small meals a day or else risk “starvation mode” and “slow metabolism”—you start questioning all the other advice they give?
It might be some of that. But a big reason why intermittent fasting and low-carb eating tend to converge is that they are synergistic. Doing one makes the other work better, and vice versa.
What are the benefits? What are the synergies?
Low-carb is easier when you fast.
Fasting is easier when you’re low-carb.
The two inputs support each other. Rather than a vicious cycle, it’s a virtuous one. Fasting promotes fat-adaptation by upregulating fat-burning mitochondria, spurring the creation of new mitochondria, and reducing your reliance on sugar. Fat-adaptation makes going low-carb easier, because you’re really good at burning body fat and don’t get so many sugar cravings. Going low-carb makes you even better at burning fat and builds even more mitochondria, which is a prerequisite for fasting for extended periods of time.
If low-carb is going to work, you have to do it. If fasting is going to work, you have to fast. Anything that makes those easier is going to be good for your health. That’s where it all starts.
A 2013 study compared low-fat dieters on an alternate day fasting schedule with low-carb dieters on an alternate day fasting schedule. Both groups lost weight and improved metabolic health markers, but the low-carbers lost more body fat.
A more recent study putting low-carbers on a fasting schedule for six months saw their body fat drop, lean mass remain stable, and fasting insulin decrease. However, there was no control group and the low-carb diet was still 30% carbs. I think you’d see better results if you dropped those carbs down even lower.
One common criticism lobbed at intermittent fasting enthusiasts is that it has the potential to cause muscle loss. This is a valid point. If you are insufficiently fat-adapted, your glucose requirements will remain elevated when fasting, and you may break down muscle tissue for the amino acids to convert to glucose.
Luckily, ketones spare muscle tissue by reducing your need for glucose. It turns out that a fair number of tissues that would otherwise run on glucose can run on ketones. Being in a ketogenic or low-carb fat-burning state before you fast accentuates this effect. It’s no wonder that the studies mentioned in the last section found that weight loss via low-carb dieting and fasting was entirely from body fat.
Bump up the muscle-sparing effect even more by lifting heavy things (even during the fast).
A major reason low-carb and keto diets work so well for weight loss is that they increase satiety and inadvertently reduce calories. Whereas your average calorie-counter is painstakingly tracking everything he or she eats and expends just to wrangle a few pounds lost, the low-carb dieter often just eats to satiety and lets the weight loss happen.
The average low-carber will fast without even thinking about it. They fast because they simply aren’t hungry and forget to eat. Someone on a higher-carb “regular” diet often must summon superhuman willpower to fast. They fast in spite of being ravenous and can think of nothing else but eating.
Forgetting to eat is a much easier way to fast than willing yourself to do it.
Eating a high-carb meal after fasting for two days will spike your postprandial blood sugar. Eating a low-carb meal after fasting for two days will likely not. Why?
Consider the problem of the long-term low-carber trying to pass a glucose tolerance test. It’s common for long-term low-carbers to “fail” glucose tolerance tests because they’re trying to handle 75 grams of pure glucose with a fat-based metabolism. The body is set up to burn fat and you suddenly introduce a bunch of glucose. It’s hard to do, and most people will fail that—even if they’re healthy.
When you’re coming off a fast, you’re burning fat. You may not have the glucose intolerance of a long-term low-carber, but you are running on fat, and that’s going to raise the chance of exaggerated postprandial blood sugar numbers.
If you do want to eat carbs after a fast, the best way to mitigate this issue is to break the fast with a hard workout and then eat the carbs. You’ll “simulate” glucose tolerance by clearing out glycogen and providing open storage depots for the incoming carbohydrates.
Or you could just be a low-carber to begin with and avoid the problem altogether.
Fasting is a great way to induce caloric deficits and thus lose weight. That’s pretty much why it works so well, as an artificial boundary to control our eating and snacking habits. However, people can tend to go a little wild on the re-feeds. They haven’t eaten all day, so of course they’re going to pig out when they break the fast and eat all sorts of foods they wouldn’t otherwise eat—and eat way more calories than they would have, thereby counteracting all the good they did not eating.
If you’re sticking to low-carb or keto principles, the re-feed is safer. You’re less likely to overeat, because low-carb is so satiating. You’re less likely to eat junk, because the “rules” of the diet eliminate most of the offending foods like chips, sweets, and fried carbohydrates.
When insulin is elevated, fat is locked away in our adipose tissue, making it very difficult to burn fat. Hyperinsulinemia, or chronically elevated insulin, also increases the risk of cancer and Alzheimer’s disease, and elevated insulin levels are linked to atherosclerosis.
Intermittent fasting is a potent antidote to hyperinsulinemia. This recent study found that, despite causing similar reductions in body weight, alternate day fasting led to greater improvements in insulin and insulin resistance than regular caloric restriction.
Low-carb eating is also a potent antidote to hyperinsulinemia. As it turns out, avoiding carbohydrates reduces one’s insulin load.
I’m no enemy of insulin. It serves many valuable purposes, and we wouldn’t be alive without it. But too much insulin at the wrong time causes undeniable problems—and combining IF with low-carb can normalize it.
Fasting while low-carb isn’t the only way to do it. Many people have great success combining high-carb dieting with fasting, provided they also lift weights. One popular (and effective) method is to go high-carb, low-fat on training days, with the workout coming at the tail end of the fast and the first meal coming after the workout. Then on rest days, you break the fast with low-carb, high-fat meals. The training stimulus increases insulin sensitivity and gives all those carbs a place to go (your muscle glycogen stores).
But combining low-carb and intermittent fasting really does increase and accentuate the benefits of both practices.
Thanks for reading, everyone. I’d love to hear about your experiences with intermittent fasting and low-carb eating. Do you find that the two go hand in hand, or have you also had success fasting with a higher-carb diet?
Klempel MC, Kroeger CM, Varady KA. Alternate day fasting (ADF) with a high-fat diet produces similar weight loss and cardio-protection as ADF with a low-fat diet. Metab Clin Exp. 2013;62(1):137-43.
Kalam F, Gabel K, Cienfuegos S, et al. Alternate day fasting combined with a low-carbohydrate diet for weight loss, weight maintenance, and metabolic disease risk reduction. Obes Sci Pract. 2019;5(6):531-539.
Mujaj B, Bos D, Kavousi M, et al. Serum insulin levels are associated with vulnerable plaque components in the carotid artery: the Rotterdam Study. Eur J Endocrinol. 2020;
Gabel K, Kroeger CM, Trepanowski JF, et al. Differential Effects of Alternate-Day Fasting Versus Daily Calorie Restriction on Insulin Resistance. Obesity (Silver Spring). 2019;27(9):1443-1450.