Keto and intermittent fasting do work for weight loss, but only if you restrict calories. But, is it worth it?
According to an internet legend, if you say, “I want to lose weight,” three times in front of the mirror, a gym bro will appear in your house and say, “have you tried keto?”
Keto and intermittent fasting (IF) have taken fitness, nutrition, and weight loss communities by storm. Avid followers of these diets all claim miraculous results, like rapid weight loss, improved cognitive function, and a healthier cardiovascular system.
However, with research finding that restrictive eating practices can make an individual more susceptible to weight gain<sup>1</sup>, the question remains: Are keto and intermittent fasting diets good for weight loss?
Before we dive in, let’s make sure we’re on the same page about these diets.
Keto, or the ketogenic diet, is a low-carb and high-fat diet. The idea with keto is that, with very little energy available from carbohydrates, the body is forced to use fat as a fuel source, which is known as ketosis. Ideally, 70% of your calories come from fat, 20% from protein, and 10% from carbs.
Intermittent Fasting, or IF, involves only eating your meals within a specified window of time. Typically, most people start with fasting for 16 hours and eating without restriction for 8 hours. The 5:2 diet (where you eat with no restrictions for five days and fast for two) is another popular IF method.
So, while keto restricts what you can eat, IF works by restricting when you can eat.
Make no mistake, though—both of these are still restrictive diets. While an IF diet allows you to eat anything you want, you’ll still be restricted on how much you can eat by your metabolism.
Plus, both of these diets still work on the basic principle of consuming fewer calories than you expend if you’re following them for weight loss. So, even though IF is unrestricted, you’ll need to pay attention to what you’re eating, otherwise, you won’t see any weight loss on this diet.
One particularly interesting study found that participants on a keto diet lost more fat around their midsections<sup>2</sup> than a control group on a different nutritional plan. A large amount of visceral belly fat has been linked with inflammation, so losing fat in this area can have other benefits to your health outside of weight loss.
In addition, other studies have found that, while in ketosis, insulin levels are minimalized<sup>3</sup>, helping to improve insulin sensitivity. Not only does this promote fat loss, but it also can reduce the risk of developing diabetes and improve your fasting glucose levels.
With keto being a relatively new diet, it hasn’t been around for long enough for studies to discover if there are any long-term benefits or effects of this nutritional plan.
While keto and weight loss seem to go hand in hand, research also shows that restricting food types often causes people to feel out of control when they are “allowed” those foods again<sup>4</sup>. This can lead to greater weight gain when people stop following the keto diet, or reduced benefits of the diet if you like to use “cheat days” to stay on track.
Changing the primary source of energy from carbohydrates to fat can help you to control your blood sugar levels. However, the keto diet could be potentially dangerous if you have type 1 diabetes<sup>5</sup> and allow your ketone levels to rise above normal.
A research review on the effects of ketosis on type 1 diabetics notes that elevated ketones parallels the impact of high glucose levels in diabetics, who have increased ketone production and decreased ketone clearance. This can be attributed to inherently low insulin levels (insulin is required for ketone clearance), the review says. The health implications of this can include oxidative stress and damage, effects on insulin secretion and resistance, inflammation, and liver dysfunction.
This is rare with type 2 diabetes; however, it can still occur<sup>6</sup>.
With that in mind, if you’re diabetic and considering the keto diet, you need to take extra care in monitoring your blood sugar levels and, ideally, testing your ketone levels, too.
Intermittent fasting has numerous documented health benefits, particularly when it comes to your blood sugar. One study found that periods of fasting can improve glucose metabolism and insulin resistance<sup>7</sup> and reduce your risk of developing type 2 diabetes. However, the study concluded that these effects appeared to be the result of calorie restriction, and not IF itself.
Similarly, intermittent fasting and weight loss aren’t a clear-cut relationship. While you can lose weight following an IF regimen, there are no significant differences between IF and other diets in terms of weight or body fat loss<sup>8</sup>.
Like we mentioned earlier, intermittent fasting works by restricting when you can eat, so naturally, you will eat less during the day.
However, given that you have no restrictions on what you can eat, this diet doesn’t help you to eat healthier foods or improve your diet. It’s entirely possible that you can follow IF and maintain, or even gain, weight. Again, as one previously mentioned study showed, psychological restriction can increase the amount of food you’re driven to eat.
As with keto, IF hasn’t been around for long enough for there to be any meaningful study of its long-term effects on weight loss.
It’s also important to note that, while intermittent fasting has been shown to improve insulin resistance, this diet can make managing your blood sugar particularly difficult. In particular, long periods without food can cause your blood sugar levels to drop too low. When this happens, the liver aims to release a surge of stored glucose.
While this study suggests that intermittent fasting is safe for people with diet-controlled diabetes<sup>9</sup>, it concludes that people who take insulin or other medications should consult a doctor before starting an IF diet and should carefully monitor their blood glucose and ketones.
One of the biggest criticisms of IF is that it doesn’t restrict what you eat, which is why many people pursuing weight loss combine IF with another diet. Keto is a particularly popular choice.
Make no mistake—combining these two diets will work on the simple premise that you’re consuming fewer calories than you’re used to eating.
One study even found that a combined approach of keto and IF helped one woman with type 2 diabetes to control her blood sugars without medication<sup>10</sup> after 14 months. The study found a combined approach helped to reduce her blood glucose levels and lose a small degree of weight.
However, this study only covered one individual. It’s also worth noting that this individual received full medical support alongside her keto and IF diet, so these same results may not be applicable outside of this case study.
Because both diets are particularly restrictive, you might also find it overwhelming to stick to them in combination. You might also find that this high degree of restriction doesn’t help you to learn healthier habits.
Keto and IF can potentially be hazardous if you have diabetes because of how they affect your blood sugar levels. You’re particularly at risk of conditions like DKA or hyperglycemia if you use medication to control your blood sugar levels. So, if you have diabetes and you’re considering keto, IF, or both, speak to your doctor.
If you’re pregnant or breastfeeding, we also don’t recommend that you attempt a keto or IF diet without consulting your doctor. Pregnancy and nursing require a higher calorific intake to sustain yourself and your child, so attempting to reduce that intake can be hazardous for you and your baby.
Similarly, anyone under 18 shouldn’t attempt intermittent fasting or keto, as they have a higher calorie requirement.
Finally, if you’ve got a history of eating disorders or disordered eating in any capacity, we don’t recommend following intermittent fasting or keto. Intermittent fasting, in particular, can trigger overeating and binge eating. Deregulating your appetite by attempting these diets can also trigger a resurgence of previous disordered eating behaviors.
Keto and intermittent fasting do work for weight loss, but it’s important to understand the risks of following these highly restrictive diets, particularly if you need to monitor your glucose levels to help you manage diabetes.
The key to long-term weight loss isn’t just finding a diet that helps you lose weight, but one that teaches you healthy habits to help you keep the weight off. Many diets only focus on helping you achieve short-term results, and many diets aren’t sustainable.
While some people find an IF, keto, or combination diet that works for them, everyone has different nutritional needs. You might find that keto and IF don’t work for you, and that’s perfectly normal. Not everyone can sustain a restrictive diet, and it’s important to find a nutritional plan that works for you and your goals.
As always, if you’re considering starting a new diet for weight loss, consult your doctor first, particularly if you need to monitor your glucose levels for medical reasons.