This is a fairly loaded topic, depending on the outcome you are looking for. Do you want to lose weight? Or are you looking to improve your health? Are you wanting a quick fix, or looking for more long term results?
Most people consider intermittent fasting to lose weight, but there are some other (lesser known benefits) that science is starting to uncover.
Firstly, let's take a look at what intermittent fasting actually is. Intermittent fasting is where you alternate between normal eating and eating a limited amount of calories (or nothing at all). Some people will fast for days on end, while others will fast every other day e.g 5:2 diet is where you eat normally for 5 days of the week then have a very low calorie intake on 2 days of the week. There are lots of different ways to do it, with the overall goal of reducing total calorie intake. For people who like the all or nothing approach, this type of dieting can be effective for weight loss (any diet leading to a calorie deficit will achieve this). What has to be considered is how will this work long term? Once you've reached your weight goal, what do you do? Whilst effective in the short term, intermittent fasting is not necessarily something you want to be doing for the rest of your life.
Is intermittent fasting more effective for weight loss than just reducing your calorie intake each day?
It's important to work with a registered nutritionist or a dietitian if you are wanting to use intermittent fasting as a weight loss approach. Any form of restrictive diet increases the risk of nutrient deficiencies, so it's important to work with a food expert to do it safely and also to come up with a long term plan to keep the weight off. You may even decide that intermittent fasting isn't right for you. For example, if you're the kind of person who gets hangry with no food, has ever suffered from an eating disorder, or is pregnant or planning pregnancy, intermittant fasting is probably not the safest approach for you. It's good to get the opinion of a registered nutritionist or dietitian who can guide you in the right direction.
Within the context of intermittent fasting, there is also something called time-restricted feeding. The aim is not necessarily to reduce calorie intake, but to restrict the time in which you consume those alories. For example, the 16:8 diet is where you eat all your usual calories within an 8 hour window (e.g 7am to 3pm) and usually nothing but water outside of these times. Most time-restricted feeding is done on an earlier schedule, aligned with our sleep cycle (circadian rhythm). There is some evidence that eating in alignment with your sleep cycle has it's own benefits, which is a super interesting topic that I'll cover in another blog. There is some interesting science behind time-restricted feeding and it's effectiveness as a weight loss strategy. One study (1) found that when people undertake time-restricted feeding without specifically trying to maintain the same calorie intake, it seems that calorie restriction occurs naturally anyway (around 300 calories per day). However, it was also found that the dropout rate was high at 26% over the 12 week study, indicating that sticking to the diet could be challenging.
A systematic review (2) found that in some controlled studies, people lost more weight on a time-restricted feeding regime than people who did not have restricted eating hours, even if they consumed the same number of calories. Across a number of studies where the effects of Ramadan time-restricted eating practices were observed, it was unclear if the observed weight loss was the result of fewer calories consumed due to eating time restrictions, or through another mechanism. It was also noted that weight loss came from a reduction in muscle mass and/or fat mass combined (not just fat by itself). This means that while time-restricted feeding could potentially be a good tool for weight loss even without having to restrict calories, it's not yet known if it's better or worse at preserving muscle than standard calorie restriction approaches.
On an even more interesting note (and outside of weight loss entirely) time-restricted feeding could help improve health measures for people with prediabetes and type 2 diabetes. In a small early time-trestricted feeding study (3) involving just 8 men with prediabetes, it was found that despite none of the participants losing weight, their 6 hour time-restricted eating window lowered insulin levels and improved insulin sensitivity, as well as lowering blood pressure, compared to a 12 hour eating window. Lowering insulin levels can also assist with weight loss, which is why this is a double-benefit for these participants. The men also reported a decreased appetite in the evenings. They had the same calorie intake across both trials. This was a very small scale study and only in men, so we aren't quite ready to start recommending intermittent fasting for all prediabetic clients just yet. However, it's an exciting window into the potential health benefits of intermitteant fasting which I will be certainly keeping my eye on.
Finally, a 2021 systematic review (4) concludes that intermittent fasting can be effective for helping to manage type 2 diabetes. However, they have yet to separate the benefits (improved fasting glucose and insulin levels) from the weight loss that occurs from intermittent fasting. Otherwise, there would be no reason for health professionals to recommend intermittent fasting over a standard healthy meal plan, which tends to be more sustainable for clients.
Overall, if you have prediabetes or type 2 diabetes (especially so for people on medications), it's very important that you do not undertake any form of restricted eating unless you are working with a health professional such as a doctor or dietitian. If you're interested in knowing if intermittent fasting or time-restricted feeding could work for you, check out my online dietitian services and get in touch if you have any questions.