Which is the best fasting diet for weight loss?

If you’ve read about intermittent fasting (IF) you’ve likely heard people singing its praises as a miracle cure to a whole host of life’s problems. Fans remark ‘you’ll feel wonderful, you’ll have mental clarity and the weight will come off without even feeling hungry!’.

Today we are going to examine what the latest science says when it comes to intermittent fasting specifically for weight loss. Is this a genuinely helpful approach or another fad diet soon to be relegated to the food bin of history. After reading this article you’ll have a much better idea if IF is the right approach for you! You’ll be able to avoid many of the common mistakes people make and understand who should definitely not try IF.

If you’re interested in this topic you're likely looking for a weight loss approach that works for you, or you're wondering if IF has unique health benefits that you shouldn’t miss out on. As you likely know, sticking to a traditional reduced calorie diet long enough to achieve any amount of significant weight loss, can be vey difficult.

Is IF worth considering?

You’ve probably noticed IF diets have become very popular over the past few years. They are often mentioned in social media posts as a one size fits all solution, owing to their simplistic nature, especially when compared to calorie counting. You would be right to assume that there is a lot of hype and unsubstantiated claims when it comes to IF. So let’s find out together if the claims are supported by the evidence. Let's explore the three most popular types of IF you can choose from.

Alternate Day Fasting (ADF)

This approach involves a fast day alternating with a feast day. On fast days you have two options. Either go zero calorie and consume only water or cut energy intake to just 25% of your needs.

The 5:2 Diet

This is a modified version of alternate day fasting which involves two fast days and five feast days. Its up to you if you want to schedule your fast days next to each other or not.

Time Restricted Eating (TRE)

This approach is a little different, in that it requires you to fast everyday for a specified amount of time. If following this plan you would keep all of your eating within a certain window of time. The least restrictive would typically be a 12 hour fast followed by a 12 hour feeding window, we would call this 12:12. The feeding window can be reduced and some popular options include 14:10, 16:8 or 20:4. The most attractive feature of this diet for many is the fact you don’t need to monitor how much you eat during these windows.

Effects on bodyweight

So now we understand some of the different approaches we can take, let’s see how they stack up when it comes to reducing bodyweight. Studies have found that alternate day fasting and the 5:2 diet can result in 4-8% weight loss over the first 12 weeks. After this weight loss typically tends to plateau. Time restricted feeding seems to result in a lower weight loss during this time of just 3-4%, although there are no studies examining this method beyond 12 weeks.

These fasting protocols have proven successful for people living with obesity, helping them to lose approximately 0.2 to 0.5kg per week. Results did not seem to vary when accounting for gender or menopausal status. People living with pre-diabetes and type 2 diabetes also lost similar amounts of weight to those without the conditions.

Weight Maintenance

OK, so let’s say you’ve lost the weight. Now let’s explore how these approaches compare when it comes to keeping the weight off. It appears both ADF and the 5:2 diet can help you maintain your weight loss, when utilising the weight maintenance phase of the diets. This allows you to approximately double your energy intake to 1000-1200kcal on fast days. This is important as utilising this phase may help you stick to this diet long term.

There is currently no evidence examining the effectiveness of TRE for weight maintenance. However, you could likely slightly increase the length of your eating window and assess for weight change to find your acceptable limit. Ultimately it appears fasting is no more effective at eliciting weight loss when compared to traditional dieting, in which you have implemented a calorie restriction of 25% below estimated needs. This result has been observed in people living with obesity or type 2 diabetes.

Body Composition

Another important factor to consider when following a weight loss diet is how it impacts on your body composition. When implementing a calorie deficit it is quite normal for around a quarter of the weight loss to come from lean mass including muscle tissue. Some have suggested that fasting is a superior form of weight loss for holding onto muscle. Unfortunately, it appears fasting results in similar reductions of fat and lean mass as traditional dieting. If you are implementing a calorie deficit, regardless of the diet used to achieve this, we see significant benefits to the amount of muscle tissue maintained if you include some form of resistance training. This does not have to involve pumping iron in the gym. Any form of movement through space that challenges your body should be beneficial.

Energy Intake

The main reason fasting can result in weight loss is due to the calorie deficit it creates. If following one of these plans you could see a reduction in consumed calories by an estimated 10 to 30%. A common objection to fasting diets is that you could make up for this deficit on feast days, as these are generally unrestricted. However, we are starting to learn that this is not the case. Although you may consume above your daily energy need on feast days, you are unlikely to make up for the deficit incurred on fasting days, unless you actively choose to overeat. Especially when we examine your energy requirements over a longer time period.

This observation extends to time restricted eating also, as you will typically not be able to eat the same amount of calories as you did in an unrestricted eating window. This form of fasting can result in an unplanned reduction of consumed calories of 10-30%. The simplicity of this approach allows many people to adhere to the diet for much longer than traditional calorie counting, or alternate day fasting, as there is no requirement to measure your intake. However, the total amount of weight lost with this approach tends to be lower, and may be insufficient to help you reach your goal weight.

So Who Shouldn’t Fast?

I would suggest caution if you have a history of eating disorder or low self esteem, poor body image, disordered eating behaviours or internalised weight bias. Although, to put this in context I would extend these cautions to variety of weight loss approaches and not just fasting. Data suggests that time restricted eating does not increase rates of binge eating, purgative behaviour, depression or fear of becoming overweight. However, studies have not included people with a history of eating disorders in their trials. This means we do not have the evidence to confidently say whether or not fasting is appropriate for people already diagnosed with an eating disorder, or even those at high risk of developing one.

We still don’t have enough evidence to examine fasting’s impact on age related muscle loss. Therefore caution is advised in elderly individuals over the age of 70. If you need to take medication with food you may also find fasting regimes difficult to implement safely. Fasting does not appear to negatively affect people ability to exercise. However, for those interested in maximising their performance nutrient timing plays a significant role and fasting may make this more difficult. As with any calorie restriction diet you should ensure you get at least 50g of protein on fast days to help minimise any muscle breakdown.

Long-Term

When considering the longer term implications of fasting, we still don’t have good evidence to apprise fasting’s ability to help people manage their weight. However, evidence appears to show that fasting does not reduce resting metabolic rate any further than traditional dieting. Therefore, if it works for you and you are confident that you are able to follow one of these approaches long-term, whilst minimising the risk of any nutrient deficiencies, negative impacts on your psychological health and social life then I’d say it’s worth a try.

This blog is based on the review article 'Clinical application of intermittent fasting for weight loss: progress and future directions' from the journal Nature Reviews Endocrinology. You can access the article here: https://www.nature.com/articles/s41574-022-00638-x.