Intermittent fasting: an update on the weight and metabolic health benefits

It was my second ever podcast episode released way back in February of 2020, but it has proven to be the most popular. It was titled The User’s Guide to Intermittent Fasting. Now, it is time to revisit the topic of intermittent fasting with an update on the clinical evidence for how fasting stacks up for its effects on body weight and metabolic health compared to other ways of eating. And that’s what I’ll be exploring in this episode by profiling the results of a recent review on this topic.

Links referred to in the podcast

Review of intermittent fasting vs calorie restricted and unrestricted diets https://pubmed.ncbi.nlm.nih.gov/35586738

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Intermittent fasting is currently one of the world's most popular health trends. But humans have been fasting for thousands of years for health or spiritual reasons. And each day we ‘break the fast’ with the first meal of the day no matter what time it is eaten. What has made intermittent fasting so popular is its claims about weight loss and other health benefits.

There is nothing complex about intermittent fasting. It is simply the practice of alternating periods of normal eating with extended periods of low to no food intake. Most diets only focus on what to eat. Intermittent fasting is different. It isn’t dogmatic about what foods you should or shouldn’t eat, only when you eat them.

So, let’s explore the different ways that people can fast. There are several different ways to do intermittent fasting but they all share the common theme of splitting the day or week into eating and fasting periods. The different methods only differ by the time spent not eating or eating very little.

Here are some of the popular fasting schedules that you may come across, but the list is by no means exhaustive.

The 16/8 plan involves eating only during an 8-hour window followed by a 16-hour fasting window. So, if you have your last meal at 8 pm, you don’t eat again until midday the next day. This way of fasting is known as time-restricted eating
The very popular 5:2 diet is another way. This involves choosing two days each week where you eat only 2 to 3000 kilojoules. You eat normally the other 5 days of the week
Another way is called the Eat-Stop-Eat method of fasting. Simply fast for a full 24 hours straight one or two days per week
Then there are extended fasts that may go for several days.

There are endless variations to fasting though. Such as 12/12 which is 12 hours of fasting followed by a 12-hour eating window. This is the simplest method as it can mean having your last meal at say 7 pm and then eating again at 7 am the next morning. A very easy way to dip your toe into the world of fasting. You could also do 14/10 which is 14 hours fasting followed by a 10-hour eating window or even 20/4 which is 20 hours fasting followed by a 4-hour eating window. Really, pick whatever time mix you like so long as there are extended times outside of sleeping that you’re not eating.

Which way of fasting is best? There isn’t a clear answer to this. Many people find the 16/8 method to be the simplest and most sustainable method to stick to. Doing something simple like 12/12 is a good way to start. You can see if fasting agrees with you and if so, slowly increase the fasting window from there. And feel free to have some days that you don’t do it to keep things flexible with your lifestyle.

During the fasting times, water is still allowed. Black coffee and tea (but, no sugar) are usually considered okay to have during fasting although some people argue against this. My view is if having a black coffee in the morning can help someone stick to the fast for a few more hours, then that is what matters. During the eating period you should eat normally with a focus on enjoying food and opting for more nutritious choices.

Now onto the main topic of today’s podcast: what sort of health benefits can fasting give someone? Back in my first podcast episode on intermittent fasting which was episode 2, I explored this question. And based on the research at the time, I couldn’t see much evidence that fasting was some next-level diet hack that would shed kilos any better or worse on average than any other approach to eating less. All fasting does is create a calorie deficit and then…..weight loss! Amazing I know.

Most studies I reviewed at the time found only minor differences in weight loss between fasting versus more traditional dieting. Intermittent fasting ‘works’ for weight loss simply because you eat less food. That means intermittent fasting doesn’t appear to be any better or worse than any other diet if weight loss is the goal.

And as for metabolic benefits on things like fasting glucose and blood pressure, these did improve with fasting but were in line with what you would expect to see because of the weight loss.

But several years have passed and more research has been done, so what is the research field saying today? This was the topic of a recent meta-analysis of 43 randomised-controlled trials that looked at the effects of intermittent fasting compared to both calorie-restricted diets or diets that didn’t control calories – what we would call just regular eating. And all sorts of different ways of fasting were included in the review which were alternate-day fasting, intermittent energy restriction, time-restricted feeding, and even dawn-to-dusk fasting during Ramadan. Studies ran for at least 1 month with a median duration of about 3 months.

The calorie-restricted diets included the Mediterranean diet, the DASH diet which is used to help with high blood pressure, and just continuous energy restriction - all used to create an energy deficit to help with weight loss. For calorie unrestricted diets, the participants continued their usual dietary habits. The key outcomes were BMI, waist circumference, fat-free mass, fat mass, insulin resistance, blood glucose, triglycerides and total cholesterol. And I’ll link to the review in the show notes. https://pubmed.ncbi.nlm.nih.gov/35586738

Let’s dive into the results. First, how did fasting compare to unrestricted calorie diets? In this case, intermittent fasting was a clear winner as it produced greater weight loss, a lower BMI, a smaller waist circumference, less fat mass, more fat-free mass, and greater reductions in triglycerides and total cholesterol. Insulin resistance also improved in the fasting groups, but there were no differences in blood glucose or blood pressure between groups. So, clearly in favour of intermittent fasting. But, remember the comparison diet here was people continuing their normal eating habits. Hardly a surprise that following a diet that can cause a calorie deficit because of having fasting periods in it would give a small amount of weight loss and many of the flow-on effects that come from it.

So, what about when you compare fasting with calorie-restricted diets? Here 10 studies could be analysed and the results were….utterly unremarkable. Body weight, blood glucose and blood lipid changes were similar between both groups. The only difference was a greater waist circumference decrease in people following intermittent fasting. Though many of those differences, while not statistically significant, were marginally in favour of intermittent fasting. But even here, it was probably because of just a slightly greater weight loss.

It was telling that over 80 percent of the results section of the paper focussed on the benefits of intermittent fasting over just regular uncontrolled eating, but there was very little to report or talk up when the researchers compared differences between fasting and traditional calorie-restricted diets. So, intermittent fasting is pretty much equal to any other type of calorie-reduced diet. But not superior. Acknowledging of course that everyone’s mileage can vary because all of these clinical trials represent the average of participants.

What this says is that it doesn’t really matter what method a person uses – they all can give similar results with some approaches connecting more with some people than others. And this is the best predictor of any long-term sustainable weight and health benefits from any change in eating – finding something that connects with you that you enjoy doing and works for you. Anyone proclaiming that intermittent fasting is some superior method to achieve this is full of…..empty calories.

So, in summary, if intermittent fasting works for you and meets your goals and you feel better for it, knock yourself out with whatever fasting protocol works for you. But you won’t be seeing me anytime soon proclaiming it as some next-level diet hack that is going to have you shredded for summer.

Other benefits of fasting

There is a growing list of health benefits being linked to fasting. While some of those benefits are flow-on effects attributed to any weight loss seen, there could just be some additional beneficial changes happening at the cellular level. And that introduces the term 'autophagy'. ‘Auto’ means self and ‘phagy’ means eat. So, autophagy is the body's way of cleaning out damaged cells literally by ‘eating itself’.

Autophagy is a natural cellular mechanism by which the cells in your body degrade unnecessary or damaged components within the cell. This is done as a way for the body to regenerate newer, healthier cells. It is happening inside you right now.n times of stress, and when cells are deprived of nutrients, such as what happens during fasting, autophagy is ramped up to provide an alternative source of energy from recycled cellular material. The benefit for your overall health means less cellular junk and a leaner, cleaner and better functioning body. In our modern times of food abundance and ‘three meals a day with snacks in between’, it is rare for this metabolic switch of autophagy to be flipped.

Most of the research work on autophagy is from animals, rather than humans. So, scientists still do not know how long a fast is needed to see a significant uptick in autophagy. So be wary of advice you may hear that you need to fast for ‘x amount of days’ to start autophagy or that a sip of coffee will break your fast and turn off autophagy. No one has the full picture here yet.

One interesting finding is that autophagy seems to decline as we age, so the benefits of fasting could be greater in older people. It makes sense as there has been a lot more time for cellular junk to build up.

Not for everyone

I need to finish today’s podcast by clearly stating that intermittent fasting is not for everyone. If you’re underweight or have a history of disorder eating, intermittent fasting is not for you unless under the supervision of a health professional. Intermittent fasting could be a dangerous triggering factor for someone vulnerable to an eating disorder.

For anyone taking medications to treat diabetes, intermittent fasting should not be done without talking to your doctor first. These medications remove glucose from the blood and without any food, they can do their job too well and put a person at risk of hypoglycaemia.

And if you’re pregnant or breastfeeding, it is best to give intermittent fasting a miss.

Hunger is the main side effect of intermittent fasting. You may also feel weak, and your brain may not perform as well as you're used to. This may only be temporary as your body adapts.

So, let’s wrap all this up. Intermittent fasting can work very well for some people, but it is not a magic bullet to improved health. It is simply one of many lifestyle habits that a person may choose to adopt. For lasting, long-term health benefits, it is best to find an eating pattern that you enjoy and can stick with.

So that’s it for today’s show. You can find the show notes either in the app you’re listening to this podcast on if it supports it, or else head over to my webpage www.thinkingnutrition.com.au and click on the podcast section to find this episode to read the show notes.

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I’m Tim Crowe and you’ve been listening to Thinking Nutrition.

Thinking Nutrition

Intermittent fasting: an update on the weight and metabolic health benefits

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