• Danielle Stephens

Intermittent Fasting & Metabolic Health

Updated: Jan 14, 2021

Multiple books, blogs and social media posts have popped up over the last few years advocating the health benefits of intermittent fasting. Despite the huge increase in popularity of this eating pattern, there still remains many questions around the health benefits of this way of eating.

In this blog post I will be exploring the metabolic effects of intermittent fasting and what this means for our health. I will be looking into the evidence behind its increase in popularity and why so many people are endorsing this way of eating.

What is intermittent fasting?

Fasting has been around for centuries and is engraved in various religious practices, such as Ramadan and Lent. It involves restricting food intake for a given amount of time. In the nutrition science world, there are several different types of fasting, these include:

  • Time restricted feeding. This is where you generally fast overnight for a minimum of 12 hours and consume food within an 8-hour window.

  • Alternate day fasting. This is where no food is consumed for a day followed by a day of your normal eating habits.

  • Modified alternate day fasting. This is also known as the 5:2 diet; this entails restricted calorie intake for 2 non-consecutive days, and then your normal food consumption for the remaining 5 days.

Why the hype around intermittent fasting?

I only came across this way of eating a few years ago when a friend asked whether I had tried it out. Since then I have seen it advertised EVERYWHERE. It has been endorsed by many celebrities and dieters and there have been hundreds of books published on how best to follow this eating regime and the so-called health benefits.

Unfortunately, I think its popularity stems from the fact that it disguises another way to restrictively eat, without having to say the dreaded word 'diet'.

Intermittent fasting has been hypothesised to improve metabolic markers of health, such as glucose metabolism (the way our body deals with sugar when we eat it) and impacts certain lifestyle behaviours such as our quality of sleep (1).

Despite this, the main headline that pops up when you search intermittent fasting on Google is weight loss. This is most likely due to reduced energy intake resulting from the restricted time you can eat within the day. However, this should DEFINITELY not be the reason to try this dietary pattern out!

The theories behind the metabolic health benefits…a little bit of science…

There are numerous theories why intermittent fasting may cause these health benefits, one of which is that restricting our food intake actually helps to regulate our inner body clock, also known as our circadian rhythm.

Our circadian rhythm controls many many important processes in our body, not only our sleep pattern.

For example, it makes sure all our important biological processes are performed at the optimal times for ideal body functioning. The time of day plays a huge part in making sure our metabolism, hormones and sleep all occur at the right time and are in sync with one another. If our circadian rhythm is not in synchronisation then it can increase our risk of chronic diseases such as obesity and developing diabetes.

Eating is an important signal to our body clock, so if we are eating outside our normal human feeding times such as at night, then this can have detrimental effects and disrupt our energy balance.

Our circadian rhythm also affects our gut microbiome. Our gut function is also controlled by and influences our body clock. Our stomachs empty and there is more blood flow to our gut in the day time compared to at night. Therefore, a disturbed gut due to consuming food throughout the night can have negative effects on our gut function and impact on our metabolism and therefore our overall health. Intermittent fasting has been shown to impact directly on our gut microbiome, such as increasing the diversity of our gut microbes, and cause a non-obese microbiome profile, meaning we absorb less energy form the food that passes to that part of our gut.

The most talked about headlines of this eating regime though, is the fact that many people consume less energy due to having a shorter period in which to eat food, this leads to weight loss. There has also been evidence to suggest that intermittent fasting improves your quality of sleep.

But are these health claims and hypotheses true and backed up by high-quality evidence? Or are we once again endorsing another diet?

I am going to focus on the 2 most popular types of intermittent fasting, the regimes which you most likely have tried or maybe have been interested in giving a go.

First up time-restricted feeding…

Despite the huge increase in popularity of this eating regime, there is a lack of high-quality human intervention trials looking at the health benefits. There are four human studies that have shown that restricting your food consumption to an 8-hour window, with a prolonged overnight fast has been shown to cause a decrease in metabolic health markers such as cholesterol 6 (2, 3). These trials also show a significant decrease in weight compared to the control groups.

Most of the studies undertaken have been carried out in mice and rats. These studies have actually shown multiple metabolic health benefits, including improved glucose metabolism, decrease in inflammation (which is associated with obesity) and a decrease in the risk of developing obesity and obesity related conditions such as diabetes (4, 5).

Obviously despite all these great results seen in animals, we can’t assume that they will also be seen in humans. We are a different species after all! We need more human intervention trials that use large numbers of people, as four studies is really not enough to see the true effects of a dietary pattern on our health!

The 5:2 diet…

I heard about this dietary pattern before time restricted eating, and I distinctly remember thinking how on earth do people cut their calories so much on their fasting days and actually survive!!! People typically restrict their calories to between 500 and 600 kcal per day on the fasting days and can then eat what they like on the remaining 5 days of the week.

There have been more human intervention trials carried out to look at the effects of this diet on metabolic health. Some studies have also shown an improvement in glucose metabolism, a decrease in cholesterol and fats that circulate in our blood as well as inflammation (inflammation is associated with obesity) (6-8); so similar findings to that of time restricted feeding. Again, the majority of the studies show significant weight loss in the groups that follow the 5:2 regime compared to the control groups.

This regime has also been compared against your traditional reduced energy intake diet. The evidence from the few trials that have looked at this, found no difference in weight loss between the 5:2 diet and the normal reduced calorie diet (9, 10).

Combined evidence on intermittent fasting

A relatively recent systematic review and meta-analysis (a type of study where researchers combine all the data and see what the combined effect is), looked at all the different types of intermittent fasting (11). They found that in the general population (metabolically healthy people), there was a significant decrease in BMI and other metabolic markers such as blood sugar concentration and insulin resistance. This indicates that intermittent fasting can actually improve our metabolic health, which may result in decreased risk of diseases, such as diabetes and obesity.

There are several limitations to this review. For example, the studies included used different types of intermittent fasting, with different calorie restrictions and fasting times. The control groups also differed a lot. This means that we can not accurately compare and analyse the results from all the different studies and come up with a definitive answer as to whether intermittent fasting actually does improve our metabolic health.

Another limitation with the evidence gathered so far are the small sample sizes of people used in the trials. One of the reasons for this is that many people drop out of the studies because they cannot sustain the restrictive diet and feeding times that the study entails.

So, what next?

We need more human intervention trials that analyse the effects of all the different types of intermittent fasting. However, these trials need to use similar calorie restrictions and fasting times, as well as similar control groups, so that we can compare the results and get definitive answers.

We also have no idea what the long-term effects of intermittent fasting are. This is because all the human trials have only been carried out for a maximum of 3 months.

What does this all mean for you and me?

So, once again in nutritional science we do not have a definitive answer to say that intermittent fasting is the go-to new eating regime. We can say that there is emerging evidence that it may have beneficial effects on our metabolic health by making sure our circadian rhythm is all aligned.

My advice regarding intermittent fasting is that if you fancy it then give it a go, you may feel as though you sleep better because you may not be eating as late as normal. And it may improve the way your body deals with glucose. It is important though to remember that it may not work for everyone.

However, I am NOT advocating intermittent fasting as a weight loss method or as another diet. You should still be consuming the same number of calories, but just within certain hours of the day.

We are all unique, so it may not work for everyone. I decided to try time-restricted feeding back in November last year as I had heard great things from friends, so I thought why not! For the first week or so I was loving it, I could have my breakfast for lunch, what more did I want! However, I quickly found out that this eating regime was not the one for me. I am a morning person and tend to get up early and exercise in the mornings which meant by the time my fast was ending, at around midday, I was STARVING. I also found that I was restricting my calorie intake because I wasn’t eating breakfast at the usual time. This meant that instead of feeling more energised, I was actually feeling tired and lethargic.

It’s a lifestyle not a diet!

For some people, this might be a regime that helps them to improve their metabolic health. However, for the majority of us we should view intermittent fasting as a lifestyle NOT a diet. Do not restrict your calorie intake. Make sure you are still consuming a well-balanced diet with lots of fruits, veggies and whole grains.

And above all let’s EAT HAPPY and LIVE WELL.



  1. Patterson RE, Sears DD. Metabolic Effects of Intermittent Fasting. Annual Review of Nutrition. 2017;37(1):371-93.

  2. Carlson O, Martin B, Stote KS, Golden E, Maudsley S, Najjar SS, et al. Impact of reduced meal frequency without caloric restriction on glucose regulation in healthy, normal-weight middle-aged men and women. Metabolism. 2007;56(12):1729-34.

  3. Stote KS, Baer DJ, Spears K, Paul DR, Harris GK, Rumpler WV, et al. A controlled trial of reduced meal frequency without caloric restriction in healthy, normal-weight, middle-aged adults. The American Journal of Clinical Nutrition. 2007;85(4):981-8.

  4. Chaix A, Zarrinpar A, Miu P, Panda S. Time-Restricted Feeding Is a Preventative and Therapeutic Intervention against Diverse Nutritional Challenges. Cell Metabolism. 2014;20(6):991-1005.

  5. Chung H, Chou W, Sears DD, Patterson RE, Webster NJG, Ellies LG. Time-restricted feeding improves insulin resistance and hepatic steatosis in a mouse model of postmenopausal obesity. Metabolism. 2016;65(12):1743-54.

  6. Johnson JB, Summer W, Cutler RG, Martin B, Hyun D-H, Dixit VD, et al. Alternate day calorie restriction improves clinical findings and reduces markers of oxidative stress and inflammation in overweight adults with moderate asthma. Free Radical Biology and Medicine. 2007;42(5):665-74.

  7. Varady KA, Bhutani S, Church EC, Klempel MC. Short-term modified alternate-day fasting: a novel dietary strategy for weight loss and cardioprotection in obese adults. The American Journal of Clinical Nutrition. 2009;90(5):1138-43.

  8. Varady KA, Bhutani S, Klempel MC, Kroeger CM, Trepanowski JF, Haus JM, et al. Alternate day fasting for weight loss in normal weight and overweight subjects: a randomized controlled trial. Nutrition Journal. 2013;12(1):146.

  9. Harvie MN, Pegington M, Mattson MP, Frystyk J, Dillon B, Evans G, et al. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. International Journal of Obesity. 2011;35(5):714-27.

  10. Harvie M, Wright C, Pegington M, McMullan D, Mitchell E, Martin B, et al. The effect of intermittent energy and carbohydrate restriction v. daily energy restriction on weight loss and metabolic disease risk markers in overweight women. British Journal of Nutrition. 2013;110(8):1534-47.

  11. Cho Y, Hong N, Kim K-w, Cho JS, Lee M, Lee Y-h, et al. The Effectiveness of Intermittent Fasting to Reduce Body Mass Index and Glucose Metabolism: A Systematic Review and Meta-Analysis. Journal of Clinical Medicine. 2019;8(10).

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