Intermittent fasting has been a hot topic in the health and fitness world for a few years now. If you search the internet, you’ll see there are a lot of good informative articles on the topic. Most of them tell you what it is and list the benefits and disadvantages. I could do the same thing here, but I’d just be recycling, or even plagiarising, the material that’s already out there.
Instead, I’ll present a short summary to set the scene, then provide my thoughts on the subject.
What is it?
Also known as time-restricted feeding, it’s all about eating during specific periods of time and remaining fasted, or near fasted, at all other times. Some of the different protocols include:
- Fast for 16 hours a day, feed during the remaining 8 (let’s refer to it as 16:8).
- There’s a more extreme 20:4 version of this.
- Fast for 24 hours, twice a week. The well known 5-2 diet is a form of this.
- The hardcore ‘alternate day’ fasting. One day fasting, next day eating.
Why would you do it?
Probably the main three reasons are:
- For health benefits
- For weight loss
- To simplify life!
What health benefits? That’s a good question. In science, a finding does not become a fact until it has been reproduced many times. In the studies that have been done you see some findings that are not reproduced in other studies. And some studies that say the opposite to other studies. Why is that? Because there are different fasting protocols (e.g., 16:4, alternate day, 5-2, 20:4 etc), different subjects (obese, weight-trained, normal-weight etc), different study designs, different study durations and so on. A finding that is observed in the short term may not be observed in the long term. An outcome observed on an obese population may not be observed on a normal-weight population. There hasn’t been a sufficient number of studies to date that make any of the health claims irrefutable. It seems to me that we can’t use the word ‘will’ just yet, and have to be content with ‘may’. It may reduce inflammation, for example. Now that said, I think it’s fair to say that some of the health benefits would be ‘probably true’ whilst others would be ‘difficult to say one way or the other right now’.
With that in mind, here are the often-cited benefits of intermittent fasting
- Helps with disease protection
- Improves heart health
- Reduces the risk of cancer
- Reduces cholesterol
- Improves mental clarity
- Improves longevity
- Reduces inflammation
- Increases energy
- Increases GH production*
- Improves insulin sensitivity*
- Increases autophagy*
- Helps with weight loss*
The last four I mark with a * to indicate that these are readily measurable and verifiable physiology or body weight changes. They are good changes and will benefit your health, but they are not unique to fasting: exercise can achieve the same changes.
I’d say weight loss is one outcome that is almost a proven benefit. Why does it work? Almost entirely because, by eating less frequently, you’re eating less. It seems that most people do not make up the calorie shortfall during the eating window and so end up in calorie deficit. Note that you can achieve the same thing without fasting simply by eating less.
But the real question is whether fasted weight loss is more effective than eating normally and simply maintaining a calorie deficit. This paper here concludes “Alternate-day fasting did not produce superior adherence, weight loss, weight maintenance, or cardioprotection vs daily calorie restriction”. On the other hand this paper here observed improved body composition with fasted weight loss compared to simple calorie restriction. It states, compared to simple calorie restriction, fasted weight loss produced “a significant modification of body composition, including reductions in fat mass; significant increases in blood pressure and in total, LDL-, and HDL-cholesterol concentrations”. So better reductions in fat mass but worsened blood pressure and cholesterol. Other studies show improved blood pressure and cholesterol. In this paper here weight trained males on a normal diet are compared with those on a 16:8 fasting protocol. “fat-free mass, muscle area of the arm and thigh, and maximal strength were maintained in both groups. Testosterone and insulin-like growth factor 1 decreased significantly in time-restricted feeding”. So, this paper suggests no difference due to a fasting protocol in terms of body composition – at least on weight trained males. And, worryingly, it shows reduced testosterone!
So, you can see there is huge variety in study design, tested populations, fasting protocols and so on. No wonder there are differences of opinion regarding the pros and cons.
This paper here from 2015 concludes “Research on time-restricted feeding is limited, and clear conclusions cannot be made at present. Future studies should examine long-term effects of intermittent fasting and the potential synergistic effects of combining intermittent fasting with exercise.”
So, my view is, in terms of health and weight loss, if you like the sound of it – give it a go! If it simplifies your life and the meal timings work for your schedule then it’s got to be a good thing. It’s probably good for your health even if there is some debate about that. And it will more than likely help you lose weight.
Why would you NOT do it?
The main downsides of intermittent fasting relate to:
- Blood sugar insulin variations – long periods of low insulin followed by high
- Eating behaviours – it can make you hungry and fixated on food
- Getting a steady supply of nutrients – long periods without sustenance or energy intake
As you’ll see in the various posts online, this makes intermittent fasting inadvisable for the following:
- Women who are pregnant or breastfeeding
- People with low blood pressure
- Anyone with an eating disorder
- Underweight people
- Women who experience amenorrhea
- Women who are trying to conceive
- People on medication that requires regular eating
- Adolescents and children
It’s also been reported that some women do not respond well to intermittent fasting. So, women should probably start with a small fasting window and proceed gradually to something more hardcore.
If you’re a grazer or you can’t bear the thought of going without food for long periods, then intermittent fasting is probably not for you either.
So, what do we think? I’m going to give you my view based on the papers and articles I’ve read, all the other research I’ve ever done and the experience I’ve gained with myself and our clients
1. If you want to lose weight and you’ve tried other approaches then give intermittent fasting a go. It’s very unlikely to do you any harm – unless you are amongst the cautioned populations listed in the previous section – and you may feel better on it. If it suits your lifestyle and schedule then it might be the best weight loss approach for you. A weight loss programme has to work for you and has to be something you can stick to. If this approach suits you, then it’s right for you.
2. It is probably no more effective than simple calorie restriction. Most of the papers show simple calorie restriction to be pretty much the same as intermittent fasting in terms of weight loss.
3. The cited health benefits almost certainly can be achieved by eating healthy food, eating less of it and exercising. For example, I like this article here for it’s no-nonsense delivery of the pros and cons. It’s the only one in the early pages of my search that talks about metabolic flexibility. This is where the body is readily able to use fat as a fuel because it is good at it. The body has flexibility to use fat, carbohydrate or, heaven forbid, protein as a fuel. Being good at using fat means the body is less likely to go for carbohydrate or protein as a fuel source, so it spares both glycogen stores and muscle. Intermittent fasting makes you better at oxidising fat. Over time it leads to increases in the activity of enzymes associated with fat oxidation. Great news. But then you can also improve fat oxidation by doing regular fasted cardio. Read these articles:
So, which is better for improving metabolic flexibility – intermittent fasting or fasted cardio? Who knows? As far as I’m aware, there’s no research on it. Just be aware that intermittent fasting is not the only choice for improving fat oxidation capabilities. Other health benefit examples such as improved heart disease risk factors, blood pressure, cholesterol, cancer risk, insulin sensitivity and so on – they’re all well-established benefits of exercising and eating healthily.
4. If you’re into building muscle and getting lean, as I am, then be aware that some studies have shown reduced testosterone, reduced thyroid output and increased cortisol. Even though there is an insufficient number of studies on intermittent fasting to make these observations ‘true’, it makes sense to me from a physiology point of view. That’s enough of a risk to put me off using intermittent fasting for myself. I’d suggest you read around to find any possible downsides that are relevant to your particular goals and health concerns.
You might get the impression that I’m an intermittent fasting naysayer. Honestly, I’m not. It’s just that I’m not convinced it’s any better than other approaches for improving health and losing weight. And I love food. I love eating. Going hungry is not for me!
If you’d like to read some more about intermittent fasting then this Intermittent Fasting Guide and Benefits article provides a nice round-up of the key points, and includes some clear infographics.
In addition, here’s a selection of the material I read when putting this article together, some of which goes into more scientific detail.