The fast on fasting

Fasting, particularly the intermittent variety, is more than a passing fad. There is plenty of scientific evidence that periods without food can be very good for us, writes Professor Grant Schofield.


  • There is scientific evidence that periods without food can be good for us and even extend lifespan
  • In order for the cells in our bodies to spring-clean and repair themselves they need periods during which they are not fed
  • Highly regular meal and snack consumption means that many of us are in an anabolic (growth) phase most of the time, rather than the catabolic phase necessary for cell repair (autophagy)
  • Sleep provides a large enforced rest period during which cell repair can occur, but additional periods of non-eating can help to optimise health
  • Intermittent fasting time-periods can vary, but around 14 hours or more without eating will usually result in the baseline glucose and insulin levels necessary for cell repair.
Not eating for a while – fasting – hardly seems like a massive health and nutrition breakthrough, does it? Of course, fasting is not new, but along with keto it’s the big thing’ in nutrition right now. Fasting has been a common practice throughout the ages. In fact, it’s only since agriculture emerged around 10,000 years ago, that going without food became considered as ‘not normal’. Before we tamed nature for our own nutrition purposes, eating would have been a pretty sporadic activity. Planned fasting is a common practice in almost every religion. From short fasting in Judaism’s 24-hour Yom Kippur, and Buddhism’s daily post-midday fast, to the prolonged 30-day dawn-to-dusk fasting for Muslims in Ramadan, fasting has long been an integral part of life from a spiritual perspective. Now it’s back in vogue for anybody who’s interested in striving for optimal health and longevity.

The science

There really is no scientific evidence that backs up the need for humans to eat three square meals a day, plus multiple snacks, for optimal health. There is plenty of scientific evidence that periods without food can be very good for us. Research has shown that just restricting calories may indeed lengthen lifespan in animals, with the amount varying depending on the size of creature. The effect may increase lifespan by 20-30% for bigger animals, and up to double it for small creatures. The animal research is easy to do, because the experimental animals are caged and have no behavioural input into when they eat or what they eat. Unfortunately, the poor animal ends up with side effects – low body temperature (cold through reduced metabolic rate), reduced sex drive, hunger, emotional issues… So, you might live longer, but your quality of life sucks. Fasting, especially intermittent fasting, allows the same biology to happen, but without the metabolic and emotional side effects associated with severe calorie restriction. How does this happen? Until now, modern nutrition science has missed or ignored some very important facts about cell repair and regeneration. Fasting taps into these repair and regeneration mechanisms. The science tells us that every single cell in our body has the ability to spring-clean and repair itself – we simply need to stop feeding these cells from time to time. Nature certainly has it sussed – as does the 2016 Nobel prize winner who figured it out, Yoshinori Ohsumi. This self-repair of immune cells, skin cells, brain cells – and more – is known as autophagy.

Autophagy: your natural recycling machinery

Like all mammals, we humans have a recycling plant in every cell in our bodies. The process is called autophagy (pronounced ‘or-toffa-gee’) – when pathogens (infectious agents), cell ‘junk’ or old and damaged structures are broken down inside a cell and the parts reused.1 Okay, so autophagy is a big word for a really simple and useful process. It is, in fact, the way that the human body is able to keep itself in optimal condition by getting rid of the old, fixing the used, and growing the new. This is just like what any decent business would do. Imagine a supermarket that never closed for cleaning or stocktaking: eventually the aisles would be cluttered, stock would get past its use-by dates, some essential items would run out, and customers would get sick. Our ancestors often had to either go without food, or exercise very hard to get more food. Exploiting these periods for the purpose of spring-cleaning is hardwired into our DNA. The key is that what we eat, and when, affects this process. Sometimes what we eat pushes cells to keep multiplying and not recycle, called an anabolic state. Sometimes our body moves into a different state – one where we tidy up cells, kill off and recycle old ones. This is called the catabolic state, and it happens when we don’t eat. For optimal human health, the balance between anabolic and catabolic processes is crucial. But a problem caused by our modern lifestyle is that many of us are in an anabolic (growth) phase most of the time.2 This is largely caused by the ‘3 meals a day plus snacks’ regimen.  The body is so finely tuned around this balance that if we go on eating all the time, we miss cleaning up – and this happens at our peril. Thank goodness Mother Nature has our backs and has made sleep non-negotiable; but still, with our modern busy lifestyles, we need more non-eating time to optimise our health. Fasting, and to an extent keto diets (also sometimes called ‘fasting mimicking diets’) allow the body to cease anabolic signalling and engage autophagic mechanisms through the lysosome in every cell. It is now recognised that this ‘nutrient stress’ (lack of food) is critical in health and longevity.

Mealtimes were irregular for Ugg and his family...

12 questions about fasting

As a fitness professional, you are likely to be asked some of the following questions about fasting by clients and members. Unless you’re also a nutritionist or dietitian, you can’t prescribe any eating behaviours or write meal plans, but you can provide general information.

1. How long should I fast for?

Getting glucose and insulin down to baseline is critical – 14 or so hours without eating will most likely get you into the right physiology. This has been the basis of ‘intermittent fasting’ and the ‘restricted eating window’. You can do 16:8 (16 hours fasting, eat in an 8-hour window), 20:4 (4-hour eating window), or even 24 hours (dinner to dinner). You might do that a few days a week or more.

2. What do I eat when I’m not fasted?

Nutrient dense whole food. Food low in human interference (low HI). Lower carb is better because it makes the transition to fasting easier, because you will be a better fat burner (see below).

3. What do I need to do to prepare for fasting?

Get ‘fat-adapted’ before you start fasting by going lower-carb and eating more healthy fats, so that you are burning fat as your main fuel. Too many carbohydrates turns off fat-burning and makes you hungry. Going LCHF will make fasting easier and more rewarding.

4. Can I drink tea and coffee while fasting?

You can drink these provided you use only very little or no milk.

5. What about alcohol?

That’s out during a fast.

6. What about electrolytes?

Extra salt, especially when getting used to fasting, may be needed.

7. What about bone broth?

This is potentially useful during longer fasts, but it has enough nutrients in it to push up insulin, reducing autophagy.

8. What about longer fasts?

In our experience, 2 to 5-day fasts are challenging and should probably be restricted to being a once or twice a year activity. They are stressful on the body, can undermine metabolic rate if you are not careful, and affect quality of sleep. They can be useful for immune system regeneration and you will learn something about yourself if you do one.

9. Won’t I just overeat in the eating window?

You will certainly eat more than you normally would have during that time, but typically not as much as you would have overall if you hadn’t fasted at all. In other words, the science shows us that this is an effective way to decrease overall intake. It is possible to overeat in the refeeding period, but being mindful and maintaining some rules around avoiding processed junk food will help prevent this.

10. Is it better to fast in the morning or evening?

Research shows a marginally better effect of eating during the morning and fasting through the afternoon and overnight. However, be aware that this is behaviourally very hard. You are always hungrier at night because of the circadian rhythms. I think a more sustainable behavioural method is to skip breakfast, get busy, miss lunch, and eat good food when you get home. Be aware that the ‘Steve Jobs rule’ applies here. Jobs wore the same clothes – no matter what – in order to reduce expending time and effort on the things that didn’t matter to him and spend more on those that did. You can apply similar thinking to fasting, although in this scenario it’s more about making it easier to stay on track and avoid sabotaging your good intentions through impulsive food choices. Having ‘go-to’ meals and establishing a pattern around what and when you eat (or don’t eat) will help you stick to the fast.

11. What happens when I eat junk food?

What happens is that you will have eaten junk food. At least enjoy it if you do! Then you’ll need a behavioural technique to make sure you don’t keep doing it from then on. I like the 3-meal rule for this. Things aren’t always going to go to plan. It’s what you do 90% of the time that counts, and enjoying celebrations without guilt is important. So, up to three treat meals a week is fine for good health, and one a week – or less frequently – for fat loss.

12. What do I tell people about my not eating?

You could tell them that you are experimenting with fasting and you’ve found this and that out. But what is more important is what you tell yourself. We all have different reasons for changing our diets. What’s yours really? Knowing that is critical, because sticking to nutrition rules and patterns is quite hard and requires effort, at least some of the time. Ask yourself the hard questions about what you want out of life, and how much effort you are prepared to put in. This isn’t about judgement: it’s about knowing whether you really want to achieve your health and/or fat loss goals or not. Find your truth, otherwise, what’s the point of putting in the effort?

Grant Schofield

Grant is Professor of Public Health at Auckland University of Technology in New Zealand. Well-known for his work around low carb and ketogenic eating, he is the author of several books including What the Fat? and What the Fast? and has a reputation for challenging conventional health wisdom. / /