The Problem with Fasting


It all seems too good to be true. Eat whatever you want, and just fast two days per week. And…voila! Weight loss! I’ve even read that it staves off ageing.

Think about that for just a moment. Fast to live longer. Fasting makes you cranky. Live a longer, yet crankier life. That just gives me all kinds of crazy mind pictures of old ‘hangry’ people… Anyway.

Fasting is quickly becoming one of the most popular methods for weight management and the pursuit of health, with many claiming they regularly ‘fast’ to reset their metabolism and keep them at a healthy weight. Along with the claims that it’ll increase your lifespan, many advocates of the eating pattern claim it cures diabetes, reduces risk of heart disease and, of course, helps you lose weight.


There are a number of different types of fasting patterns, the most popular of which is the 5:2 diet or the diet outlined in Mosley and Spencer’s best-selling book ‘The Fast Diet’. This diet involves dramatically reducing energy intake to 20-25% of your daily energy needs on two, non-consecutive days of the week (fast days) and then eating normally the rest of the time.


It is well established in the scientific literature that when you restrict food intake or fast for a period of time there’ll be changes in your metabolism. The most notable of those changes, for the purposes of this article, are the reductions seen in blood glucose and insulin levels.

Human metabolism is extremely complex biochemistry and even with two nutrition degrees under my belt I would not, for a second, claim that I was an expert on the stuff. I’m absolutely not. That’s how complex it is. However, I do know this: Rather than thinking about metabolism as an on/off switch, which many diet books and articles can lead us to believe — think common phrases used in marketing like: ‘kick start your metabolism’ or ‘fire up your fat burning’ — metabolism is more like a dimmer switch.

To make complex biochemistry as simple as possible, glucose is absorbed via the gut from the food we eat. It’s used as energy and burnt by the body. The body uses a hormone called insulin to regulate this process. When you consume more food than your body needs, you create a situation where you’ve got too much glucose in your bloodstream and so your body needs to produce more insulin to take care of it. If this happens constantly and all the time, this state of over-consumption leads to metabolic disturbances, like insulin resistance.

Research tells us that even one week of eating too much and our body becomes insulin resistant – a sign of metabolic disturbance. Don’t fret, the reverse of the dimmer switch is also true. Just one exercise session, or a few days of reducing your food intake, and your body becomes sensitive to insulin again and metabolism is restored. Hmmmm.

So fasting can help decrease your insulin levels and help you lose weight. But so do other dietary patterns that decrease your daily food intake. You don’t have to fully ‘fast’.


Yes. It does. But there’s a catch.

Fasting just so happens to be one of the ‘fad diets’ that has a fairly decent amount of research behind it. However, these studies only include eight that are conducted with humans. The rest have been done in animals. Here are the main conclusions from the research:

Fasting can’t harm you. Isn’t that nice to know? So give it a shot if you’re keen, it won’t do you any damage.

Fasting can leave you feeling extremely hungry on fast days. Yeah…pretty obvious, that one.

Any intermittent fasting regime will result in some weight loss, but it’s not superior to other weight loss diets that reduce energy intake consistently each day.


I’ve helped clients follow an intermittent fasting dietary pattern and they’ve lost weight at the same rate as clients that I’ve help lose weight with my program and are consistent with sticking at it. This is honestly the key.

Any weight loss program will work if you STICK TO IT.

Of the people I’ve met who have tried intermittent fasting without success, they’ve failed because they didn’t fast properly on their fast days and/or they overate on their non-fast days. A total of 500 calories (fast day quota) can quickly turn into 800 calories when you’re hungry, tired and staring at the fundraising chocolate box at work.

Overeating non-fast days is also a problem. Eating ‘normally’ is not ‘going to town on the chocolate aisle at Coles. If you’re continually overeating on your non-fast days, then the diet won’t work for you either. Remember the dimmer switch analogy?

Also, both the fast and the non-fast days need to include healthy food. You may only be interested in losing weight, but your body still needs fibre (to poo properly), vitamins and minerals (to function properly) and other nutrients to be healthy. It’s impossible to meet your nutrient needs on fast days, so you’ve got to make a really concerted effort that your non-fast days are full of healthy, nutrient rich foods.

In the end, you’ve got to figure out a dietary pattern that you can sustain long term. If that’s fasting, great, but if you can’t see yourself doing it forever, then perhaps it’s best to lose weight in the same way you intend on maintaining it. If you need help with that, I’m your girl.

I offer individualised advice to help you implement nutrition advice into your everyday life!

Article by Kate Freeman

Registered Nutritionist. Writer. Presenter. Home cook. Mother. Wife. Runner. Hiker. Amateur photographer.