The Intermittent Fasting Diet (5:2 Diet), by Caroline Noonan

Forget depriving yourself every day, the new diet on the block suggests you only have to limit food intake 2 days a week to lose weight and improve your health. Too good to be true?

Everywhere you look dieting is big business, whether it be online, bookstores or magazines. The global diet industry is currently worth a staggering €55 billion per year and it shows no signs of slowing down as more and more of us try desperately to curb our weight.

We are bombarded on a daily basis with diets such as the maple syrup diet, grapefruit diet, gut buster diet, the eat and burn diet and so on. The one thing that all of these diets have in common is long term deprivation of foods we love to eat! Deprivation makes us want restricted food even more which is why most diets do not work long-term. It is this basic psychology that may explain the popularity of a relatively new approach to weight loss – intermittent fasting. The idea that you restrict calorie intake two days per week and for the rest of the week eat what you want while losing weight has been backed up by some interesting recent research.

This research hit the mainstream with a Horizon documentary on BBC with Dr. Michael Mosley. The programme followed the broadcaster on a weight loss journey during which he lost a stone in six weeks, as well as a reduction in LDL (bad) cholesterol, increase in HDL (good) cholesterol, reductions in blood glucose levels and, most importantly, Insulin-like Growth Factor 1 (more on this later). For 2 days per week he reduced his calorie intake to 600 calories a day (500 for women) while on the other 5 days he consumed what he wanted. This approach is more commonly known as 5:2 fasting, one of the main pioneers of this being nutritionist Brad Pilon based on his research at the University of Guelph. He states that ‘dieting is a slow, inevitable march to failure. With intermittent fasting, if you stick to it for just 24 hours, you’ve won. Once you have done it three or four times, you know you’ll be able to do it again and again.

Sounds interesting so far – so how does it work?

One of the links between 5:2 fasting and longevity seems to be a hormone called Insulin-like Growth Factor 1 (IGF-1). IGF-1 and other growth factors keep our cells constantly active. You need adequate levels of IGF-1 and other growth factors when you are growing, but high levels later in life appear to lead to accelerated ageing. 5:2 fasting may lower levels of IGF-1 and also appears to switch on a number of DNA repair genes. The reason seems to be that when we run out of food our bodies change from “growth” to “repair” mode. Professor Valter Longo, the director of the Longevity Institute at the University of Southern California agrees that reduced levels of IGF-1 slows down the production of new cells, diverting energy into repairing existing ones. He likens our normal pattern of eating ‘to driving a car all the time and not taking it for a service’. Intermittent fasting allows the body to go into repair mode. And how is this achieved? By eating less through intermittent fasting.

A 2007 clinical review looking at the effects of intermittent fasting in humans in terms of health outcomes concluded that, specifically, alternative day fasting may have a protective effect against heart disease, type 2 diabetes and cancer. Dr Michelle Harvie, a research dietician at the Genesis Breast Cancer Prevention Centre in Manchester completed research with British women at risk and concluded ‘those on the fast had improved insulin resistance and markers for oxidative stress and inflammation. One further study suggests that intermittent fasting can also improve mood as it appears to boost serotonin levels and Brain derived neurotrophic factor.

According to Patrick Holford, an advocator of intermittent fasting, ‘if your goal is to live long and be healthy it’s better to have lower IGF-1 levels, achieved by eating a more plant-based, less meat and dairy based diet. The key is to keep blood sugar levels stable, which means less insulin release. These are the key principles of a low GL diet’.

A word of caution, however, as most studies into intermittent fasting have been completed on mice. Much work remains to be done to understand this dietary strategy fully. If you are fasting, you may want to think about how fasting will impact on your life during your fasting days. You are likely to be very hungry and have less energy and this could affect your ability to function, in particular it may affect your ability to exercise which is an important part of maintaining a healthy weight. Also, intermittent fasting may not be suitable for pregnant women and people with specific health conditions, such as diabetes or a history of eating disorders. If you are considering trying IF for yourself, it is wise to speak to your GP first to see if it is safe to do so.

Tips on how to approach the 5:2 Diet

How to Fast

  • Pick 2 days in the week that would be suitable for fasting. Some people prefer to break up these 2 days within the week.
  • Do not plan to do anything too strenuous on those days.
  • Have your menu prepared for the fasting days so you are not tempted to eat over the 500 to 600 calorie allowance.
  • Choose the best time for you to start. You might find 2pm on the first day to 2pm the next more tolerable as you will be asleep for a large chunk it. Others find it disrupts sleep so find the best way that works for you.

When to Eat

There is no research as yet that shows when the best time to eat is. Most individuals, however, tend to eat twice on the fasting day – morning and evening time.

Feed Days

In theory you can eat what you like on ‘feed’ days. However, according to research, most individuals ate on average only 10 to 15% more than their normal calorie intake the day after a fasting day. From a nutritional point of view it is not necessarily the best approach to eat processed, high sugar, hydrogenated rich foods on feed days. As mentioned previously a GL based diet is probably the best approach to long term health combined with intermittent fasting.

How to Stick to It

  • Keep busy on the fasting days but do not over exert yourself.
  • Have a purpose for completing the fast. Research shows individuals that do it for religious purposes or moral reasons tend to be more successful. So have a focus for doing it- reduce weight, reduce high cholesterol etc.
  • Keep your goals short term ones. If you look too far ahead your chances of succeeding reduce.
  • Reward system- if you succeed treat yourself to something you want.
  • On fasting days you can drink herbal teas, coffee and tea in moderation and water.

1 day example menu


Banana and Walnut Yogurt Compote (150 cals)

1 medium banana

4-5 walnuts chopped

170g of 0% fat Greek yogurt

1 tsp of runny acacia honey


Large vegetable salad with toasted sunflower seeds and vinaigrette dressing (150 cals)

Salad leaves

Sliced tomatoes & cucumber

Steamed peas

French beans

Asparagus & spring onions

Pinch of chilli

Chopped herbs

Handful of toasted sunflower seeds

1 tablespoon of vinaigrette dressing


Roast fresh mackerel (200 cals, serves 2)

1 tbsp olive oil

1 tbsp sherry vinegar

½ tbsp fresh thyme leaves

Sea salt and black pepper

2 large or 3-4 small mackerel

Salad to serve

Mix the oil, sherry vinegar and thyme in a bowl.

Spoon over the mackerel and cook in the oven for 15 to 20 minutes.

Serve with salad.