Dietary Analysis – Intermittent Fasting, by Tania Logan, Dietitan and Diabetes Educator

Christina ReynoldsUncategorized

This instalment of my analysis of popular dieting trends investigates Intermittent Fasting. In this article I’ll discuss what the diet is about, analyse the scientific evidence available and give you my verdict on whether it is worthwhile or not.

The diet

Intermittent fasting is essentially a method of reducing energy intake.  These deliberate periods of energy imbalance encourage the body to burn more fat.  During fasting, the body burns fat stores for energy instead of the usual pathway where is utilises glucose for energy.  Like with the ketogenic diet, this means that less insulin is required to deliver energy to our body which may reduce inflammation and oxidative stress.

Intermittent fasting focuses on how much you eat (quantity) rather than how nutritious your food choices are (quality).

There are two types of intermittent fasting that are popular at the moment.  

Alternate Day Fastinghas been popular for many years.  It encourages restricting energy intake to 500 – 600 calories (2000 – 2500 kJ) on two, non-consecutive days each week, while maintaining a healthy, balanced diet on the remaining five days of the week.  Food intake on a fasting day is minimal. An example of this might be:

  • Breakfast 

1 thin slice of wholegrain toast topped with 1 egg served with pan fried mushrooms and asparagus

  • Lunch

A salad of lettuce, tomato, cucumber, carrot, celery and ½ cup of blueberries

  • Dinner

100g of lean chicken stir fried with shallots, garlic, ginger, celery, carrot, snow peas, broccoli and cauliflower

Time Restricted Feedinghas been gaining popularity more recently.  This approach is based on providing a smaller window of time to eat each day.  Many adults typically eat over a 14 hour period each day, for example from 6 am to 8 pm. Time restricted feeding limits eating to 8 hours each day (for example 9 am to 5 pm) with 16 hours of fasting in between.

The evidence

The majority of research on intermittent fasting is in animals.  There is some great evidence in rats that shows successful weight loss as well as a reduction in blood pressure, lipid levels and blood glucose levels.  The evidence for health improvements in humans isn’t as dramatic.  There is evidence that shows intermittent fasting does lead to weight loss, but no more than other approaches that reduce energy intake. It does not appear to improve efficiency of weight loss. 

Most studies seem to show no additional benefit to health parameters like blood pressure, lipid levels and blood glucose levels, other than improvements gained through weight loss.

Further research is required on time restricted feeding.  One small study over a short 5-week period, showed that 18 hours of fasting reduced insulin levels and improved insulin sensitivity compared with 12 hours of fasting.  

There is some suggestion that intermittent fasting may have benefits on the gut microbiome, but this is mostly from animal studies.

More research is needed on the effects and benefits of intermittent fasting.

Possible benefits

Evidence shows that intermittent fasting is effective, but no better than other methods that reduce energy intake.  Restricting energy intake every day is challenging, so alternate day fasting may be useful for those who are able to focus on a reduced energy intake on a smaller number of days each week.  Time restricted feeding, particularly if meals are eaten earlier in the day with fasting in the evening, may put our eating more in sync with the body’s natural circadian rhythm.

Possible side effects

Fasting can cause a number of side effects including headache, fatigue, weakness, dehydration, diarrhoea and vitamin and mineral deficiencies.  More information on the side effects of fasting can be found in my previous blog on detox and cleansing diets.   These effects should not be as significant with intermittent fasting but the risks increase if it is practised too frequently or on too many consecutive days.

Intermittent fasting does encourage a starve/binge attitude, so it is not suitable for people at risk of eating disorders or with a history of an eating disorder.  It is also not suitable for women who are pregnant or breastfeeding or children and adolescents.

People with diabetes who are taking diabetes tablets or insulin need to be particularly aware of an increased risk of hypoglycaemia when fasting.  Medication may need to be adjusted before starting a diet based on alternate day fasting or time restricted feeding.  Please see your GP or Endocrinologist, Diabetes Educator or Dietitian before trying intermittent fasting.

In a nutshell

Intermittent fasting is an effective strategy to reduce energy intake for weight loss, however it is not superior to other strategies.  It does not consider the types of foods that are beneficial for health and wellbeing.  Despite this, there are some principles of intermittent fasting that may be beneficial to incorporate into our lifestyle.  These include:

  • Eating earlier in the day
  • Reducing snacking, particularly at night
  • Prolonging the amount of time you don’t eat overnight.  Instead of eating from 6 am to 8 pm each day, try eating from 8 am to 6 pm.

If you would like help with developing healthy eating or lifestyle behaviours, or if you would like practical assistance for making any of these suggested changes to improve your health, please contact me. You can find my contact details and how to book an appointment on our website at

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