Intermittent fasting is certainly in the spotlight at the moment, but did you know that fasting was used for therapeutic purposes way back in ancient China?
In this blog, I’ll be exploring What is intermittent fasting?, What happens when you fast?, What are the benefits?, as well as why Intermittent fasting isn’t for everyone. Skip to the highlights box below for the 60-second summary!
First of all, it’s definitely not about starving yourself. There are several types of intermittent fasting, including alternate-day fasting and 24-hour fasting. The most popular and practical approaches are:
• Time-restricted eating – this involves eating during a specific period of the day, generally lasting between 8 and 12 hours. So for example, the 16:8 diet means that you’d eat between say 10am and 6pm and then fast for the next 16 hours, during which you would only consume calorie-free drinks such as water, herbal teas or tea/coffee with no milk.
• Modified fasting – with this type of intermittent fasting you consume a low-calorie diet on 1 or more days and then eat a balanced diet with no calorie restrictions for the rest of the week. The most common version of this is the 5:2 diet where you consume between 500 and 800 calories per day for 2 days each week
The rest of this blog focusses mainly on these two types of intermittent fasting.
Fasting results in a ‘metabolic switch’ flipping over from using glucose to fats as an energy source. Normally your cells use glucose for energy. But after 10 to 12 hours of fasting, your metabolism flips over to using fat stores. These break down resulting in the production of compounds called ‘ketones’, which become the key energy source for most of your tissues, including your brain, during fasting.
So how does this feel? Well perhaps not surprisingly, you may feel hungry during periods of fasting! A small proportion of people may also feel cold, irritable and may find it more difficult to concentrate, although this is usually temporary. On the plus side, intermittent fasters have reported feeling more positive and self-confident.
In particular, this type of diet should not be followed if you are pregnant or breast feeding. You should also check with your GP before making any significant changes to your diet if you have a long-term health condition (including raised blood pressure and/or cholesterol levels, diabetes or heart problems) or any history of an eating disorder or mental health condition.
Bear in mind also that while a lot of research has been done in this area, most of the studies are short-term and in overweight or obese people. More research is needed to understand the effects and safety of this type of dietary approach over the long term and in normal weight people.
• The two most well-known forms of intermittent fasting are the 16:8 diet (consume food for 8 hours, fast for 16 hours each day) and the 5:2 diet (eat 500-800 calories on 2 days and a healthy, balanced for 5 days).
• After about 10-12 hours of fasting your metabolism flips over from using glucose as the body’s main energy source to your fat stores, which are broken down into compounds called ‘ketones’.
• Potential benefits include – weight loss (mainly fat not muscle), improved blood sugar balance, less inflammation, better quality sleep.
• Intermittent fasting should not be followed if you are pregnant or breast feeding and is not recommended for other conditions.
1. De Cabo and Mattson (2019) Effects of intermittent fasting on health, aging, and disease. New England Journal of Medicine, 381 (26): 2541-2551.
2. Longo and Mattson (2014) Fasting: molecular mechanisms and clinical applications. Cell Metabolism, 19: 181-192.
3. Anton et al. (2018) Flipping the metabolic switch: understanding and applying the health benefits of fasting. Obesity Biology and Integrated Physiology, 26: 254-268.
4. Gill and Panda (2015) A smartphone app reveals erratic diurnal eating patterns in humans that can be modulated for health benefits. Cell Metabolism, 22: 789-798.