The idea of fasting – that is, abstaining from all or some kinds of food and drink – has become more common in recent years.
Historically, there are religious and spiritual reasons for fasting, along with the fact that at times there simply wasn’t enough food to eat.
But in this context, we will look at fasting from a ‘choice’ and physiological point of view. Does it help you lose weight? Is it a good for your health?
There are a number of “fasting diets” around that are not fasting in the true sense. For example, the 5:2, the two-meal-a-day diet, the UpDayDownDay diet.
The 5:2 diet involves eating normally for five days per week and restricting your calorie intake to 500-600calories on the other two days.
The UpDayDownDay diet is about restricting calories every other day.
The two-meal-day diet is exactly that – eating two meals a day.
Although these all differ slightly, they all involve reducing your calorie intake over a set period.
This is when there is a set period for fasting in between eating periods.
For example, there is the “16/8 Method” which involves skipping breakfast and restricting your daily eating period to eight hours, for example from 1pm to 9pm. Then you essentially “fast” for 16 hours in between. But that could be described as cheating a bit due to the fact many of those hours are asleep.
Or, you could fast for the entire day, taking in nothing except water, black tea, or weak coffee. If you count the previous and following nights while you were sleeping, this would add up to 36 hours without food.
Again, such periods of fasting can result in a calorie reduction over a period of time.
Why has fasting become more popular?
It’s certainly one way to help reduce calorie intake. Such a reduction in energy intake over a period of time should help in weight loss. But that is the crux of it – what is consumed over “a period of time”.
There have also, however, been some studies that show possible potential health benefits. For example:
- In one rodent study, fasting was shown to promote the reduction of toxins in the body by activating detoxifying liver enzymes 
- Fasting has also been linked to improved cardiovascular health as a study found obese individuals who completed alternate day fasting had lower ‘bad’ cholesterol levels 
Potential issues with fasting
A reduced window in which food can be consumed can decrease your oral intake but it does not guarantee that you will eat the right foods when you do eat. Foods eaten when not fasting must be nutrient dense to avoid developing any mineral or vitamin deficiencies .
Further rodent studies have discovered that fasting reduces the levels of cytochrome P-450 and glutathione, which are detoxifying enzymes, so decreasing the liver’s detoxifying abilities . This, in essence, contradicts other studies.
While one study found that alternate fasting reduced ‘bad’ cholesterol levels, further studies in humans have found negative health effects from fasting for over 24 hours, including:
- loss of muscle mass and physical work capacity
- occurrence of headaches in 50% of participants
- development of insulin resistance (as occurs in type 2 diabetes) .
Is fasting right for me?
Fasting is like all weight-loss diets, in that the idea is to have a reduction in calorie intake, so followed by weight loss. Following this pattern of eating does not mean you are eating ‘well’ or meeting your nutrient requirements.
Fasting is not a dietary pattern that will work for everybody and requires a large amount of commitment if you want to avoid deficiencies. More research is required that investigates the long-term effect of fasting-type diets on our health.
Note: This style of eating can also result in disordered eating habits as the flexibility for when you can eat is reduced and a larger amount of planning is required for meals and snacks. If you want to give fasting a go, consider talking to a health professional, such as a dietitian or your GP, about the potential benefits or ill-effects it could have on your health.
1. Hong JY, Pan JM, Gonzalez FJ, Gelboin HV, Yang CS. The induction of a specific form of cytochrome P-450 (P-450j) by fasting. Biochem Biophys Res Commun. 1987 Feb 13;142(3):1077-83. Abstract available from: http://www.ncbi.nlm.nih.gov/pubmed/3827 895
2. Bhutani S, Klempel MC, Berger RA, Varady KA. Improvements in coronary heart disease risk indicators by alternate-day fasting involve adipose tissue modulations. Obesity (Silver Spring). 2010 Nov ;18(11):2152-9. Abstract available from: http://www.ncbi.nlm.nih.gov/pubmed/2030 0080
3. Nikki Bezzant. Behind the headlines: Fasting diets. Healthy Food Guide. June 2013 – [cited 8 March 2018]. Available from: https://www.healthyfood.co.nz/articles/2013/june/behind-the-headlines-fasting-diets
4. Dietitians of Canada. Evidence clip – Is fasting healthy? 2013. PEN Database.