Intermittent Fasting (IF) diets: what is it?
Please note ‘diets’ during active cancer treatment is not recommended. You can read more about why here
Have you heard about ‘fasting diets’? I was asked about these diets from a cancer survivor who wanted to know which ‘diet’ she should follow after cancer treatment. I regularly write for a magazine called NHD Magazine which aimed at Dietitians and health care professionals so was really interested in examining the research for their February issue. The article is aimed at Dietitian so I have written this blog to share some of the key highlights with you.
What is intermitting fasting?
Intermittent fasting or IF involves changing your eating pattern to include periods of ‘feeding’ and ‘fasting’.
The idea of ‘fasting’ isn’t new and has been around for a long time. Historical scientific data details extreme fasting which occurred during hunger strikes, famine situations or induced experimental starvation. Fasting has also been practised throughout history for religious, cultural and spiritual reasons.
Over the last 10 years fasting diets have become popular in diet culture, especially following the 2012 BBC Horizon documentary and diet book ‘The fast Diet’. Social media is also filled with ‘miracle’ ‘before’ and ‘after’ phots from people following these diets so it’s been hard not to be curious to learn more.
Different various of intermittent fasting diets
There are many different types of fasting or intermittent fasting diets which are all based on the idea of limiting your calorie intake for some of the week.
5:2 or 2-day diet
In this diet you ‘fast’ by following a low-calorie diet (around 500kcal for women and 600kcal for men) on 2 consecutive or non-consecutive days of the week and then eat a normal diet for the remaining five days a week (feast days).
Interestingly the original 2-day intermitting fasting diet was developed by Dr Michelle Harvie and Professor Tony Howell through their research looking at the preventative effect of calorie restriction in overweight women with a family history of breast cancer. In their version the ‘fasting’ days occur on two consecutive days and 5 days of unrestrictive eating.
Alternate Day Fasting (ADF) or Alternative-Day Modified Fasting
75% energy restriction on ‘fast’ days, alternating with an unrestrictive feed day.
Time-Restricted Feeding (TRF)
16/18/20 hours of fasting leaving 8/6/4 hours of feeding or eating each day.
Fast during the day followed by 1 large meal in the evening.
‘Fast’ for up to 24 hours 1-2 days a week and the rest of the week following the usual diet.
So why follow these diets?
All of these types of diets have 1 thing in common- they aim to encourage a reduced energy intake. Some people believe these diets are simpler to follow compared to other ‘weight loss diets’ and maybe easier too? But how does the science measure up with these diets??
A lot the research has been conducted in people following the 5:2 type of diet which involves following 2 days of fasting where you consume a low-calorie diet 1 (500kcal women, 600kal for men) and then the rest of the week you have your usual meals. Research from several randomised controlled trials (where there are 2 groups matched in their characteristics and then randomised assigned to a test or non-test group) showed both groups limiting their calories through the fasting diet and usual low calorie diet had similar weight loss 2. There also was no difference in other measurements such as blood pressure, blood lipids (cholesterol, HDL, triglycerides) in both groups.
Most research has been completed over a 6-month period and during these studies they received regular support following their diets. There are limited studies testing people on these diets longer term but one study looked at the impact after 12-24 months 3-4. Yet again there was no difference between the groups This time the study also included a ‘week-on-week off’ group, ‘fasting’ group and conventional a low energy group. All groups had similar outcomes in terms of weight loss (they all lost weight) and blood lipids and fasting blood sugar level. Interesting after 24 months all groups regained some of the initial weight they lost but still kept a small amount of weight loss from their starting weight. This does support the idea that ‘diets’ don’t work longer term- maybe longer lifestyle choices may have better long-term benefits.
What about IF diets during cancer treatment?
There have been a few studies conducted in animals to investigate fasting diets and the impact on cancer. Some of these studies showed less gastrointestinal symptoms during cancer treatment (chemotherapy or radiotherapy) but the studies had a very small sample size and was not tested in humans. These are important variables means we cannot assume the same effect will be seen in humans. Therefore, intermitting fasting diets are NOT RECOMMENDED FOR ANYONE UNDERGOING CANCER TREATMENT. These types of diets can promote weight loss as you have seen above, which can lead to malnutrition, which has a negative effect on treatment tolerance. Some oral chemotherapy medications also require you to take them with food so fasting may reduce their effect.
To reduce the risk of cancer or recurrence intermittent fasting type diets may be helpful to some. However, these can only be explored after active treatment and recovery and only under guidance from a medical professional.
Most of the studies discussed provided the study members a lot of support from a dietitian- once a fortnight appointments at least which you could argue adds to their success. Would these results be seen without this? We don’t know.
Research shows that IF diets may help people to lose weight and have positives health improvement (better blood lipids and overall improvements in cardiovascular health) however this was similar to conventional lower calorie diets. There is also a question whether these diets would work as a long-term lifestyle change. We are also missing data that specially looks at whether these diets are risky to our health. As they have simply not explored this question yet.
So the bottom line is these types of restrictive diets may be another tool for weight loss but they are not the ‘magic bullet’ they are perceived to be.
If any Dietitians/ nutritional professionals would like to read the health professional version of this article you can find it here NB. Nutrition Health professionals will need to sign up and then you will receive a password to access the magazine
If you are looking for information on cancer nutrition or diets book your complementary call
1 Sundfør TM, Svendsen M, Tonstad S. Effect of intermittent versus continuous energy restriction on weight loss, maintenance and cardiometabolic risk: A randomized 1-year trial. Nutr Metab Cardiovasc Dis. 2018 Jul;28(7):698-706.
2 Harvie MN, Pegington M, Mattson MP, Frystyk J, Dillon B, Evans G, Cuzick J, Jebb SA, Martin B, Cutler RG, Son TG, Maudsley S, Carlson OD, Egan JM, Flyvbjerg A, Howell A. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. Int J Obes (Lond). 2011 May;35(5):714-27
3 Headland ML, Clifton PM, Keogh JB. Effect of intermittent compared to continuous energy restriction on weight loss and weight maintenance after 12 months in healthy overweight or obese adults. Int J Obes (Lond). 2019 Oct;43(10):2028-2036.
4 Headland, M.L., Clifton, P.M. & Keogh, J.B. Impact of intermittent vs. continuous energy restriction on weight and cardiometabolic factors: a 12-month follow-up. Int J Obes 44, 1236–1242 (2020).