OMAD & 20:4 intermittent fasting: 6 things you need to know

By The Gut Health Doctor Team

Dr Megan Rossi in a lab looking through a microscope

You’ve probably heard of intermittent fasting, such as “the 5:2 diet”, 16:8 or alternate day fasting – but recently two new types of fasting have been in the headlines, termed “OMAD” and “20:4” intermittent fasting.


So what’s it all about and what does the science say?

The many versions of intermittent fasting are all designed to limit eating to set periods during the day and fasting for the rest (aka time-restricted eating).

OMAD stands for ‘one meal a day’ and is one of the most restricted forms of fasting, whereby you usually eat only one single meal all day in less than an hour (i.e. in a 24-hour period).

The 20:4 diet has been given a few different definitions over the years, but most recently it’s been defined as limiting your eating time to a 4 hour window during the day (e.g. 4pm – 8pm) and eating freely during that period, and fasting for the rest of the day.

Advocates support these types of fasting for helping with weight management and improving overall health – however, studies on OMAD and 20:4 so far are very limited (especially in humans) and the findings on fasting overall are somewhat mixed.


Here are 6 things to know based on the research so far…

1. There is some evidence from animal studies that some types of intermittent fasting (typically less restrictive) could have some benefit on metabolism, via our GM (gut microbiota, the trillions of microbes in our gut) – although we can’t necessarily translate this directly to humans (we’re very different!)


2. It would be tricky to get all your daily nutrition needs from just one meal on OMAD fasting, which could increase the risk of nutritional deficiencies that may come with further health implications. This goes for the all-important fibre goodness too, whereby not getting enough could also lead to constipation and poor gut health, among many other chronic conditions in the long term.


3. There are likely to be other potential side effects too, as having only one meal a day can also make people feel irritable, dizzy, extremely hungry, fatigued and have difficulty concentrating (as you’d probably expect! Hanger, anyone?)


4. One small human study, on a group of men and women aged 40-50 years old, suggested that eating one meal a day between 5pm – 9pm (with the same calories as three meals a day) could negatively impact morning blood sugar levels and delay insulin response, which is linked with increased risk of type 2 diabetes. This wasn’t a long-lasting effect and returned to original levels when they went back to three meals a day – suggesting spreading our food intake throughout the day (versus an evening feast) is perhaps a better option for most.


5. When it comes to weight management, there is some evidence for intermittent fasting, largely based on the overall reduction in calorie intake as there’s less opportunity to eat. It’s worth knowing that studies suggest there may not be any significant difference in results of intermittent fasting compared to simply managing overall energy intake across the day. 


6. Research has suggested this type of restricted eating could increase the chances of developing disordered eating habits too, so it’s important to be careful and keep an eye on how it impacts your relationship with food if you do want to try any form of fasting.


The bottom line

Time-restricted diets aren’t something I generally recommend (although there may be some instances where a less extreme version, such as 16:8 i.e. fasting for 16 hours each day, could be suggested on a case-by-case basis in clinic). Because we certainly are all different. If it’s support with weight management you’re after, it’s worth looking for a sustainable – and enjoyable! – balanced option or having a chat with your healthcare provider to find what works for you personally.

If you’re considering any type of intermittent fasting, or already find it works well for you, try to make sure you’re still getting all the nutrients your body needs and 30g fibre a day to keep your gut microbes well fed.

For those with IBS, particularly if you’re susceptible to loose poops and bloating, it’s a good idea to spread your meals out as much as possible instead of large loads in restricted periods.


Taking medication?

Always check with your GP or physician, as longer fasts may affect them. 


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