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Intermittent Fasting: A Dietitian's Review

One of the key difficulties with dieting and nutrition overall for many is trying to make sense of all the information available and determining what is right for you. Rather than giving yourself new food rules and choices to make, just eating within a specific time- thinking what when rather than what can seem so simple. For this reason, it’s easy to see why intermittent fasting (IF) has become so popular.

IF involves periods of alternating fasting and feasting. The fasting period typically lasts anywhere from 12 to 24 hours to be considered an intermittent fast. This is where some set rules do actually come into play.

There are two main types of IF:

  • Alternate day fasting: this is where you restrict your intake and fast for a few days per week. A common version of this is the 5:2 diet, where there are two non-consecutive days that you consume very low calories, and then eat normally for the remaining 5 days.

  • Time restricted feeding: this is where you shorten your eating window. By far the most common example, and often easiest to fit into someone’s day is the 16:8 diet. This involves an 8-hour window where you may eat and a 16 hour fast.

So, let’s get to the reason most people ask me about IF…weight loss

Weight Loss

To lose weight, you need to consume less energy than you burn to maintain your current weight and activity levels.

Generally, restricting your eating window, or overall intake may result in an overall calorie reduction, and so, weight loss. Quite consistently, IF has been found to be comparable to “traditional” calorie reduced diets when it comes to weight loss- not being any superior, faster, or easier to adhere to. This is also consistent in terms of measurements such as waist and hip circumference, and fat loss. The current findings regarding loss of fat free mass, including muscle varies.

In terms of adherence to IF does not seem to be superior and the evidence is mixed. Some studies have reported increased hunger reported by those following IF diets compared to standard calorie reduction, but even this isn’t consistent. Like with anything diet and lifestyle related, it very likely depends on and individual’s preferences and habits.

For some, the ideas of shortening their eating window works well and tends to fit their personal preferences e.g., those that feel unwell at the thought of eating breakfast (which I have written about before). For others, the delayed start, or even rules are eating times means they are hungrier at eating times and struggle the manage portions or may compensate the next day. In these cases, eating more regularly and portioning those options would be better suited.

IF may make weight loss easier for some, but more difficult for others. Unsurprisingly, it seems to come down to preference as IF stacks up comparably to standard calorie reduced diets for weight loss.

Chronic Diseases and Metabolic Markers

In studies looking at metabolic health including markers like glucose control, lipid profiles (including cholesterol and triglyceride levels), as well as blood pressure, improvements are often observed. The catch is, these improvements have not been found to be uniquely different to that of a calorie reduced diet.

These changes do relate to reducing risk of developing type 2 diabetes and heart disease (though only the factors that increase risk, not heart disease itself has been studied). As with weight loss however, the results of IF has resulted in comparable changes in these markers, not making it a superior option.

Interestingly, the value of eating in line with our circadian rhythm, which may follow an IF time restricted feeding pattern (finishing eating earlier in the day) has been associated with improvements in glucose control, reducing risk of Type 2 Diabetes, and lowering blood pressure. This evidence is not only IF-specific however, in that the area of ‘chrono nutrition’ is fast growing, which supports not eating into “biological night” and the associated risks of shift workers for developing chronic diseases like diabetes and heart disease.

When it comes to risk of type 2 diabetes and heart disease, following an IF-style diet may help in reducing overall risk, however, following general healthy eating principles in a calorie reduced diet can do the same, and may suit some better.


This is one of the most interesting and eye-catching arguments that IF supporters will make.

In animal studies, there is some evidence that any form of calorie restriction (including intermittent fasting) can result in slowing the ageing process and extend the lifespan. This is partially related to autophagy, a process triggering the body to bump up regenerative process of cell repair and recycling damaged cells. However, not only are these studies only applicable to the animal models they’ve been tested on, they also don’t apply exclusively to IF, but rather calorie reduced diets in general.

Really, far more research is required, not only in the “life extending” benefits that IF is so sought after for humans, but also in comparing IF and general calorie restriction for the same.

Digestive Health

The research in this area is even more limited than the others. Meaning: despite how it may be sold to you, there is nothing yet supporting IF to solve your digestive woes. In fact, it may worsen digestive issues like IBS with the possibility of larger meal sizes, FODMAP stacking and increased stresses around your food choices often being culprits for worsening symptoms.

Anecdotally, many who suffer from IBS-like symptoms including bloating and abdominal pain may report improvements when fasting. However, this may be due to removing common triggers, adjusting their meal timing or just an overall reduction in intake. There are many other options to trial before starting a restrictive diet.

In those with chronic conditions like Inflammatory Bowel Disease (IBD), there is already a risk of nutrient deficiencies and so, further limiting your nutrient intake, particularly during a flare could put you at greater risk here. When it comes to the microbiome itself, IF hasn’t been linked to significant changes, and so, again, this area would also need further research

If you have chronic digestive condition, IF may put you at risk of worsening symptoms and nutrient deficiencies. If it is something you would like to trial, I strongly encourage working with a Dietitian and your health care team here. Overall, there is much more evidence needed in this space to support the talk regarding IF and digestive health.


From a nutrition standpoint, it can be difficult to meet all your nutrient requirements consistently when following an IF pattern and having less eating occasions during the day. Many struggle to do this when eating “normally”, while this is further emphasised when in a calorie deficit and intake is restricted, so is not unique either.

IF doesn't specify your food choices and diet quality, and this is still an important consideration for health.

If looking to undertake an IF-style diet, I would recommend consulting with a health professional to guide you in planning your days.

Who shouldn't fast?

IF does not suit everyone, and that’s OK.

If you are pregnant or breastfeeding, have a history of disordered eating or a poor relationship with food, you are younger than 18 or older than 75, have a complex medical history or are on medications that may cause low blood sugar, then IF is not recommended for you.

If you have poor glycaemic control, fasting has been linked with worsened glucose response, instead having a steady intake through the day is more appropriate.

Finally, if you like breakfast, the idea of small, frequent meals or don’t like food rules…IF isn’t for you either. I think this is most important to consider. Even if there were evidence proving IF extended life a little, or made weight loss a little easier, if it does not suit you and makes your life very uncomfortable, there is no benefit and there are other options.


IF has been linked to reducing weight and the risk of chronic diseases like diabetes. However, this has not been found to be a superior method over general healthy eating and a calorie reduced diet.

As with any way of eating, which is very personal, it really seems to be suitable for some and not others. So, do whatever suits you best and keeps you well physically, mentally, and emotionally.


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