What is Fasting?  

Technically speaking, to 'fast' is to "abstain from all or some kinds of food or drink, especially as a religious observance". The term is loosely assigned to any period where an individual goes without food for a number of consecutive hours, such as when we are sleeping. This has paved the way for the loose definitions of 'intermittent fasting' and 'fasted cardio' when really, a fast should only be used for periods without food of 24 hours or more.

What you'll learn:

• What the term 'fasting' means
• The difference between popular fasting approaches
• Assessing the claims about the benefits - from a scientific perspective
• Takeaways - Should I try/recommend it to clients?

What are the different types of intermittent fasting? 

There are a variety of fasting modalities and approaches, and we would be here all day if we were to cover all of them. Instead, let's focus on those that have been used as dietary strategies for either improving health or improving body composition. These include:

• 5/2 fasting (Eat 5 days, fast 2 days)
• Alternate-day fasting (Eat every second day, fast the others)
• Intermittent fasting (Fast regularly for 24 - 36 hours)
• Time-restricted feeding (fast for 12 - 16 hours, eat for 8 - 12)

"Wait, I thought the 16/8 model was called intermittent fasting?"

Is that what you're thinking right now? Not to worry, it is a common misconception, and even I will refer to time-restricted feeding (TRF) as intermittent fasting (IF) at times. However, the distinction is critically important, namely because most if not all of the physiological benefits that can be achieved from 'fasting', are found in studies with a lot longer fasts than 16 hours. Even scarier, although the topic of fasting is exploding, the majority of studies to date are conducted in animals.

Yep, animals!

Think about a two-day fast for a rodent that lives for approximately 1 - 2 months in total, it is more like a year of fasting for us, humans. Of course, their physiology would be impacted to a greater extent for that portion of their survival.

What are the benefits of intermittent fasting? 

Before answering this question, we need to identify for what individual we are considering fasting. Are they looking to lose body fat, increase overall mass, improve performance in a sport, or simply to improve health. My personal recommendations to my clients would be very different depending on which of these categories they fell into.

Weight/fat loss - less time to eat

For weight loss, there have not been any studies to date showing that TRF (16/8) or IF are more beneficial than a normal calorie-restricted diet when the calories are matched. This tells us that the main way in which people successfully lose weight on these plans is by achieving a caloric deficit. Why? Well, simply put, allocating only 8 hours in a day for eating tends to reduce the daily calorie intake for most people.

Unfortunately, it's not the case for everyone, and some people will be unsuccessful in their weight loss, or may even gain weight. This can occur when calories are not loosely tracked and the individual mindlessly consumes food when their "window is open", out of fear they will be hungry "when it closes". We will discuss this topic a bit further down.

Muscle preservation while dieting

One of the inescapable phenomenons about weight loss is the loss of muscle that will occur alongside fat. This ultimately reduces the amount of energy an individual burns on a daily basis and can be a major reason for weight loss failure, dropout, rebounding, etc. Time-restricted feeding has been put forward by a couple of studies, researchers claiming that when fasting for longer durations (not 16/8) the body preserves muscle as a survival mechanism. Again, this would not likely occur in a 16/8 model and a study showed exactly that here (Source: NCBI).

Blood glucose & Insulin

Another intriguing finding that has cropped up in fasting trials is slight improvements in both fasting glucose (amount of sugar circulating in the blood when fasted) and insulin responsiveness (the response of insulin to carbohydrate intake). As insulin resistance, pre-diabetes and diabetes are major global health issues right now, these measures are valid markers of long-term health.

Again, there haven't been a huge amount of studies assessing these markers, but one study that compared an early-morning to a late-night fast, found that both modalities led to slightly improved blood glucose levels.

Appetite

This is one of the primary reasons that TRF has become widely popular in diet culture. Many individuals will claim that their appetite is much more manageable in TRF than a traditional diet, even when calories are matched. There are a few reasons for this, namely that our bodies are operating off of fatty acids (fat) as they have depleted the stored carbohydrate (while we slept) in our liver. When we are operating off of fatty acids, we tend to have lower hunger levels than when we are burning carbohydrates. One study also showed a decrease in circulating ghrelin, the hormone that makes us hungry, when fasting.

Psychological Factors

People enjoy TRF. I enjoyed TRF. That is all there is to it. It is definitely a more entertaining approach to reducing caloric intake on a regular basis. This can make successful weight loss achievable, and if it works for you or your client in this regard, there is nothing wrong with that. Another widely reported experience is improved focus and concentration in the mornings prior to the initial meal. These anecdotal reports are most likely due to the steady blood sugar levels as your body is not dealing with ingested carbohydrates.

So should I or my client try Intermittent fasting?

As mentioned above, this is a decision that is entirely up to you. The 16/8 model is not necessarily a dangerous or risky diet, such as the ketogenic or carnivore diets can be, so there is not much concern there. However, longer-term fasts should be done under the guidance or supervision of a professional.

Here are some tips for trying out a fasting diet -

• Start small - Eat and fast for 12/12 hours for a week. See how it feels, this might be all you need to achieve the progress you want. If not, and you feel ready to step it up, widen your fasting window by 1 - 2 hours max. Try this out for a brief period of 1 - 2 weeks

• Break up the morning - The idea of 'fasting' is to not consume energy (kCals) that will cause an insulin spike and downregulate the burning of fatty acids for energy. You can still enjoy black coffee, mints, chewing gum, zero-calorie drinks in moderation to help break up your morning.

• Plan your meals - If doing TRF, it is likely you will be consuming only 1 - 2 meals each day. That means that it is crucial to ensure you are getting sufficient protein, carbohydrates and fats into each meal. Tracking for the initial couple of days can be a useful method of assessing where you currently are, and which meals you might need to add in a serving of protein, for example.

• Hydrate - One common mistake when people are undertaking long-term fasts is to overlook hydration. Make sure you are getting plenty of fluids, and not just that, but electrolytes too. If you are fasting for 24 hours or more, you need to ensure that you're getting some sodium, potassium, magnesium and the other electrolytes through a fluid mix.

• Seek professional guidance - Please please don't a long-term (>24 hours) fast without some form of professional guidance from a nutritionist, dietician or other qualified individuals. For TRF, you may not need to seek out guidance, but it can never hurt and will improve your understanding.