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  • Writer's pictureMatt Crowe

A Complete Guide To Intermittent Fasting

Updated: May 6, 2023

A phenomenon called intermittent fasting (IF) is currently one of the world's most popular health and fitness trends. Whilst research is really only in early stages many studies are showing that intermittent fasting can have significant benefits including weight & fat loss, improved metabolic health, protection against disease, improved focus & concentration and it can even help you live longer!

Is it true? Does it work? Is it healthy? How do you do it? What is the best method?

All good questions so let’s take a deep dive into all you need to know about Intermittent Fasting!

What Is Intermittent Fasting?

Intermittent fasting is an eating pattern where you cycle between periods of eating and periods of fasting (not eating anything).

It does not say anything about which foods to eat, but rather when you should eat them.

There are several different intermittent fasting methods, all of which split the day or week into eating periods and fasting periods. Most people already "fast" every day, while they sleep. Intermittent fasting can be as simple as extending that fast a little longer. You can do this by skipping breakfast, eating your first meal at noon and your last meal at 8 pm.

Then you're technically fasting for 16 hours every day, and restricting your eating to an 8-hour eating window. This is the most popular form of intermittent fasting, known as the 16/8 method.

No food is allowed during the fasting period, but you can drink water, coffee, tea and other natural non-caloric beverages.

Why Fast?

Humans have actually been fasting for thousands of years. Sometimes it was done out of necessity, when there simply wasn't any food available. In other instances, it was done for religious reasons. Humans and other animals also often instinctively fast when sick.

Clearly, there is nothing "unnatural" about fasting, and our bodies are very well equipped to handle extended periods of not eating. All sorts of processes in the body change when we don't eat for a while, in order to thrive during periods of famine.

11 Evidence-Based Health Benefits of Intermittent Fasting

1. Intermittent Fasting Changes The Function of Cells, Genes and Hormones

When you don't eat for a while, several things happen in your body.

·Insulin levels: Blood levels of insulin drop significantly, which facilitates fat burning.

·Human growth hormone: The blood levels of growth hormone may increase as much as 5-fold. Higher levels of this hormone facilitate fat burning and muscle gain, and have numerous other benefits.

· Cellular repair: The body induces important cellular repair processes, such as removing waste material from cells.

· Gene expression: There are beneficial changes in several genes and molecules related to longevity and protection against disease.

2. Intermittent Fasting Can Help You Lose Weight and Belly Fat

Many of those who try intermittent fasting are doing it in order to lose weight. Putting all the science aside intermittent fasting is at its heart another way to reduce calorie intake throughout the day. When you eat fewer meals there is a high likelihood you will eat less calories! Some people may compensate by eating more during the other meals but ideally you can maintain your usual meal quantities!

Additionally, intermittent fasting enhances hormone function to facilitate weight loss. Lower insulin levels, higher growth hormone levels and increased amounts of norepinephrine (noradrenaline) all increase the breakdown of body fat and facilitate its use for energy. A further advantage of this process is that short-term fasting actually increases your metabolic rate helping you burn even more calories.

In other words, intermittent fasting works on both sides of the calorie equation. It boosts your metabolic rate (increases calories out) and reduces the amount of food you eat (reduces calories in).

All things considered, intermittent fasting can be an incredibly powerful weight loss tool. However, keep in mind that the main reason for its success is that intermittent fasting helps you eat fewer calories overall. If you binge and eat massive amounts during your eating periods, you may not lose any weight at all.

3. Intermittent Fasting Can Reduce Insulin Resistance, Lowering Your Risk of Type 2 Diabetes

Type 2 diabetes has become incredibly common in recent decades. Its main feature is high blood sugar levels in the context of insulin resistance. Anything that reduces insulin resistance should help lower blood sugar levels and protect against type 2 diabetes.

In human studies on intermittent fasting, fasting blood sugar has been reduced by 3-6%, while fasting insulin has been reduced by 20-31%.

What this implies, is that intermittent fasting may be highly protective for people who are at risk of developing type 2 diabetes.

4. Intermittent Fasting Can Reduce Oxidative Stress and Inflammation in The Body

Oxidative stress is one of the steps towards aging and many chronic diseases. It involves unstable molecules called free radicals, which react with other important molecules (like protein and DNA) and damage them.

Several studies show that intermittent fasting may enhance the body's resistance to oxidative stress. Additionally, studies show that intermittent fasting can help fight inflammation, another key driver of all sorts of common diseases.

5. Intermittent Fasting May be Beneficial For Heart Health

Intermittent fasting has been shown to improve numerous different risk factors, including blood pressure, total and LDL cholesterol, blood triglycerides, inflammatory markers and blood sugar levels.

However, a lot of this is based on animal studies. The effects on heart health need to be studied a lot further in humans before recommendations can be made.

6. Intermittent Fasting Induces Various Cellular Repair Processes

When we fast, the cells in the body initiate a cellular "waste removal" process called autophagy. This involves the cells breaking down and metabolizing broken and dysfunctional proteins that build up inside cells over time.

Increased autophagy may provide protection against several diseases, including cancer and Alzheimer's disease.

7. Intermittent Fasting May Help Prevent Cancer

Fasting has been shown to have several beneficial effects on metabolism that may lead to reduced risk of cancer. Although human studies are needed promising evidence from animal studies indicates that intermittent fasting may help prevent cancer.

There is also some evidence on human cancer patients, showing that fasting reduced various side effects of chemotherapy

8. Intermittent Fasting is Good For Your Brain

Several studies in rats have shown that intermittent fasting may increase the growth of new nerve cells, which should have benefits for brain function.

It also increases levels of a brain hormone called brain-derived neurotrophic factor (BDNF), a deficiency of which has been implicated in depression and various other brain problems. Animal studies have also shown that intermittent fasting protects against brain damage due to strokes.

9. Intermittent Fasting May Help Prevent Alzheimer's Disease

There is no cure available for Alzheimer's, so preventing it from showing up in the first place is critical. A study in rats shows that intermittent fasting may delay the onset of Alzheimer's disease or reduce its severity.

In a series of case reports, a lifestyle intervention that included daily short-term fasts was able to significantly improve Alzheimer's symptoms in 9 out of 10 patients.

10. Intermittent Fasting May Extend Your Lifespan, Helping You Live Longer

One of the most exciting applications of intermittent fasting may be its ability to extend lifespan. Studies in rats have shown that intermittent fasting extends lifespan in a similar way as continuous calorie restriction.

In some of these studies, the effects were quite dramatic. In one of them, rats that fasted every other day lived 83% longer than rats who weren't fasted.

Although this is far from being proven in humans, intermittent fasting has become very popular among the anti-aging crowd. Given the known benefits for metabolism and all sorts of health markers, it makes sense that intermittent fasting could help you live a longer and healthier life.

11. Makes Your Healthy Lifestyle Simpler

Eating healthy is simple, but it can be incredibly hard to maintain.

One of the main obstacles is all the work required to plan for and cook healthy meals. Intermittent fasting can make things easier, as you don't need to plan, cook or clean up after as many meals as before.

For this reason, intermittent fasting is very popular among the life-hacking crowd, as it improves your health while simplifying your life at the same time.

Types of Intermittent Fasting

Intermittent fasting has become very trendy in the past few years, and several different types/methods have emerged.

Here are some of the most popular ones:

The 16/8 Method: Fast for 16 hours each day, for example by only eating between noon and 8pm. This can be as simple as not eating breakfast and only eating from 12pm-8pm where you have 2 or 3 meals.

Like all IF diet plans it is very important to eat mostly healthy foods during your eating window. This won't work if you eat lots of junk food or excessive amounts of calories. In my experience this is easily the most natural and ‘doable’ IF method and works extremely well.

Eat-Stop-Eat: Once or twice a week, don't eat anything from dinner one day, until dinner the next day (a 24 hour fast). Start with one day and move to a maximum of two.

The 5:2 Diet: Eat normally for 5 days and then for 2 days of the week, eat only about 500–600 calories. Ensure rest of the week is quality eating as always.

Alternate-Day Fasting: Fast every other day: Alternate-Day fasting means fasting every other day. There are several different versions of this. Some of them allow about 500 calories during the fasting days.

With this method, you will be going to bed very hungry several times per week, which is not very pleasant and probably unsustainable in the long-term.

The Warrior Diet: Fast during the day, eat a huge meal at night: Eat small amounts of raw fruit and vegetables during the day, then eat one huge meal at night. Basically, you "fast" all day and "feast" at night within a 4 hour eating window.

This diet also emphasizes food choices that are quite similar to a paleo diet - whole, unprocessed foods that resemble what they looked like in nature

Spontaneous Meal Skipping: Skip meals when convenient: You don't actually need to follow a structured intermittent fasting plan to reap some of the benefits. Another option is to simply skip meals from time to time, when you don't feel hungry or are too busy to cook and eat.

Skipping 1 or 2 meals when you feel so inclined is basically a spontaneous intermittent fast.

 Intermittent Fasting Safety and Side Effects

Hunger is the main side effect of intermittent fasting. If you have a medical condition, you should consult with your doctor before trying intermittent fasting.

This is particularly important if you:

· Have diabetes.

· Have problems with blood sugar regulation.

· Have low blood pressure.

· Take medications.

· Are underweight.

· Have a history of eating disorders.

· Are a woman who is trying to conceive.

· Are a woman with a history of amenorrhea.

· Are pregnant or breastfeeding.

All that being said, intermittent fasting has an outstanding safety profile. There is nothing dangerous about not eating for a while if you’re healthy and well-nourished overall.

Frequently Asked Questions

Here are answers to the most common questions about intermittent fasting.

1. How Long Do I stay on Intermittent Fasting?

Intermittent fasting can be a safe and effective continual lifestyle choice if you choose.  You can stick to one plan only and do periods of on/off such as one month on and one month off or you can swap and change between different IF plans. There are no set rules so experiment, see what fits into your preferred lifestyle plan and achieves the desired results.

2. Can I Drink Liquids During the Fast?

Yes, water, tea, coffee and other non-caloric beverages are fine. Do not add sugar to your coffee. Small amounts of milk or cream may be okay. Coffee can be particularly beneficial during a fast, as it can blunt hunger.

3. Can I Take Supplements While Fasting?

Yes. However, keep in mind that some supplements like fat-soluble vitamins may work better when taken with meals.

4. Can I Work out While Fasted?

Yes, fasted workouts are fine and can even be beneficial for fat loss as your body uses fat for fuel more readily due to the fasted reduced glycogen state you start the workout in! But this can also work in reverse if you do not have enough energy to perform at usual levels meaning you don’t burn as many calories as usual and have a reduced EPOC (post exercise calorie burn) because the intensity was not high enough. Bottom line – trial & error!

5. Should Kids Fast?

Allowing your child to fast is probably a bad idea.

Dispelling the Myths

For many decades there have been commonly held beliefs amongst the greater society in regard to nutrition that are simply not supported by scientific research. Intermittent fasting highlights a few of these!

1. Skipping Breakfast Will Make You Fat

"Breakfast is the most important meal of the day." There is an ongoing myth that there is something "special" about breakfast.

People believe that breakfast skipping leads to excessive hunger, cravings and weight gain.

Although many observational studies have found statistical links between breakfast skipping and overweight/obesity, this may be explained by the fact that the stereotypical breakfast skipper is less health-conscious overall.

In a recent scientific study published in 2014 that compared eating breakfast vs skipping breakfast in 283 overweight and obese adults the results were clear. After a 16-week study period, there was no difference in weight between groups.

Breakfast is definitely an individual preference topic and what works for some people will not work for others. There are other factors to consider as well including work and training performance when skipping breakfast but there is little doubt that breakfast is not essential and there is nothing "magical" about it.

2. Eating Frequently Boosts Your Metabolism

"Eat many, small meals to stoke the metabolic flame." For as long as I can remember everyone has been taught to believe that eating more meals leads to increased metabolic rate, so that your body burns more calories overall.

This is simply not supported by clinical science.

It is true that the body expends a certain amount of energy digesting and assimilating the nutrients in a meal. This is termed the thermic effect of food (TEF), and amounts to about 20-30% of calories for protein, 5-10% for carbs and 0-3% for fat calories.

On average, the thermic effect of food is somewhere around 10% of the total calorie intake. However, what matters here is the total amount of calories consumed, not how many meals you eat.

Eating six 500-calorie meals has the exact same effect as eating three 1000-calorie meals. Given an average thermic effect of 10%, it is 300 calories in both cases.

This is supported by numerous feeding studies in humans, showing that increasing or decreasing meal frequency has no effect on total calories burned.

3. Eating Frequently Helps Reduce Hunger

Some people believe that snacking helps prevent cravings and excessive hunger. Interestingly, several studies have looked at this, and the evidence is mixed.

If snacking helps you experience fewer cravings and makes you less likely to binge, then it is probably a good idea. However, there is no evidence that snacking or eating more often reduces hunger for everyone. This is another individual preference debate.

In my experience this misconception about eating regularly to reduce hunger and boost metabolism can result in unnecessary calorie consumption. The real benefit in snacking is allowing the intake of high quality vitamins and minerals from foods that are not typically eaten at main meals such as quality fruits and nuts.

4. The Brain Needs a Constant Supply of Glucose

Some people believe that if we don't eat carbs every few hours, that our brains will stop functioning. This is based on the belief that the brain can only use glucose (blood sugar) for fuel.

If that were true, then humans would have become extinct a long time ago because carbohydrates were not typically a main food source for the first couple of million years!

The body can easily produce the glucose it needs via a process called gluconeogenesis or used stored liver glycogen for many hours.

In contrast to our brains needing glycogen to perform properly, there has been a recent trend of individuals going on high fat diets (ketogenic diets) and doing prolonged fasting periods (24-48hrs) to allow the body to ‘feed the brain’ on ketones produced from dietary fats. The observational evidence being that concentration and focus is enhanced in this state and individuals are more productive.

Give it a try if you wish but the research is still not there on this!

5. Fasting Puts Your Body in "Starvation Mode"

One common argument against intermittent fasting is that it can put your body in "starvation mode." According to the claims, not eating makes your body think it is starving, so it shuts down its metabolism and prevents you from burning fat.

It is actually true that long-term weight loss can reduce the amount of calories you burn. This is the true ‘starvation mode’ (the technical term is adaptive thermogenesis). This is a real effect, and can amount to hundreds of fewer calories burned per day.

However, this happens with weight loss no matter what method you use. There is no evidence that this happens more with intermittent fasting than other weight loss strategies.

In fact, the evidence actually shows that short-term fasts increase metabolic rate. This is due to a drastic increase in blood levels of norepinephrine (noradrenaline), which tells the fat cells to break down body fat and stimulates metabolism.

The key here is obviously short term fasting is good but more than 2-3 days and the body will start to slow the system to enhance survival so limit your fasting bouts accordingly.

6. The Body Can Only Use a Certain Amount of Protein Per Meal

There are some who claim that we can only digest 30 grams of protein per meal, and that we should eat every 2-3 hours to maximize muscle gain.

Studies do not show a difference in muscle mass if you eat your protein in more frequent doses. The most important factor for most people is the total amount of protein consumed, not how many meals it is spread over.

7. Intermittent Fasting Makes You Lose Muscle

Some believe that if we fast, that our bodies will start burning muscle and using it for fuel. It is true that this happens with dieting in general, but there is no evidence that this happens more with intermittent fasting than other methods.

In fact, some studies even suggest that intermittent fasting is better for maintaining muscle mass.

Intermittent fasting is becoming popular among many bodybuilders, who find it to be an effective way to maintain high amounts of muscle with a low body fat percentage.

8. Intermittent Fasting is Bad For Your Health

Some people think that fasting can be downright harmful, but nothing could be farther from the truth. As shown previously, numerous studies show that intermittent fasting, and intermittent calorie restriction, can have incredible health benefits. 

For example, intermittent fasting changes the expression of genes related to longevity and protection against disease, and has been shown to prolong lifespan in test animals.

It also has major benefits for metabolic health, such as improved insulin sensitivity, reduced oxidative stress and inflammation, a reduction in various risk factors for heart disease and improved brain health.

9. Intermittent Fasting Makes You Overeat

Some claim that intermittent fasting won't cause weight loss, because it causes you to overeat during the eating periods. This is partly true. After a fast, people automatically tend to eat a little bit more than if they hadn't been fasting.

In other words, they compensate for the calories "lost" during the fast by eating more during the next few meals. However, most studies show that even if you do eat a little more following the fasted period your calorie deficit still significantly outweighs the extra calorie consumption.

Some studies show an average of about a 2000 calorie deficit over a 2 day period split between 24hrs of fasting and 24 eating. This is very significant.

The Verdict

As an ‘evidence based only’ fitness professional I find any diet that is not a good lifelong eating plan difficult to support but the scientific evidence behind intermittent fasting is definitely mounting. What is different about IF is that this is an eating pattern as opposed to a food diet.

The benefits are derived from when you eat not what you eat and as an exercise scientist I can not ignore the evidence. Putting aside the fat loss component IF does seem to have some very significant benefits for a variety of health factors. Not the least of which is that it may actually help you live longer!

In regards to fat loss I believe the real benefit comes down to individual make up or mindset because at the end of the day IF (like every diet ever) is about calorie restriction. If you are highly disciplined and can continually restrict your calorie intake every day and eat the standard 3 square meals than you will be able to lose weight or maintain a healthy weight throughout your life. In fact most studies show very little difference between standard calorie restricted diets and IF in regard to weight loss.

If though, like most long-term overweight people, you need to believe you are doing something ‘special’ to motivate you and discipline you to restrict calorie intake than IF may be ideal for you. You will not only get the benefits of reduced calories in but also the proven extra bonus of periods of higher metabolism meaning more calories out!

Important – no matter if what IF plan you choose it is essential to remember that the usual nutrition rules apply. Quality of food is always the key meaning you should be eating whole real foods wherever possible such as fruits, vegies, grains, lean meats and healthy fats.

Intermittent fasting is not something that anyone needs to do.

It’s simply one of many lifestyle strategies that can improve your health. Eating nutritious whole real foods, exercising and getting quality sleep are still the most important factors to focus on.

Start with the 16/8 plan and if you find it to be a sustainable way of eating try to do one day per week of 500-600 calories to enhance your weight loss results. It is ok to cycle in and out of the IF plan and mix up your IF methods.

Bottom line is Intermittent fasting can be a very powerful tool to lose weight and improve your health and there is no reason why if done sensibly a good IF plan cannot be sustainable for life.

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