Intermittent Fasting: Everything You Need To Know

Fasting was initially a ritual followed during many festivals. But if you see it in terms of nutrition it is a form of calorie restriction.

However in this case you don’t typically count the calories or decide what type of nutrient to include in your diet. You just abstain from eating for a certain interval of time.

Intermittent fasting is a technique derived from the practice of fasting in order to regulate eating habits and induce the body’s metabolism to make an efficient use of resources and stores. Scientists are looking forward to this as a new weight loss tool.

However this type of fasting does not mean that you have to starve yourself for some period. Lets find out more about this.

What is intermittent fasting?

IF image1A common definition you will find all over the net is : ‘Intermittent fasting is not a diet, it is a pattern of eating.’ And indeed that is the best part about this weight loss intervention. You don’t have to literally count your calories you just have to observe the time you consume it.

Intermittent fasting involves severe calorie restriction (0-25% of individual’s daily calorie intake) for a period of 16-24 hours followed by a normal energy intake for 8-24 hours depending on the type of intermittent fasting you chose.

Intermittent fasting typically involves periods where you don’t eat food followed by periods when you are allowed to eat foods. There is no such restriction of the type of food you eat but eating healthy food is always a plus point. What this technique does is it makes your body utilise the food you consume efficiently.

Also since you are limiting the time you eat, you are in turn limiting your calories. This puts your body in a calorie deficit which forces it to make use of stored fat.

How does fasting work?

It is pretty logical to think that if you are not eating anything you will lose weight eventually. But a number of things occur simultaneously to give you that result. We are typically going to investigate how the old school definition of fasting impacts our body.

An old study  dated back to 1982 pretty much describes everything that goes on in your body during a fast. In this study the fast was to be conducted for 40 days with little or rather negligible calorie intake (we do not advise this We are quoting this study to explain the mechanism of fasting).

However the individual completed 36 days of the fast at the end of which he started experiencing weakness. At the end of 36 days he lost 22.9 % of initial weight. The rate of weight loss initially was 0.9kg per day and gradually stabilised at 0.3 kg per day in the third week.

For the first 18-24 hours of fasting, liver glycogen is depleted. Glycogen is the form in which we store glucose. Protein constitutes 15% of total body energy stores and muscles have glycogen but they cant release glucose immediately into the blood stream. Fat reserves on the other hand constitute 85% of the body’s total energy, thus making them easiest accessible form of energy.

Initially the glucose levels drop and insulin levels drop correspondingly. However glucagon, a hormone that acts exactly opposite to that of insulin is raised. The change in form of energy occurs in three phases:

  • Initial 6 to 24 hours: Glycogen reserves are broken down
  • 2-10 days of fasting: Glucose is produced from protein precursors-amino acids
  • Beyond 10 days of fasting: Fat reserves are used instead of proteins and ketones are produced.

You must be reminded of ketone bodies from low carbohydrate diets.  Along with production of ketone bodies, degradation of fats or lipolysis occurs. Basic calorie requirements of a man weighing 70 kg can be met only by utilisation of fat reserves for 2-3 months.

Another noteworthy change is increase in growth hormone levels. Hunger is said to be an initial obstruction to the attempt to fast, but after 2-4 days of fasting a loss of appetite and sense of well being is reported.

What does this mean?
In fasting initially glucose reserves are utilised followed by production of glucose from amino acids. Then fat reserves are utilised and energy derived from this is used to prepare ketone bodies which then replaces glucose as the primary form of energy.

Drop in insulin while rise in glucagon and growth hormone is seen. Hunger which initially thwarts fasting attempt reduces with time.

Does science back the idea of intermittent fasting for weight loss?

Now intermittent fasting is not as rigorous as fasting continuously for a long period of time but it puts the body into a fasted state for a short interval of time repeatedly.

Gradually the same physical changes occur in your body and finally your body gets used to the periods of fasting and eating. A lot of active research is being conducted on intermittent fasting and it has revealed a number of facts to us.

1.Intermittent fasting burns fat rapidly in normal and overweight individuals

In this study , alternate day fasting (ADF) included a day where only 25% daily energy was consumed followed by a day where you were free to eat anything.  Body weight decreased by 5.2 kg and fat mass decreased by 3.6 kg within 12 weeks. Level of hormones that metabolize fat also improved.

It was concluded that ADF is an effective tool for weight loss and cardio protection in normal weight and overweight individuals.

A clinical trial  investigating the effect of ADF on non obese subjects demonstrated reported an average daily increase in fat oxidation of approximately 15g. Weight loss of around 2.5% was observed.

A review of intermittent fasting protocols was conducted.  This review included alternate day fasting, whole day fasting and time restricted food consumption. ADF trials lasting for 3-12 weeks were found to cause a weight loss of 3-7% and loss of 3-5.5 kg of body fat.

Whole day fasting trials lasting for 12-24 weeks reduced 3-9% of body weight and body fat. Improvement in cholesterol was also observed with intermittent fasting.

Another review study  covering all intermittent fasting trials conducted from 1975 to 2014 reported a weight loss ranging from 2-8% over 6 months increasing gradually followed by average weight loss of 8% after a month of weight maintenance.

Improvement in blood pressure and insulin sensitivity was observed in all trials. Attrition rates were lower than that seen in other weight management techniques.

What does this mean?
Intermittent fasting brings about weight loss of around 2-8% of body weight much of which is body fat.

2.Fasting elevates the level of glucagon and growth hormone in order to get rid of body fat

Glucagon is a hormone secreted by the pancreas whose effect is opposite of that of insulin. It raises glucose concentrations.

Research shows that glucagon levels rise two folds and reaches its peak on 3rd day of fasting.   Glucagon stimulates breakdown of fats and production of ketone bodies during fasting.

Surprisingly growth hormone levels are proven to increase by a whopping 5 fold within 2 days of fasting.  Growth hormone also stimulates breakdown of fats and preserves lean mass.

What does this mean?
Glucagon and growth hormone levels rise with fasting that stimulate breakdown of fats.

3.Fasting alters glucose and fat metabolism

A study  demonstrated that varying duration of fasting alters glucose and fat metabolism with a 70% decline in insulin levels in 24 hours and decrease in glucose. Breakdown of fats was also observed and fat burning potential increases by 50%.

Research also suggests that intermittent fasting on a regular basis reduces insulin resistance and supporting favourably utilisation of glucose reserves.   Fasting also activates genes that increase fat burning potential.

What does this mean?
Studies show that fasting influences glucose and fat metabolism, reduces insulin resistance and activates genes that aid in breakdown of fats.

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4.In alternate day fasting, the time of meal on fast day does not matter

Typically in alternate day fasting, on the day of the fast individuals consume their fast meal at lunch time. Researchers investigated whether change in time of this meal would affect the weight loss outcome.

Three groups were made where all followed alternate day fast but the meal times were either lunch or dinner or three small meals. (The last one typically voids the idea of a fast).

At the end of 8 weeks, the weight loss was similar in all groups- highest being in the dinner group (4.1kg ).  Reduction in fast mass, visceral mass and improvement in cholesterol was seen. It was concluded that since weight loss was similar in all groups, flexibility in meal timing can increase adherence to alternate day fasting.

Combining fasting with weight loss diets results in better weight loss results.

What does this mean?
In alternate day fasting, you do not have to be rigid with meal timings. Research states that meals whether consumed at lunch or dinner or as three meals on fast day delivers similar weight loss results.

5.Alternate day fasting with high fat diet is as effective as alternate day fasting with low fat diet for weight loss

In this study obese individuals were randomly split into high fat group (45% fat) or low fat group (25% fat). Results were almost similar but the high fat group fared slightly better.

The best outcome was that both diets resulted in approximately 7 cm decrease in waist circumference within 8 weeks!

What does this mean?
Alternate day fasting combined with low fat or high fat diet improves weight loss outcomes. The high fat diet option is slightly better.

6.Intermittent fasting combined with calorie restriction improves weight loss and heart health in obese women

Fasting already restricts calories; imagine an additional calorie restriction! Sounds crazy right!? In this study intermittent fasting meant severe calorie restriction for 1 day where calorie intake was in the form of liquid or food.

Body weight decreased more with IF and liquid diet (3.9kg) in comparison to IF with food based calorie restriction (2.5kg). Also a greater reduction in visceral fat was seen in liquid diet (0.7 kg) compared to food based diet (0.3kg).

For those who are brave enough to try this out, yep IF combined with calorie restriction and liquid diet indeed helps to lose more weight.

What does this mean?
Intermittent fasting combined with only liquid meals on fasting days in a way that calories are under control results in better weight loss.

7.Intermittent fasting versus Dieting: Who wins?

IF image 2A review study  was conducted to examine the effectiveness of dieting and change in pattern of eating or intermittent fasting. 3-12 weeks of IF resulted in 4-8 % loss of body weight and daily calorie restriction resulted in a weight loss of 5-8%.

Even fat loss was better with IF (11-16%) compared to dieting (10-20%).  According to researchers these findings suggested that both techniques were equally effective for weight loss but IF even preserved lean mass.

A clinical trial  was conducted to compare effects of IF (25% calorie restriction on 2 days of the week) with daily calorie restriction (6276 kJ 7 days a week). Weight loss was around 6.4 kg for IF and 5.6 kg for daily calorie restriction. Improvement in metabolic parameters was also observed and they were greater in IF group. If definitely proves itself as a great alternative to dieting.

A study  was conducted to compare IF with carbohydrate restriction and daily calorie restriction.  IF with carbohydrate restriction regimen involved 600-650 calories a day with less than 40g of carbohydrate for 2 days every week. Daily restriction involved 25% calorie reduction.  Another IF diet with carbohydrate restriction and zero restriction on protein and fats was also studied.

Weight reductions in IF groups was far better than daily energy restriction and improvement in insulin activity was also observed.

What does this mean?
Intermittent fasting is as effective as regular dieting when it comes to losing weight.

8.Intermittent fasting increases longevity and reduces the risk of other diseases

IF is proven to increase lifespan in mice and humans.   Short term fasting is reported to increase chemotherapy outcome, improve quality of life in cancer patients and reduce side effects of chemotherapy.

Modified versions of fasting are proven to be effective in treating rheumatic diseases, high blood pressure, metabolic syndrome, inflammatory and degenerative conditions as well as brain related disorders.

What does this mean?
Apart from weight loss, IF improves cure rate in cancer, inflammatory conditions as well as brain related disorders.

What are the different techniques of intermittent fasting?

Variations in intermittent fasting are available and you can take a pick from these. There are three basic types:

  • Daily intermittent fasting:  You fast for 16 hours and eat within 8 hour window period. The best time to eat is around 1pm to 8pm so that it doesn’t hamper your social life and you can fast for the rest of the day.
  • Weekly intermittent fasting: You can do this once or twice a week. Lunch can be your last meal on Monday and your next meal is on Tuesday at lunch time. This is only for 1 or days of the week.
  • Alternate day fasting: Dinner on Monday night is your last meal and then you eat nothing till Tuesday night. Then on Wednesday you consume food normally and again begin the fast after dinner.

Nutritionists and researchers have developed different meal plans and workout schedules based on these techniques to suit everyone’s day to day life. Lets go over them one by one.


This is started by Martain Berkhan, a Swedish personal trainer and nutrition writer. This plan is suitable for those who hit the gym regularly.

Fasting is around 14 hours for women and 16 hours for men. Food consumption is in 3 meals during the 8-10 hr window period.

On exercise days, meals should include protein, vegetables, and carbohydrates; on non-exercise days, meals should include protein, vegetables, and fat. If a workout is scheduled for that day, then it should be before the first meal which should be a high protein meal.

What are the advantages and disadvantages of this plan?

The idea of splitting the feeding period into 3 meals gives you flexibility and avoids the pain of consuming extra large meals to meet calorie requirements. However there is a lot of restriction on what to eat and what not to eat.

Eat Stop Eat/ Periodic Fast

This plan is covered by Brad Pilon and also described by Dr. John Berardi in his article. It is a 24 hour fast done typically 1-2 times a week. Brad refers to the fast as a ’24 break from eating’. No food is to be consumed but calorie free beverages are permitted.

This plan limits our calorie intake and prepares our body to make use of fat reserves as form of energy. If resistance training is combined the plan will definitely reap success.

What are the advantages and disadvantages of this plan?
The advantage of this plan is that you can increase the fasting phase gradually till it reaches 24 hours. Also there are no food restrictions. The disadvantage is that going without food for 24 hours is likely to cause irritation, fatigue, headaches etc. So you need to exert self control.

5:2 Intermittent fasting

This one has been described by Dr. Michael Mosley in his book, The Fast Diet: Lose Weight, Stay Healthy, and Live Longer with the Simple Secret of Intermittent Fasting. It involves eating normally for 5 days a week and reducing calorie intake for 2 days.

This is kind of similar to the above plan. On fasting days calorie intake should be around 600 calories for men and 500 for women.

What are the advantages or disadvantages of this plan?
The 500-600 calories on fasting days can be split up into different meals or can be enjoyed at one go. This makes the plan psychologically attractive. However a restriction on carbs should be exercised and this plan might just interfere with your social life.

The Warrior diet

This one is started by Ori Hofmelker. It is a strict plan for highly devoted individuals who have a good sense of self control. This plan is said to be in sync with the human body clock.

Warriors are trained to fast for 20 hours and eat one large meal at night. During the 20 hour fast servings of raw fruit, vegetables and even protein is permitted. The four hour window period helps the body recuperate and assimilate nutrients for growth and repair. This plan stimulates hormones that cause fat burning.

What are the advantages and disadvantages?
This plan permits healthy snacks within the 20 hour fast but eating large meals at one go and especially at night might become difficult for some.

Alternated Day Fast/ UpDayDownDay

This technique is majorly exploited in clinical trials. On one day you consume only 25% of your daily calorie requirements (To find out your calorie requirement check out How many calories are ideal for overweight individuals?) and the next day you consume your regular calories.

On low calorie days you can start initially by consuming meal replacement shakes which can be sipped throughout the day. But that is just for the beginning later you have to start consuming low calorie foods.

What are the advantages and disadvantages of this plan?
Cutting down greatly on calories causes a weight loss of 1-2 pounds in a week. The drawback is that one can tend to binge after such fasting period. Again self control solves this problem.

Fat loss Forever

This plan combines the advantages of most of the above mentioned IF plans. This plan incorporates different fasting protocols over the week with one day’s rest. However that one day of freedom is followed by a 36 hour fast! It also incorporates workouts for better results.

What are the advantages and disadvantages?
A scheduled timetable provided with this plan helps us develop a structure approach towards fasting thus delivering great benefits. However that one day of freedom needs to be utilised with caution.

What is the impact of exercise on intermittent fasting?

A practical thought process would be that can exercising in fasting state be possible? Already your body is coping up with the deficit so would it actually permit exercise or any other training? Now there is mixed evidence available on this one.

One study demonstrates that fasting prior to resistance training will increase the potential of subsequent carbohydrate or protein meal in developing muscle mass.  A number of other studies suggest that short term fasts (even those lasting for 3.5 days) do not affect physical performance.

However long term endurance training is likely to be affected by fasting.

Does intermittent fasting affect performance or cognitive performance?

One thing that is likely to pop up in your mind is that probably not eating for so long is going to affect your mood or disturb your work performance. And this does stand true because hunger is a feeling that makes you irritable.

But in case of intermittent fasting, this rage of appetite control occurs in the initial days after which your body eventually gets used to it.

Interestingly animal studies prove that intermittent fasting improves cognitive performance.  A clinical trial demonstrated that short term fasting, which in this case was little or no food for 24 hours, did not affect attention or immediate memory.

Another study demonstrated that even 2 day calorie deprivation does not seem to affect cognitive performance, activity, sleep or mood.

Are there any drawbacks or precautions linked with intermittent fasting?

Actually its the other way round..Intermittent fasting is proven to reduce the risk of cardiovascular diseases, metabolic syndrome and even improves brain function. However there a few things you should keep in mind:

  • If you are suffering from diabetes or metabolic diseases, consult a doctor before fasting.
  • Nursing and pregnant women should not even think about this.
  • Elderly as well as adolescents should not attempt fasting.
  • Intermittent fasting is all about watching when you eat. However if you don’t keep an eye on as to what you eat, fasting is not going to help.
  • Results differ for men and women. So if you are a female go for wider eating window.


The idea of intermittent fasting disrupts some traditional rules like ‘Dont skip breakfast!’ , ‘Eat three meals a day!’, ‘Eat frequently and regularly to curb hunger’ etc. It does not mean that these rules do not apply any more, they still do.

The thing about fasting is that it does not work for everyone. In that case strictly adhering to old rules should help you. There are different techniques for intermittent fasting and it definitely does not mean that you have to starve yourself. Find one that suits you and make it a habit.

If you want some information on this two good reads are The Fast Diet by Micheal Mosley and Eat Stop Eat by Brad Pilon.

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