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We cannot pinpoint the beginning of fasting, as it has been a part of several cultures since time immemorial. The last few decades have seen the rise of diets like the Paleo diet, the Mediterranean diet, the South Beach diet, and the Atkins diet, each with its unique practices and guidelines. The current trend is Intermittent Fasting (IF). Before jumping on the IF bandwagon, let us dig deeper into this trend. 
What is Intermittent Fasting (IF)?

Intermittent Fasting (IF) involves alternating between periods of fasting and eating. IF entered the mainstream because of the TV documentary Eat Fast, Live Longer (2012) [1]. However, fasting is nothing new and has been a part of various cultural and religious traditions for centuries. [2] 
There are various methods for practicing intermittent fasting:

The 5:2 or Fast diet involves five days of eating normally and two days of fasting. The 5:2 diet incorporates a planned intake of around 500-600 calories on fasting days.  
Eat Stop Eat diet involves a 24-hour fast once or twice a week.
Time-restricted fasting is the most commonly followed IF approach. There is a broad palette for this type of IF. To get the best results, it is recommended that people fast for more than 13 hours. For example, the Warriors’ diet, or the 20:4 fasting method, involves 20 hours of fasting and four hours of normal eating. Similarly, variations like 12:12, 14:10, 15:9, and 16:8 exist.

16:8 Intermittent Fasting 
In 16:8 IF, you chow down on your regular meals in an eight-hour time frame and say ‘Thanks, but no thanks’ the other 16 hours of the day. Piece of cake, right? You will likely skip one meal per day on this diet. You can consume zero-calorie beverages like green tea and water during fasting. Most people follow an 11–7 eating timeframe.
However, you can choose your own routine based on the following factors:

Are you a morning person, or do you need coffee to kickstart your engine?
Are you grumpy when you skip your breakfast?
Do you hit the sack early or stay up till late?
Do you start your day with an intense workout?

People who generally sleep late can start their eight-hour timeframe later. On the other hand, individuals who cannot function without breakfast can skip dinner and have breakfast. Keeping yourself busy can take your mind off food. This can help align your fasting window with your working schedule.
Read more on 16:8 Intermittent Fasting 7-Day Meal Plan

How Do You Fast?
Pick your eight-hour slot. As mentioned earlier, it depends on your lifestyle and preferences. Once you cross this task off your list, you can light the fuse on your new food groove and try to stick to it. If it doesn’t work for you, experiment with different time slots until you find the most sustainable one. 
The golden rule is to eat strictly within your chosen eight-hour timeframe. Remember, it does not mean you must eat all three or four meals in the eight-hour window. Skipping a meal that now lies in the eight-hour fasting window can help you enter a calorie deficit.
Whether you do this for a few days or all the days of a week is up to you. That said, this diet is not for children, pregnant women, or people with eating disorders. [3] 
How Does Intermittent Fasting Work?
The primary mechanism behind intermittent fasting is ketosis. Our body depends on glucose from food for energy. When we fast, the primary source of energy is systematically depleted. So our body turns to the glycogen stored in our liver and muscles for energy, which also runs out in a few days. At this point, the liver breaks down the fat stored in the body into ketones to use as fuel. [4]
When we fast regularly, our body reduces reliance on glucose for energy, and fat gets burned faster. Ketones enhance satiety and fat metabolism. All of this contributes to better weight loss and health.
How Many Calories Should You Consume?
There are no dietary restrictions on this diet regimen. This diet is not about what you eat; it’s about when you eat. You can continue eating what you usually eat. But since most people start this diet to lose weight or become healthier, it is better to stop eating processed or junk food.
IF is not a dry fast. You must keep yourself hydrated throughout the day. To maintain energy levels, you can have low-calorie beverages like green tea or other herbal teas and unsweetened black coffee. 
For Weight Loss
If losing weight is your endgame, it is smart to keep tabs on your calorie intake. To lose weight, you should burn more calories than you consume. That’s a no-brainer, right?
Our bodies carry out vital processes continuously, even when we are asleep. Breathing, heartbeat, muscle relaxation or contraction, nutrient absorption, temperature regulation, hormone regulation, and detoxification are just a few of our body’s activities that require energy. The food we consume is digested, broken down, absorbed, and transported to our thirty-something trillion cells. They are further broken down to generate energy for all the metabolic activities. The excess energy is converted to fat and stored as an energy reserve.
Weight Loss
Fat
Before we talk about calories and metabolism, let’s talk about fat.
Fat has a bad reputation among those trying to lose weight. But here’s the thing, our body needs fat. The fat under our skin insulates our body and prevents heat loss on cold days. Our vital organs have a layer of fat protecting them. Without fat, our bodies cannot absorb fat-soluble vitamins like vitamins A, D, E, and K. Every nerve and cell in our body has a fatty layer surrounding them to enable their functionality. However, the fat reserves have to be regularly emptied and refilled. It is when there is no healthy balance between usage, storage, and consumption that problems arise. In short, fat is necessary in healthy amounts. [5]
How Many Calories Does Your Body Need?
Our body needs calories for all the basic metabolic activities mentioned earlier. Everyone needs a minimum amount of calories in their daily diet to keep them alive. This is different for different people. It depends on age, gender, height, weight, sleep patterns, stress, etc.

Basal Metabolic Rate (BMR) is the minimum calories your body needs for basic metabolic activities at rest. You can find this out by using an online calculator like this one. Alternatively, you can use the following equations to calculate your TDEE: [6]
Men: (13.397 x weight in kg + 4.799 x height in cm – 5.677 x age in years) + 88.362 
Women: (9.247 x weight in kg + 3.098 x height in cm – 4.330 x age in years) + 447.593 
Total Daily Energy Expenditure (TDEE) is the number of calories your body requires or burns daily based on your activity levels.

Amount of Exercise/Activity
Description
TDEE/Maintenance

Sedentary
Little or no Exercise/desk job
TDEE = 1.2 x BMR

Lightly Active
Light exercise/sports 1-3 days/week
TDEE = 1.375 x BMR

Moderately Active
Moderate Exercise, sports 3-5 days/week
TDEE = 1.55 x BMR

Very Active
Heavy Exercise/sports 6-7 days/week
TDEE = 1.725 x BMR

Extremely Active
Very heavy exercise/physical job/ training twice a day
TDEE = 1.9 x BMR

So if you are a 6-foot-1, 35-year-old male weighing 95 kilograms and living a lightly active lifestyle, then:

Your BMR = (13.397 × 95) + (4.799 × 185.42) – (5.677 × 35) + 88.362 = approximately 2,052 calories.
Your TDEE = 2052 × 1.375 = 2,821.5 calories. 

You need approximately 2,821.5 calories to stay in shape. To lose weight, you must enter a calorie deficit.
Related: Should You Exercise While Intermittent Fasting? 
How Many Calories Should You Cut?
Try not to consume less than your BMR. Losing around 1-2 pounds (approximately 0.5–1 kg) per week is considered safe and healthy Attempting to lose more can have severe repercussions.
One pound of fat equals 3,500 calories. As a general rule of thumb, shaving off 500 calories from your daily intake can cause a weight loss of 1 pound per week. 
What Should You Eat?

More protein and whole grains: More energy is required to digest or metabolize proteins and whole grains. Also, consuming these foods makes you feel full and can curb your hunger.
Less processed food: A small serving (70g)  of French fries has over 200 calories and still doesn’t make you feel full, because of which you might end up ordering more food. By the time you are satiated, you’ll have mostly likely overshot your daily calorie intake goal. 
More healthy food: Fruits and vegetables are an excellent option. They are nutrient-dense, have low calories, and are very filling.
Combine with other diets: You can practice intermittent fasting with other weight-loss diets like the keto and low-carb diets. Despite their intense calorie restriction, such diets allow you to meet your daily macro and micronutrient intake goals. Losing fat may be good, but vitamin or mineral deficiency can harm your overall health and well-being.

Related: How long does it take to see results from Intermittent Fasting? 
Benefits of Intermittent Fasting
Here are the advantages of practicing intermittent fasting:

Easier weight loss: IF might improve your eating habits. You don’t binge-eat before bed or snack when you are bored at any point when you practice IF. Plus, IF can train your body to use its stored fat. You will experience weight loss even if you don’t restrict your calories. [7] [8]
Reduces insulin resistance: Our food is broken down into glucose and absorbed into the bloodstream. Insulin is responsible for glucose absorption by the body’s cells (they need it to generate energy for cell metabolism). When there is Insulin resistance, the cells cannot absorb glucose easily. Because of this, more Insulin has to be produced, and blood sugar levels increase. Studies have shown that IF can reduce insulin resistance and, as a consequence, reduce the risk of type 2 diabetes. [9] [10]
Reduces inflammation: A study by the University of Toronto showed that IF reduces oxidative stress. This, in turn, reduced inflammation induced by oxidative stress. Inflammation is responsible for several chronic diseases like asthma and arthritis that can be avoided through IF. [11]
Supports brain function: Healthy food gives you more mental clarity and energy. Some studies suggest that IF promotes the growth of new nerve cells and can help prevent Alzheimer’s. [12]
Detoxification: Even your stomach and intestines need to take a breather. With time-restricted eating, it is easier for the body to remove toxins that are generated due to metabolic activities and other external factors.
Improves heart health: IF reduces bad cholesterol and promotes good cholesterol in your body. This can lower your blood pressure and improve your heart health. Improved blood sugar levels and a healthy body weight reduce heart disease risk. [10]

Frequently Asked Questions
Are there any side effects to Intermittent Fasting?
A sudden change in diet and reduced food intake can cause nasty side effects like hunger, headache, dizziness, irritability, digestion problems, fatigue, and dehydration. Some of these may disappear after you get used to the new diet regimen or make changes to suit your needs. However, extreme and prolonged calorie restriction can cause malnutrition.
Is 16 hours the minimum fasting duration?
Sixteen hours is not mandatory for intermittent fasting. There are other variations to choose from. However, you need to fast for at least 13 hours to squeeze every drop of goodness out of it. Plus, 13 hours doesn’t feel long since you will be sleeping through most of it.
What should I avoid doing while fasting?
You should avoid:

Drastic dietary changes without consulting a dietician or a nutritionist
Overeating to make up for the lost meal
Snacks, sugary drinks, and processed food
Not drinking enough water
Eating heavy dinners
Consuming very-low calories and starving yourself
Hardcore exercise if you are in a calorie deficit

Can I exercise while fasting?
Yes, you can. It accelerates fat metabolism and weight loss. However, it is better to stick to low-intensity workouts to avoid side effects like dizziness, fatigue, and low blood sugar. Stay hydrated and try exercising during the eating window.
How long does intermittent fasting take to work?
This depends on your goal, diet, calorie intake, health, sleep routine, etc. Your body takes a few days or weeks to switch to your fat reserves for energy, so don’t step on your scale right away. Within a month, you will have improved gut health. Most people notice weight loss results after 10 weeks. Longer fasting durations offer better benefits.
Conclusion
The number of calories you need to eat during intermittent fasting depends on several factors, including your current physique, lifestyle, and goals. But remember that a significant calorie deficit will make you lose more muscle mass than fat. 
Muscles are essential for movement and metabolism. Decreased muscle strength will reduce metabolism and make losing fat much more challenging.
References

Intermittent fasting: The positive news continues – Harvard Health. (2018, June 29). Harvard Health. 
Patterson, R. E., Laughlin, G. A., LaCroix, A. Z., Hartman, S. J., Natarajan, L., Senger, C. M., Martínez, M. E., Villaseñor, A., Sears, D. D., Marinac, C. R., & Gallo, L. C. (2015, August). Intermittent Fasting and Human Metabolic Health. Journal of the Academy of Nutrition and Dietetics, 115(8), 1203–1212.
Not so fast: Pros and cons of the newest diet trend – Harvard Health. (2017, October 1). Harvard Health. 
Wilhelmi de Toledo, F., Grundler, F., Sirtori, C. R., & Ruscica, M. (2020, June 10). Unravelling the health effects of fasting: a long road from obesity treatment to healthy life span increase and improved cognition. Annals of Medicine, 52(5), 147–161.
Calder, P. C. (2015, July 15). Functional Roles of Fatty Acids and Their Effects on Human Health. Journal of Parenteral and Enteral Nutrition, 39(1_suppl), 18S-32S.
Roza, A. M., & Shizgal, H. M. (1984, July). The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. The American Journal of Clinical Nutrition, 40(1), 168–182.
Klempel, M. C., Kroeger, C. M., Bhutani, S., Trepanowski, J. F., & Varady, K. A. (2012, November 21). Intermittent fasting combined with calorie restriction is effective for weight loss and cardio-protection in obese women. Nutrition Journal, 11(1).
Gabel, K., Hoddy, K. K., Haggerty, N., Song, J., Kroeger, C. M., Trepanowski, J. F., Panda, S., & Varady, K. A. (2018, June 15). Effects of 8-hour time restricted feeding on body weight and metabolic disease risk factors in obese adults: A pilot study. Nutrition and Healthy Aging, 4(4), 345–353.
Sutton, E. F., Beyl, R., Early, K. S., Cefalu, W. T., Ravussin, E., & Peterson, C. M. (2018, June). Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metabolism, 27(6), 1212-1221.e3.
Ahmed, A., Saeed, F., Arshad, M. U., Afzaal, M., Imran, A., Ali, S. W., Niaz, B., Ahmad, A., & Imran, M. (2018, January 1). Impact of intermittent fasting on human health: an extended review of metabolic cascades. International Journal of Food Properties, 21(1), 2700–2713.
Mandal, S., Simmons, N., Awan, S., Chamari, K., & Ahmed, I. (2022, January). Intermittent fasting: eating by the clock for health and exercise performance. BMJ Open Sport & Exercise Medicine, 8(1), e001206. 
Gudden, J., Arias Vasquez, A., & Bloemendaal, M. (2021, September 10). The Effects of Intermittent Fasting on Brain and Cognitive Function. Nutrients, 13(9), 3166.