Intermittent Fasting 101 — The Ultimate Beginner's Guide (2024)

Some research suggests intermittent fasting can help with weight loss and benefit brain and heart health. However, if you decide to try it, it’s important to choose the right option for you and do it safely.

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Intermittent fasting (IF) is currently one of the world’s most popular health and fitness trends. Some people claim it’s helped them lose weight, improve their health, and simplify their lifestyles.

There is also some research that has linked IF to better cognitive function, heart health, and digestion.

Read on to learn more about intermittent fasting and how to do it safely.

Intermittent fasting (IF) is an eating pattern that cycles between periods of fasting and eating. It’s currently very popular in the health and fitness community.

IF doesn’t specify which foods you should eat but rather when you should eat them. This means it’s not really a diet — it’s more accurately described as an eating pattern.

Common IF methods involve fasting for 16 hours and having an 8-hour eating window. Some people do 24-hour fasts twice per week.

Fasting has been a practice throughout human evolution. As a result, humans evolved to be able to function without food for extended periods of time.

People also commonly fast for religious or spiritual reasons, including in Islam, Christianity, Judaism, and Buddhism.

You can practice IF in several different ways, though they all involve splitting the day or week into eating and fasting periods.

During the fasting periods, you won’t eat anything. You can usually drink calorie-free beverages like water, herbal tea, and black coffee.

These are the most popular methods:

  • The 16/8 method: Also called the Leangains protocol, this involves an 8-hour eating period and a 16-hour fasting period. Some people do this by skipping breakfast, but you can also do the reverse and skip dinner or eat early.
  • Eat-stop-eat: This involves fasting for 24 hours once or twice per week. Because this is a slightly more advanced version of fasting, you should only try this if you’re pretty comfortable with the practice.
  • The 5:2 diet: With this method, you consume only 500–600 calories on two nonconsecutive days of the week but eat as usual the other 5 days.

For many people, reducing their eating period also reduces the number of calories they eat on a daily basis. Over time, this can contribute to weight loss.

When you fast, several things happen in your body on the cellular and molecular level.

First, human growth hormone (HGH) levels go up and insulin levels go down. Your body’s cells also change the expression of genes and initiate important cellular repair processes.

Here are some changes that occur in your body when you fast:

  • HGH: Your levels of HGH increase. This can help with both fat loss and muscle growth.
  • Insulin: When you fast, insulin sensitivity improves and levels of insulin decrease. Lower insulin levels make stored body fat more accessible.
  • Cellular repair: When fasted, your cells initiate cellular repair processes. This includes autophagy, which means cells digest and remove old and dysfunctional proteins that build up inside them.
  • Gene expression: Fasting can cause changes in the function of genes related to longevity and disease prevention.

Weight loss is the most common reason that people try IF. By reducing the amount of time you can eat, you’ll likely consume fewer calories without realizing it.

Additionally, IF can affect hormone levels and aid in weight loss.

In addition to lowering insulin and increasing HGH levels, IF increases the release of the fat-burning hormone norepinephrine (noradrenaline). Because of these changes, short-term fasting may temporarily increase your metabolic rate.

According to research, IF can be a powerful weight loss tool when done safely. A 2020 systematic review of studies on people with overweight and obesity showed rates of weight loss ranging from 0.8% to 13% of participants’ starting weights. However, most of these studies were small and done over a short period of time.

Keep in mind that the main reason IF helps with weight loss is that it makes it easier to eat fewer calories overall. If you eat massive amounts during your eating periods, you may not lose any weight at all.

Studies have found that IF may have benefits for weight management and overall health. However, keep in mind that much of the existing research includes, small, short-term studies. This means researchers need more information to understand how IF can help with sustainable, healthy weight loss.

Here are the main health benefits of intermittent fasting:

  • Weight loss: IF can help you lose weight and body fat without having to consciously restrict calories.
  • Insulin resistance: IF can reduce insulin resistance, which happens when your body doesn’t respond properly to insulin, increasing your blood sugar levels. Managing your blood sugar can protect you against type 2 diabetes.
  • Inflammation: Some studies show reductions in markers of inflammation, a key driver of many chronic diseases.
  • Brain health: Studies in rats have shown that fasting increases the brain hormone BDNF and may aid the growth of new nerve cells.

Keep in mind that research is still in its early stages. Many of the studies were small or short-term or conducted in animals. Many questions have yet to be answered in higher quality human studies.

Fasting is certainly not for everyone. If you’re underweight or have a history of eating disorders, you should not fast without speaking with your doctor first.

Should females fast?

Some evidence shows that IF may not be as beneficial for females.

There are a number of anecdotal reports of females whose menstrual period stopped when they started doing IF and went back to usual when they resumed their previous eating pattern. This is likely because fasting can disrupt female hormones, affecting menstruation and overall health.

However, IF may be a useful tool for some people. Research has shown that it can help treat hyperandrogenism in people with polycystic ovarian syndrome (PCOS), which could potentially affect their ovulation and fertility.

According to a 2023 research review, studies have shown that, for females who are an average weight, IF could harm reproductive health and performance if it causes insufficient calorie intake, especially in athletes.

Experts suggest that females take care with IF. Overall, the research is very limited, and there is a lack of long-term data. More research is needed before we can really understand the implications.

Females should follow separate guidelines, like easing into the practice and stopping immediately if they have any concerns like missing periods. If you are considering IF, it’s best to talk with a doctor or dietitian to determine whether it is right for you.

Hunger is the main side effect of IF. You may also feel weak, and your brain may not perform as well as you’re used to. This might only be temporary, as it can take some time for your body to adapt to the new meal schedule.

If you have a medical condition, you should speak with your doctor before trying IF.

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 trying to conceive
  • have a history of amenorrhea
  • are pregnant or nursing

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

Can I drink liquids during the fast?

Yes. Water, coffee, tea and other calorie-free beverages are fine. Coffee can be particularly beneficial during a fast as it can suppress appetite.

Isn’t it unhealthy to skip breakfast?

No, skipping breakfast won’t harm your health. If you make sure to eat healthy food for the rest of the day then the practice is perfectly healthy.

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 containing a fat source. Many supplements can also cause an upset stomach when you take them without food.

Can I work out while fasted?

Yes, fasted workouts are fine. Expect to feel a bit more tired and weaker than usual, so take it easy and listen to your body.

Will fasting cause muscle loss?

All weight loss methods can cause muscle loss. Aim to eat plenty of protein and engage in strengthening exercises like lifting weights.

Chances are that you’ve already done many intermittent fasts in your life.

If you’ve ever eaten dinner, then slept late and not eaten until lunch the next day, you’ve already fasted for over 16 hours. Some people intuitively eat this way because they simply don’t feel hungry in the morning.

Many people consider the 16/8 method the simplest and most sustainable form of IF — you might want to try this practice first.

If you find it easy and feel good during the fast, then maybe try moving on to more advanced fasting methods. Another approach is to fast whenever it’s convenient. Simply skip meals from time to time when you’re not hungry or don’t have time to cook.

There is no need to follow a structured IF plan to derive at least some of the benefits. Experiment with the different approaches and find something that you enjoy and fits your schedule.

IF is not something that anyone needs to do. It’s simply one of many lifestyle strategies that can improve your health. Eating nutrient-dense food, exercising, and getting enough sleep are still the most important factors to focus on.

At the end of the day, there is no one-size-fits-all solution when it comes to nutrition. The best diet for you is the one you can stick to in the long run.

If you feel good when fasting and find it to be a sustainable way of eating, it can be a very powerful tool to lose weight and improve your health.

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Intermittent Fasting 101 — The Ultimate Beginner's Guide (2024)
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