How Intermittent Fasting Works In The Body: The Lowdown


Diets come and go, and few stay in vogue for long. Intermittent fasting (IF) has potential in this sense—it’s a healthy, sustainable eating approach designed to keep weight off and promote good health.

There are no special meals or restrictive diets involved with IF, which you undertake at the times of your choosing. Devotees, like talk show host Jimmy Kimmel and actress Halle Berry, often find that IF helps them lose weight quickly and keeps their energy levels high.

But recent studies suggest that the benefits of IF may extend beyond weight loss and extra energy levels. Early studies of animals and humans suggest that IF provides a number of health benefits – such as a lower risk of cancer, diabetes, and heart disease.

While much more research is needed, if you are going to try the very promising approach, Ahmet Ergin, MD, an endocrinologist at Lawnwood Regional Medical Center & Heart Institute in Fort Pierce, Florida suggests that it is important to follow certain rules. To paraphrase him regarding this, he says that doing it right may improve your wellbeing, but cautions that doing it incorrectly “may actually harm you”.

So here is a short summary of what we currently know about IF and how it affects your body. We’ll also give you some starting tips in case you decide to try it.

Your body on intermittent fasting

Fasting is not new. Humans have been practicing it for ages, often as part of a cultural celebration or religious observance, such as the Muslim holy month of Ramadan. People fast for medical reasons too, before certain tests or procedures.

There are different types of fasting with different rules. Intermittent fasting typically follows the same basic principles: You restrict your calorie intake for some periods, and then eat normally for others. This decreases your overall calorie count, but it helps you lose weight in another way also.

How it works – Whenever you eat, you get energy from the glucose (sugar) you consume. Insulin, a hormone produced in the pancreas, helps release the glucose into the bloodstream. From there, glucose is either used immediately for energy or stockpiled for later.

Without any carbs or any other form of energy, your body will immediately drastically reduce your insulin production. Once your insulin levels have fallen low enough, your body will start devouring fat for energy. The result is a sudden loss of weight – that is exactly what you want if you are attempting to lose those extra pounds of fat you are carrying around.

And many people do lose weight on IF, at least initially. However, as with all diets, the weight tends to come back once the diet is over. That is nothing specific about the IF diet, it is just an important issue with all approaches to losing weight. So there is no magical panacea here in that regard.

In 2018, a randomized controlled trial found that overweight or obese participants lost 7.1 percent of thier body weight over 12 weeks on IF. It also found that participants had regained almost 2 percent back about 9 months later. In another 2018 meta-analysis study, participants on average lost just 0.38  percent of their body weight within 24 weeks. So the take out lesson here is that IF doesn’t work for everyone.

Of course, there are proponents of the diet that point to a range of other positive metabolic effects over and above the actual weight loss itself.  

Popular Types Of IF

There is no one way to do IF and there is no sound scientific based evidence that any method is more effective than another for short-term weight loss. This being said, many people employ one of these methods, or some variation of them.

  • On a time-restricted diet, you fast most of the time and eat per usual during a set time frame such as 11am to 5pm. Since we naturally fast at night when sleeping, prolonging those hours is the easiest way to achieve this.
  • The Five-Two diet typically involves restricting yourself to 500 calories two days per week. The other five days, you either eat like you normally would, or within a certain time window, you eat normally. Your fasting days are separated by at least a day, so you never fast for two days in a row.
  • Alternate day fasting switches no- or low-calorie days with regular eating days.
  • Fasting several days at a time followed by a normal diet is known as a periodic fast.

Here are some things to know if you’re interested in IF: Long periods of deprivation could make it harder to stick to than other diets. Trials show dropout rates as high as 40 percent. Of course this depends on if there is a good alignment between the IF diet version and the unique characteristics of the person undertaking the diet.

There’s also concern people may overeat when a fasting period ends. This can result in an unhealthy approach that negates many of the benefits that come from the fasting process.

So far, there is limited scientific evidence that IF is a better approach to dieting than other calorie restriction diets. Most studies indicate that people tend to lose similar amounts of weight on both types of diet.

How IF may improve your health

Animal studies have found that intermittent fasting may boost health in multiple ways. Human research, however, is still in its infancy and limited.

Furthermore, at the current time, it is still unverifiable how much advantage comes from IF’s effects on body chemistry, versus how much benefit comes from weight loss. Although many claim IF provided a multitude of benefits over and above other weight loss plans, there is still limited scientific evidence to support this.

That said, IF may help the following:

  • Your heart: Some studies indicate that doing IF on a regular basis may positively impact certain heart disease risk factors. This could result in significant improvements in cholesterol, triglycerides, blood pressure, and inflammation levels.
  • Your brain: Research suggests IF could boost memory and help defend against degenerative disorders like Alzheimer’s and Parkinson’s disease.
  • Your liver: Drawing again from studies on animals, some experts think intermittent fasting may help reduce the chances of non-alcoholic fatty liver disease, a serious condition that may lead to scarring (cirrhosis), and eventually liver failure or cancer.
  • The sensitivity of your insulin: Prediabetes and type 2 diabetes occur in those who have a low sensitivity to insulin—who are insulin resistant. Intermittent fasting may improve that sensitivity.
  • Cancer risk: In rats and mice, some studies suggest that intermittent fasting is connected to a decrease in some types of cancer. However, there is currently no data on the influence of IF on cancer rates in humans, and more research is needed.

Intermittent Fasting Cautionary Advice

Please speak with your healthcare provider before starting any new diet.

They can help you with deciding whether IF is right for you and can give you a clear picture of the potential pitfalls and health hazards. Let your HCP know if you’re taking any medications, as well, since IF can interfere with them.

If you decide to try intermittent fasting, experts recommend that you take it slow and work your way up to longer periods without food.

 Some people may experience side effects such as headaches, lightheadedness or mood changes at first, but proponents say you should get used to these as you continue on your IF journey.

Stay hydrated while fasting. Fasters of all kinds often neglect drinking water during their fast, which can lead to dehydration and kidney problems, especially in patients with existing kidney issues.

Also bear in mind you need to follow a nutritious eating plan when you are not fasting.

Incorporate salads, vegetables, lean protein, legumes, whole grains, and heart-healthy fats into your diet.

Try to avoid binging on junk food when you resume eating. Overeating is harmful to the progress you have made.

Be aware: Intermittent Fasting, although likely safe for most of us in the short term, stopping meals and massively reducing calories is not suitable for everyone, including:

  • Children and adolescents who are growing
  • Women who are nursing or pregnant
  • People who have diabetes (without a doctor’s constant assistance)
  • People who take medications that require food
  • People who take medication for heart disease or high blood pressure
  • Those who have histories of eating disorders

It is important to know the long-term safety and effects of IF at this point are not well understood. And, like any diet, there is the issue of short term results not being maintained for the long term. So if you try IF, do it with knowing these things.

Tips On Getting Started with Intermittent Fasting

With that little bit of background out of the way and some general caution about the general pitfalls of dieting, let’s look at some tips on getting starting with IF.

Starting With A Less Rigorous Regime

It may be tempting to start out with a long fast to lose as much weight as possible. As we’ve already outlined, it may not prove the ideal method for you. If, like the rest of us,  you’re in the groove of a high-carb, high-sugar, and processed foods diet, it may be tricky to fast for 36 hours right off the bat.

If you find your very first fast impossible, you probably won’t try intermittent fasting again anytime soon. Even if you don’t, intermittent fasting is unlikely to become a part of your ordinary routine.

It’s best to work with an intermittent fasting plan for at least a month to see if it works for you. There is no way to sugar coat it (excuse the pun – no sugar to be seen for miles when you’re fasting), it will be tough to stick to a long term fasting plan if you have never fasted before.

I advise you to opt for one of the less rigorous regimes to start with. The 5:2 diet allows you to eat some food every day.

On five days of the week you are allowed to eat your regular meals. On the other two days you can eat 500 or 600 calories. As long as you choose healthy options, this should give you ample choices.You will experience the benefits of eating wisely without ever feeling hungry.

Alternatively, you can follow the popular 16:8 method. For a large portion of your fasting time you will be sleeping. After that, you’ll be able to eat whatever you want (within reason) for 8 hours. Many people enjoy the freedom that this offers. Once they get accustomed to the 16-hour fast, many people tend to find this way of eating quite simple and effective.

Once you’re accustomed to fasting, you may work up to longer fasts. However, many people continue with their initial plan over the long term and succeed.

Staying Hydrated

No matter what intermittent fasting plan you attempt, you should stay well hydrated. Fasting means not consuming food or beverages that contain calories. It doesn’t mean you can’t drink water and other calorie-free liquids. In fact, you should drink more of them.

Drinking enough water will help your body to flush toxins effectively. This will contribute to your weight loss and health goals. It will also keep you healthy in other ways. Your skin will be healthier. You will avoid headaches and other problems associated with dehydration. Your bowel movements will be more regular too.

Drinking calorie free liquids while you are fasting will help prevent you from feeling hungry. Sometimes, feelings of hunger are actually more about thirst instead.  If you drink a glass of water when you’re beginning to feel hungry, you’ll maintain your fasting for longer.

Try Experimenting with Different Eating Patterns

We have outlined some eating plan schedules here, however that does not mean you need to stick with them. The times and days we suggested are just examples. They may or may not work for you. It is best to choose the best days and eating patterns to fit your lifestyle, preferences, and needs, thereby providing you the most likelihood of long-term success.

Your body might be better suited for eating as soon as you get up in the morning and then starting  your fast early in the afternoon. Or alternatively, it may work better for you to start eating in the early afternoon and have your last meal before bed. Experiment and find out what works best for your unique circumstances.

You might want to fast on the weekends when you have fewer distractions. Or you may benefit from fasting on a weekend so you don’t feel fatigued during the workweek.

There isn’t just one perfect IF plan for everyone. That means you will need to do a little experimenting. Think carefully about the pros and cons of each regime we suggested. Think about which you like best, and try it out. Give it a month to see how well it works for you. If you’re having trouble, it’s time to go back to the drawing board. See if another intermittent fasting schedule is more compatible with your lifestyle. You can try moving your eating windows around a little to see if that makes it more manageable. You are the boss of what works best for you so take control.

Don’t be afraid to experiment – experimentation may very well be the key to success.

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