Fasting — A Path To Better Health?

Sometimes, less is more. It’s an adage that applies in many domains within our lives…and in the modern world, it most certainly applies to food. A large volume of research demonstrates that consuming fewer calories leads to better health.

Of course, there is a limit to how low you can go in terms of calories and I’m certainly not advocating starvation, but the research supporting fasting and calorie restriction is impressive.  

But before we dive into the subject of fasting and caloric restriction…you should know that the source of the calories consume matters, no matter how many you decide to eat. I hope that I don’t need to tell you that the 200 calories of root vegetables are better for you than 200 calories of root beer.

Fasting Techniques Gain Popularity

There’s a great deal of evidence, though, that keeping your calorie intake in check can provide significant health benefits. This is particularly true in the case of fasting, a practice that is gaining media attention after celebrities like Jimmy Kimmel, Beyonce, and Hugh Jackman have used it to lose weight and improve wellbeing.

It may not be the right thing for everyone but there’s no doubt that fasting has been has been used as a medical treatment for centuries and is a rich tradition in virtually all the major world religions as well.  

Fasting and Faith

There are fast days scattered throughout the year in many religious traditions. They are seen as a way of cleansing the body and the mind so that one can better focus on spirituality. Ash Wednesday, Good Friday, and Lent are all examples of fast days within Christianity. Jewish people don’t eat or drink for 24 hours during Yom Kippur as a part of the day of atonement. Muslims fast from sunrise to sunset during the month of Ramadan. You’d be hard-pressed to find a religious tradition that does not include some form of dietary abstinence as a way of improving a person’s relationship with a higher power.

The Physical Benefits of Fasting

Metaphysical matters aside, there is plenty of evidence that fasting does provide some important biochemical and metabolic benefits. Studies show that fasting triggers an array of fundamentally healthy changes in the body:

  • Fasting is associated with decreased levels of the hormone IGF-1, which is linked to cellular chemistry that promotes disease and aging.
  • Periodic fasting is linked to lower levels of C-reactive protein, which is implicated in inflammatory responses.
  • Fasting is correlated with lower blood glucose levels, which can help prevent diabetes, cardiovascular disease, and perhaps even some cancers.

Fasting Enhances Immunity

Here’s another interesting byproduct of the fasting process. According to Dr. Valter Longo, of the University of Southern California, abstaining from food can help improve or restore the immune system. As he explains, “When you starve, the system tries to save energy, and one of the things it can do to save energy is to recycle a lot of the immune cells that are not needed, especially those that may be damaged.” Research on mice confirms that fasting is correlated with immune regeneration. And studies involving chemotherapy patients showed that patients who fasted three days prior to their treatments were better protected against immune system damage.

Fasting and Weight Loss

The evidence is more mixed when it comes to the relationship between fasting and weight loss. Many people use short-term fasts to shed pounds, but I tend to agree with Dr. Madelyn Fernstrom, of the University of Pittsburgh Medical Center’s Weight Loss Management Center, when she explains that fasting will lead to weight loss, “but it is quick fluid loss, not substantial weight loss. If it’s easy off, [but] it will come back quickly – as soon as you start eating normally again.”

The advantages of fasting can be significant, but that does not mean the practice is appropriate for everyone. And not all fasts are the same…. so let’s take a brief look at some of the most common fasting techniques.

Caloric Restriction

There is a reason why the “caloric restriction society” does not have lots of photos of their members on their website. These folks look shockingly thin…some of them even look sickly. Proponents of this diet/lifestyle eat usually about 25% fewer calories than typically recommended (less than 1500/day) and usually have body fat percentages less than 12% and BMI under 20 which is extremely lean.         

Despite their gaunt look, advocates of CR claim that they feel great, and that they can expect to live long healthy lives as a result of their long term fast. There is some research to support their claims; multiple animal studies as well as some basic science research demonstrate the longevity enhancing effects of a lifestyle of caloric restriction. An increasingly popular (and arguably more sensible) approach to caloric restriction is called CROM which stands for caloric restriction with optimum nutrition.

The Fast-Mimicking Diet

A practice called Fast-Mimicking Diet (FMD) is gaining ground recently as a result of good publicity around some new research into benefits for weight loss. FMD is a technique that “tricks” the body into thinking it is fasting by cutting one’s calorie intake in half over the course of several days. Adherents claim that it helps lower the biomarkers for aging and many common diseases. Here are two other common fasting methods you hear a lot about today.

Intermittent Fasting: (The 16:8 Diet)

This strategy involves consuming all your calories during an eight-hour period while abstaining from food for the remaining sixteen hours of each day. This form of Intermittent Fasting (IM) is being touted as a weight loss method and it was popularized by the actor Hugh Jackman, who used it to slim his waistline while bulking up his muscles.

The basic premise behind the IM fasting method seems both simple and sensible. Experiments where rodents had access to food 24/7 show that they gained weight (no surprise there) had higher cholesterol, and higher blood sugar levels.

Findings like these likely apply to humans too. Constant feeding taxes the liver, keeps blood glucose levels elevated, and hinders the body from repairing itself. That’s because when you refrain from eating for a number of hours the liver can divert glucose to cellular repair, which in turn reduces inflammation.

One of the chief benefits of the 16:8 regimen is that it is similar to what most people are already accustomed to — you can have breakfast at 9 am, lunch at about 1 am, and dinner at 5 pm. But by avoiding snacks and additional meals during the remaining sixteen hours, people tend to lose impressive amounts of weight in a relatively short period of time.  This happens for several reasons:  

  • Most people eat fewer calories per day on a 16:8 regimen.
  • Intermittent fasting turns on the NRF2 gene pathway, which increases the levels of antioxidants available while reducing inflammation.
  • Periodic fasting helps shifts metabolism and encourages the body to burn fat.
  • Digesting food takes a lot of energy. When you take a break from eating your body can use its resources for cellular regeneration and repair.
  • There is emerging evidence that indicates that fasting improves mitochondrial health.  Mitochondria are cellular energy generators and improving their energy generating capacity will improve health (and metabolism) across the board.

The 12-Hour Fast

This is the same premise as the 16:8 fast described above, but less intense. The basic idea behind this fast is to leave twelve hours between your evening meal and your morning one. If you normally have dinner at 6 pm you simply wait until at least 6 am before having breakfast.

Once again, intermittent fast methods like this are associated with numerous health benefits including: 1) Generally, when we only eat during 8 hours of each day, we consume less overall calories than we would if we ate from morning till night. 2) Studies show that in addition to overall caloric reduction, a 12 hour “rest” from food can improve metabolism (the rate the we burn calories), lower inflammation, and improve insulin sensitivity.

Whether it’s a trend like FMD (or other fasting techniques), longevity research suggests that calorie-restricted diets may help people live longer. To date, most of the studies have involved laboratory animals, especially rodents and rhesus monkeys, but the results have been universally impressive. Animals that ate calorie-restricted diets had “younger” genes as evidenced by DNA methylation levels, which biologists use as an epigenetic marker for the aging process.


Fasting is a time-honored and powerful way of cleansing the body and improving health, but it is not for everybody (or without risk). Extreme fasting or long-term caloric restriction is generally not advisable for young people, pregnant women, those who are underweight, people with diabetes or other blood sugar control disorders.  

Also, there are serious risks associated with extreme methods of fasting that include dehydration, inadequate nutrition, and a tendency towards binge eating. If you do plan to abstain from food for an extended period, then be sure to drink plenty of water during the process.

For many people (and with some caveats) some types of fasting can be an appropriate part of their health regimen. Health is about balance, not extremes. It’s important to get an abundance of micronutrients, vitamins, and minerals and the best way to do that is to eat a variety of whole, natural, and plant-based foods. Once again, quality is more important than quantity.

The same less is more attitude that can apply to eating also applies to fasting — more extreme forms of caloric restriction do not necessarily lead to better health outcomes. The trick is to balance your nutritional requirements along with the needs of your metabolic system to take a breather from time to time.

Religious traditions that include fasting recognize that there is an intrinsic link between dietary restraint and mental clarity, physical revitalization, and overall wellbeing. And although I don’t advocate any form of extreme dieting…but I am impressed with the results I see in my patients who make intermittent fasting part of their lifestyle. Whether you call it fasting or not…most of us could benefit from eating fewer calories every day.  

Take good care,

Dr. Joshua Levitt