The History:

The practice of fasting dates back thousands of years, and has been recorded in time since the time of Hippocrates and Plato. Even long before that, food scarcity was a common occurrence – it’s highly unlikely our predecessors ate three balanced meals per day plus min-meals. Of course, there were and always have been natural occurrences that presented situations that created food scarcity and periods of fasting.

Over the past several decades, fasts have been used to express social and political views throughout history, as well as traditional fasting patterns practiced among many faiths, including Judaism, Islam, Christianity, Hinduism, and Catholicism..

In my nutrition practice, I have seen many people fast as a way to lose weight before a big event – like a wedding or a class reunion. Today, more than for religious reasons, fasting has become a popular dietary practice. In fact the news has been exploding with a variety of fasting diets defined as the abstinence from some or all food or drink, or both, for a specified period of time. Fasting can include the currently popular 5:2 diet (eat normally, but healthfully, for five days of the week and consume only 500 to 600 kcal on two nonconsecutive days), or the 16-18 hour per day fast.

Intermittent fasting got its big revival a few years back when British journalists Michael Mosley and Mimi Spencer published their book, The Fast Diet. In it, they advocate the 5:2 intermittent fasting: Two days per week you limit yourself to less than 500 kcal if you’re female, and 600 kcal if you’re male. The remaining five days you eat as you normally would. Basically, fasting works, because the dieter eats less in one week than they normally would; any time a person consumes fewer calories than he or she burns, that person should lose weight. The key to intermittent fasting is not to overeat on “feed” days.

Even this past Monday, research conducted at the University of South Australia was published in the Journal of the American Medical Association on the effects of the so-called 5:2 diet for controlling the diabetes. This study is the first long-term clinical trial, which compared the diets of people with type II diabetes. The researchers wrote that intermittent energy restriction is an effective alternative diet strategy for the reduction of HbA1c levels (3-month blood sugar average) comparable to continuous energy restriction in patients with type II diabetes. The reasoning is due to the fact that people have trouble sticking with restricting calories day after day after day.

But wait – isn’t this contrary to what we have been told over the last few years – like you should eat breakfast (“breaking your fast”) and that eating every 3-4 hours speeds up your metabolism? If all this is true, then won’t fasting and skipping meals slow your metabolism and cause you to gain weight?

Since I am a big proponent of nutrient timing throughout the day, I looked at some research to point out the Pro’s and Con’s to Intermittent Fasting …

The Pros vs The Cons:

Pro’s: Proponents of intermittent fasting believe it’s a good way to not only shed pounds quickly but also to reduce risk factors for diabetes and heart disease.

Con’s: Others see intermittent fasting as a poor choice for dieting and harmful to one’s health. Some worry that fasting robs the body of important nutrients and could send it into starvation mode, making sustained weight loss more difficult. Plus there is research showing that skipping breakfast increases mortality and CVD. Additional research has also shown that fasting for more than 12 to 14 hours results in a 50% increase in the risk of having a cholecystectomy (removal of the gallbladder).

There have been several studies weighing the pro’s and con’s of intermittent fasting. Most research reports found significant reduction in weight from intermittent fasting, but reductions in fasting blood glucose, and improvements in LDL (bad cholesterol) and HDL (good cholesterol) levels have been inconsistent.

There has also been some research done with animals. This research suggests that intermittent fasting can reduce obesity, and reduce chronic diseases such as diabetes and cancer, and conditions such as nonalcoholic fatty liver disease.

Opponents of intermittent fasting warn that periods of intermittent fasting can lead to periods of overeating. Although, this has not been proven – I highly recommend avoiding intermittent fasting if you have a history of eating disorders or disordered eating.

There is also no data in regards to the effects of intermittent fasting on other health behaviors such as diet quality, sleep, and physical activity — all important factors in overall health.

Should You Try Intermittent Fasting?

Although fasting may be a viable weight loss option for individuals who can’t stick to a daily calorie restriction, as a registered dietitian nutritionist, I am not a fan.

I don’t agree with the metabolic claims – like how the body will dip into its energy reserves (fat storage) during periods of intermittent fasting. When you go long periods of time without eating, the body first breaks down muscle. After several days of not eating, then the body taps into its long-term storage (aka fat storage). If you are fasting 2 days, you are using muscle for energy. If you use muscle for energy, you are slowing your metabolism. Furthermore, think of a sumo wrestler – they starve themselves all day long (to slow their metabolism) and eat one very, very large meal, and then go to sleep. Many proponents of intermittent fasting do just this when they follow the 16-18 hour fasting diet.

If used by those who have a history of eating disorders, or disordered eating, there is a potential to create more of that destructive “black and white” thinking or “good food vs bad food”. Seeing food as ‘foods you can eat when you are not fasting’ vs ‘foods you eat when you are fasting’ creates food duplicity which has the potential to create a problem for people struggling with and eating disorder or disordered eating.  Additionally, there is a potential for impulsive eating, or a binge, because there is such intense hunger on days or hours spent fasting.

If used by people who have diabetes, and who are using medicine or insulin to manage their blood sugar, they can further create imbalances in their blood sugar – and potentially create periods where blood sugar reaches dangerously low levels.

If you are training for an athletic competition, you run the risk of having your workouts suffer, having a slower recovery rate, and creating a potential for injury.

Likewise, if someone is pregnant, has a certain medical condition, or takes certain medicines that rely on food to work, intermittent fasting could be dangerous.

In fact, no one should start an intermittent fasting diet without first consulting with his or her doctor.

If you have been given the OK and want to fast, Mark P. Mattson, PhD, principal investigator in the Laboratory of Neurosciences at the National Institute on Aging recommends either of the following two approaches:

  1. Fasting two nonconsecutive days each week, eat only one meal, and the other five days eat normally; or
  2. Five days each week, fast between the hours of 6 PM and 10 AM.

Mattson believes that short fasts of 16 to 24 hours may activate adaptive stress responses that protect against disease. However, as noted, not everyone agrees with this assessment.

Like the “Blood Type Diet” at this time Intermittent Fasting does seem to be more of a trend than a sustainable way of eating. In my practice, I always try to stress how the best eating pattern is the one that comes natural, and that which you can create a lifestyle around and that you will enjoy.

If you’re reading this information and aren’t sure what way of eating fits best into your lifestyle, work with a Registered Dietitian to create a successful nutrition plan tailored to meet not only your physical needs, but also your social, and emotional your needs.