Intermittent Fasting: Effective or Dangerous?

There’s a new diet craze taking hold. It’s called intermittent fasting and it refers to an eating style where you eat within a specific time period and fast the rest of the time. Some say intermittent fasting is an effective way to lose weight. Other’s say it’s not safe.

Here to give us some straight talk on intermittent fasting are Bridget Shea, RD, and Emily Clairmont, RD, both registered dietitians at the University of Vermont Medical Center.

What is intermittent fasting?

Bridget Shea: Fasting has been part of the human experience throughout history and is often tied to religious and spiritual rituals. Intermittent fasting has been a part of our evolution. It just didn’t have the fancy name until now.

A fast is defined as consuming only zero to 25% of your energy requirements in a day. Intermittent fasting refers to the fact that it’s not continuous, meaning that there are days of normal eating, then intermittently there are fast days mixed in. So simply put, intermittent fasting is a pattern of eating, not a diet. A diet would generallyfocusmore on what is being consumed.

How do you do intermittent fasting?

BS: There are various ways to carry out an intermittent fasting plan. The two most common are alternate day fasting and time restricted feeding.

• In alternate day fasting one fasts every other day and then eats normally onthe days in between. A similar fasting plan is known as 5:2, where one would fast two times a week and then eat normally the other five days.
• Time restricted feeding is where food is only consumed during defined periods during the day and there is a longer overnight fast. For example, someone may only eat for four, six or ten hours a day and then have a longer period in which they don’t eat.

Emily Clairmont: For some, this could look as restrictive as only eating between noon and 4 o’clock in the afternoon, or four hours. For others, this may look relatively normal, such as eating between 8AM and 6PM, which would be 10 hours of eating.

What does medical research say about intermittent fasting?

BS: The research is mixed as it always is with nutrition, but weight loss is generally the desired outcome when people attempt intermittent fasting, and like all other diet plans, eating patterns and programs, intermittent fasting works for some people and does not work for others.

We know that the alternative, which is chronic daily energy restriction is difficult and adherence is the key factor for any weight loss initiative. So, it’s been proposed that it may be easier to adhere to an energy restriction if it’s only a few days a week versus every day, but that hasn’t been supported by research yet.

There are limited studies in humans within intermittent fasting and they’re often small, so it’s important to understand that.

• Some studies show that intermittent fasting results in the loss of visceral fat, the type of fat around the internal organs that is associated with poor health outcomes.
• It may also produce a weight loss of about 4 to 10% over a short term intervention, which is similar to most weight loss studies in different eating patterns.
• Some recent studies suggest that intermittent fasting helps reduce the loss of lean muscle mass when producing a weight loss.That’s an important factor because it would help you maintain your metabolically active tissue as weight loss is achieved. Again, these results are only suggestive, and more research is needed.
• One other purported benefit of intermittent fasting is that it could help prevent metabolic adaptations that occur during prolonged low calorie diets for weight loss. It’s theorized that cycling from fasting to normal eating could make it more difficult forthe body to adapt, which it usually does by lowering the metabolic rate when there is weight loss. The reduced metabolic ratecould prevent further weight loss from happening, the diet plateau or even encourage weight regain. More research is needed, but this is could be another promising  benefit of intermittent fasting as well.

What are benefits of intermittent fasting?

BS: The benefits of intermittent fasting may include weight loss due to calorie restriction really, reduced cancer growth, reduce waist circumference in visceral fat mass, increased triglycerides, a reduction in CRP, which is a marker of inflammation, increased insulin resistance and reduced insulin sensitivity. All studies done in intermittent fasting so far have been short-term, so at this point more research is needed to determine which benefits are lasting.

Has the research found anything surprising?

BS: A concern with intermittent fasting is that people will overeat on non-fast days, which would be a risk, but so far that hasn’t been shown in research. So, that is a benefit because people aren’t overeating on the other days. With time-restricted feeding studies, results show improved sleep, reduced calorie intake, and weight loss in addition to improvements in blood pressure.

Researchers believe that there could also be a role for intermittent fasting in situations where usual weight loss efforts have not been successful. So, for people like overnight shift workers, or in weight regain after loss, and also in metabolic issues like dyslipidemia and diabetes. There may be positive benefits for people who have achieved weight loss or are at a normal weight, but still want to improve metabolic parameters like A1C or Cholesterol.

What are the potential downsides to intermittent fasting?

EC: In research people who have been assigned to an intermittent fasting diet have reported higher levels of hunger, especially after a few weeks on the diet. Interestingly, people with obesity reported less hunger and higher diet satisfaction in these studies.

Intermittent fasting is that it is not appropriate for everyone. It boils down to who is choosing this dietary pattern and to what extent they are restricting food intake.

To be clear, intermittent fasting is inappropriate for people who are on medications, which impact metabolism, including those for diabetes and also medications that should be taken with food.

I would also never recommend intermittent fasting during pregnancy or breastfeeding, in youth who are growing, and with anyone who has a history of restrictive food behaviors or eating disorders. Food cravings may increase after periods of food restriction. Over time this may lead to an unintentional and unhealthy obsession with food and eating.

I would also not recommend intermittent fasting for people who know they tend to graze on food throughout the day, changing your eating pattern to such a severe degree would likely result poor adherence and results.

With all this said, we just don’t know enough about intermittent fasting to know the long term health effects and the potential risks, both physiological and psychological. With any significant change to the diet or lifestyle, it’s important to establish nutrition care with a dietician or another health professional who can really help you determine what weight loss approach is a best match for you.

What are the health impacts of few to no nutrients on fasting days?

BS: The studies are in healthy, normal, overweight and obese individuals. They’re not in chronically ill populations, so in theory people should be okay with a 24-hour period of no intake and shouldn’t have any issues with macro-nutrients or micro-nutrients during those times. At this time, we don’t have enough information to know if that’s true in the long term.

But, this is part of the reason why no one would recommend intermittent fasting for anyone with any metabolic abnormalities, such as a patient who has diabetes or kidney disease – it would not be safe for these patient’s to fast without being medically supervised.

How do you work with patients who may want to try a diet or a trend like this?

BS: First and foremost, we review the patient’s medical and weight history, medications and eating patterns and also their goals and motivations to determine if they’re a good candidate to try and intermittent fasting

Then we try to lead a patient in a discussion of the potential pros and cons for them, and just try to get an idea of if this would even be manageable for them given their lifestyle.

Then we think it’s also important to encourage healthy eating when they are eating. It is important to meet patients where they are and help them achieve their goals. Focusing on what gets them motivated or excited is important and if that’s intermittent fasting and there are no contraindications to it, then I’d say go for it.

EC: Part of dietetics is helping patients determine which route is best from a personal, medical, and metabolic standpoint, while being realistic about the plan.

Get help in determining what your baseline calorie needs are so that if the approach is to reduce or restrict calories, then you’ll have an idea of how much energy to consume, what foods are the most nutrientdense and different ways to measure that energy consumption and total calorie consumption.

I challenge you all to think about what your current eating habits are because some of you may be surprised to learn that you’re already following some version of intermittent fasting.