The allure of not counting macros or calories coupled with the promise of weight loss and longevity has thrust intermittent fasting into the limelight. It's a trend that doesn't appear to be going anywhere anytime soon.
While the research on the potential health benefits and risks of this structured way of eating continues to unfold, some dietitians are heeding caution before jumping on the One Meal a Day (OMAD), Alternate Day Fasting (ADF) or Warrior Diet bandwagon.
Here are six things dietitians want you to know before you give fasting a try:
1. You’ll Likely Be at a Greater Risk for a Nutritional Deficiency
Most Americans are currently eating consistently throughout the day. According to the Centers for Disease Control and Prevention (CDC), only 10 percent of us are getting enough fruits and vegetables and 95 percent are falling short on fiber. It's no wonder why there's cause for concern that so many want to shorten this "feeding window" to just six or eight hours — or every other day.
"Depending on how long you fast and what you eat when you are eating, fasting could lead to vitamin and mineral deficiencies in addition to an inadequate amount of macronutrients like protein, carbohydrates and fats," says Bonnie Taub-Dix, RDN and author of Read It Before You Eat It - Taking You from Label to Table. "It's also likely that you won't meet your needs for fiber."
2. Your Gut Might Not Be Happy About It
It's human nature to look forward to meal time and perhaps even overdo it a bit when you've gone for so long without eating.
"[When intermittent fasting], people are usually prone to eat very large meals during eating times, which can cause gastrointestinal (GI) stress leading to indigestion and bloating," says dietitian Patricia Bannan, RDN. "This is particularly concerning for people with irritable bowel syndrome (IBS), who already have a more sensitive gut."
While no one likes to feel stuffed or bloated after a meal, those with IBS are especially vulnerable. According to the International Foundation for Gastrointestinal Disorders, about one out of every 10 people suffers from this condition.
3. A Healthy Diet Will Get You the Same Weight-Loss Results
You don't have to go to extremes to get results. You'll probably get better, lasting results if you don't.
Researchers in a July 2017 study published in the Journal of American Medical Association of Internal Medicine looked at both intermittent fasters and those who just restricted calories and found that both groups lost similar amounts of weight, but the fasting group had a higher dropout rate.
By making smaller changes to our diets like monitoring our portions and incorporating more whole foods like whole grains, fruits, vegetables, legumes and lean proteins, we're making less room for the unhealthy stuff with added sugars and salt.
4. You’ll Probably Feel Uncomfortable (at Least at First)
If you think missing your snack between lunch and dinner makes you hangry, try going 24 hours (or longer) without food. While not all types of fasting are this extreme, ADF allows you to eat unlimited amounts one day and then no food or caloric beverages the next.
"Besides the discomfort of a rumbling belly, fasting could bring uncomfortable side effects," says Taub-Dix. "Skipping meals could cause blood sugar levels to plummet making some people feel lightheaded, dizzy and weak. It can also lead to irritability and a lack of focus."
Proponents of fasting claim that hunger pangs subside over time and actually list mental clarity as a benefit of this way of eating, but these are definitely concerns worth considering.
5. It Can Screw With Your Hormones
Intermittent fasting can do a number on our hormones. A March 2019 study published in Nutrients validates that excessive calorie restriction can lead a disturbance in women's menstrual cycles and a decrease in testosterone for men.
Additionally, animal studies (including this January 2013 research published in PLOS ONE) have found that intermittent fasting disrupts the reproductive cycle in females and lowers testosterone levels in males.
6. It’s Not Appropriate for Everyone
As with most diets, one size does not fit all and fasting it no different. Because this type of eating is so structured with set times of when to eat and when not to eat —ultimately forcing you to ignore your hunger cues — it is not appropriate for anyone with a history of an eating disorder.
There are also safety concerns with certain conditions. "People with diabetes or low blood sugar need glucose throughout the day and fasting for periods of time can have dangerous effects," adds Bannan.
According to Harvard Health, people on blood pressure medication, or people with electrolyte imbalances should avoid fasting. Additionally, kids, pregnant women or women who are trying to conceive should refrain from this type of eating pattern.
- Centers for Disease Control and Prevention: "Only 1 in 10 Adults Get Enough Fruits or Vegetables."
- International Foundation For Gastrointestinal Disorders: "Facts About IBS"
- Journal of American Medical Association of Internal Medicine: " Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial."
- American Journal of Lifestyle Medicine: "Closing America’s Fiber Intake Gap"
- Journal of the Academy of Nutrition and Dietetics: "Added Sugars Intake Across the Distribution of US Children and Adult Consumers: 1977-2012."
- American Heart Association: "9 out of 10 Americans Eat Too Much Sodium Infographic"
- PLOS ONE: "Intermittent Fasting Dietary Restriction Regimen Negatively Influences Reproduction in Young Rats: A Study of Hypothalamo-Hypophysial-Gonadal Axis"
- Nutrients: "Intermittent Fasting in Cardiovascular Disorders—An Overview"
- Harvard Health: Not so Fast: Pros and Cons of the Newest Diet Trend"