The metabolic changes that intermittent fasting can spur are generally healthy. However, going long periods of time without eating, or strictly controlling your intake, can have adverse effects for some people. “There is no one-size-fits-all or blanket dietary recommendation that can be made for all people, and that includes intermittent fasting,” says Dana Ellis Hunnes, PhD, RD, senior dietitian at the Ronald Reagan UCLA Medical Center in Los Angeles and author of Recipe for Survival. “What may be very helpful to one person may not work for another.” Determining whether intermittent fasting is right for you often comes down to trial and error on an individual level — but there are some general categories of people who may want to opt out of IF altogether. Keep reading to learn who should avoid this method.
Skip it when… You Have a History of Eating Disorders
Simply having disordered behavior around eating in your past doesn’t automatically mean you should skip intermittent fasting. But according to Hunnes, if you try the eating style, it’s very important to be aware of whether you’re adjusting to the factors — specifically in relation to past negative behaviors you’ve overcome.
“With intermittent fasting, many people also tend to reduce calories at the same time,” says Hunnes. This isn’t a requirement for fasting, but it can happen naturally because of the smaller time frame in which you’re eating during the day. “Unfortunately, people who have a history of eating disorders may begin to use fasting as a way to revisit slashing their calories.” Reducing consumption through fasting can be problematic, so if you have a history of this type of disorder but want to try IF anyway, Hunnes suggests meal planning in advance. This can help ensure you’re getting enough calories, even with a smaller eating time frame.
Skip it when… You’re a Kid
Because research on the effects of fasting in children is limited and shows inconsistent results, the current recommendation is that IF isn’t a good approach for children and adolescents, according to Leah Blalock, RD, pediatric dietitian at Children’s Health of Orange County in California. “Kids and teens are in periods of rapid growth and as such, have very specific needs in terms of calories, which may not get met with fasting,” she states. “A better approach is to emphasize intentional eating, such as having planned meals and snacks at certain times throughout the day to help manage hunger and portion control.”
Also, children can be at higher risk for problems from fasting because of their smaller size and increased metabolic needs, adds Omar Jaber, MD, attending physician with the Children’s National Health System in Washington, DC, and Virginia Hospital Center in Arlington. Those issues can include fatigue, weakness, and irritability. Even with short fasts, children show decreased cognitive function, he adds.
Skip it when… You’re Pregnant or Breastfeeding
When Hunnes was pregnant, she wasn’t just nauseous during the first trimester — she was nauseous the entire time. It was a challenge to get enough nutrients and calories. If she’d cut the time frame for eating, she suspects that would have been even tougher.
“Even if you’re not struggling with nausea, it can still be hard to get all the nutrients you need when you’re pregnant, and attempting to get those and the right amount of calories into just a couple of meals per day is definitely not recommended,” she explains.
The same goes for breastfeeding, she adds. According to the Centers for Disease Control and Prevention, breastfeeding moms need about 300 to 500 calories more per day to meet their nutritional needs. Adding those calories to your regular needs is hard, but squishing it all into a shorter time frame is even harder. Since baby rules the clock and you can’t really know exactly when you’ll be breastfeeding, this can put you at risk of falling short of needed energy.
What about if you’re trying to get pregnant? That depends on your situation. For example, a 2016 study in the Journal of Mid-Life Health found that women who have polycystic ovary syndrome, or PCOS, saw an increase in their levels of luteinizing hormone — which plays a key role in ovulation — through fasting. The same study noted that for those without PCOS, fasting may be beneficial for fertility if it leads to a reduction in excess weight, since that can have an effect on reproductive health overall. If you’re working with a reproductive endocrinologist, check with them to see if it could be helping or hindering your efforts to conceive.
Skip it when… You’re an Older Adult With Taste or Digestion Issues
If you’re over age 65, you may find that fasting is a great fit for an overall healthy lifestyle — but that’s often true only if there aren’t confounding variables, says Hunnes. If fasting causes you to lower your calorie intake, for example, that could be problematic.
“Older adults already have difficulty getting nutrients and enough total calories due to loss
of smell and taste, digestive issues, and lack of hunger signals,” Hunnes says. “Adding one more factor that can lower nutrient intake isn’t usually a good idea.” Older adults who aren’t eating enough — especially adequate protein — are at higher risk for sarcopenia, which is the age-related loss of muscle mass. That can affect a range of issues like mobility, strength, and even longevity.
Another tricky aspect to consider with IF can be medication use. There are several drugs — such as some types of antibiotics, aspirin, anti-inflammatory drugs, or metformin for diabetes, among others — that must be taken with food to prevent stomach upset. According to the CDC, about 22 percent of people aged 40-79 take five or more prescription medications regularly. It can be a lot to coordinate medications with your eating window.
Skip it when… You Feel Bad Doing It
No matter what combination of time frames you attempt, fasting simply might not be your groove. If it seems like IF makes you feel ravenous, irritable, exhausted, or gives you headaches, it could be best to opt out. In addition to these, you could experience side effects like digestive upset, lack of focus or concentration, light-headedness, difficulty sleeping soundly, intense cravings, or anxiety. Just keep in mind that some problems, like irritability and trouble concentrating, are usually temporary as your body adjusts, according to Jason Fung, MD, author of The Complete Guide to Fasting.
One way to speed the transition is make sure you’re getting enough calories, he suggests. “If you’re cutting way back on how much you eat, it’s more likely to backfire, especially in the beginning,” he says. “That also puts you at higher risk for swinging the other way and bingeing during your eating window.”
If your calories are at the appropriate level and you’re still struggling, stop fasting to see if the issues resolve. If they do, fasting may not be for you, says Dr. Fung. Or, you could try shortening your fasting time frame to a 12:12 plan (fast for 12 hours, then have a 12-hour eating window). As with any shift, if you have a chronic condition or concerns about your health, talk with your doctor first to make sure fasting will be helpful instead of harmful.
What if You Have Diabetes?
Although it may seem like intermittent fasting isn’t a good idea for those with diabetes since it could make it harder to regulate your blood sugar, studies indicate the opposite may be true. A research review on the effects of fasting for people with type 2 diabetes, published in Clinical Diabetes and Endocrinology in 2021, showed that the majority of studies indicated positive outcomes, including reduced body weight, decreased fasting glucose, decreased fasting insulin, and less insulin resistance. The researchers noted that some patients were even able to reverse their need for insulin therapy during intermittent fasting; this result is compelling enough to suggest physicians should discuss fasting with their patients who have type 2 diabetes.
People with type 1 diabetes will need to adjust their insulin dosing, which should be done
with your doctor’s supervision. No matter what kind of diabetes you have, always check with your doctor before starting a new diet or eating style and report any problems.
A version of this article appeared in our partner magazine The Complete Guide to Intermittent Fasting For Beginners.