By now, you’ve probably heard that intermittent fasting is a popular way to lose weight. However, any diet—especially one that involves fasting for extended periods of time—can have varying side effects, which brings up the question: is intermittent fasting safe for everyone? We spoke with Patricia Bannan, MS, RDN, and LA-based nutritionist and healthy cooking expert, Cedrina Calder, MD, and registered dietitian Cynthia Sass to find out why some people shouldn’t try intermittent fasting (also know as IF) because of varying health reasons.
But first, Bannan explains exactly how IF is a good method for losing weight. “Intermittent fasting causes glucose (sugar) concentrations to decrease and lipolysis (fatty acid oxidation) to increase significantly during the first 24 hours, which helps the body break down stored fat.” In other words, IF works to blast fat and fast. While it’s effective, it’s definitely not for everyone. Here are 11 reasons why you shouldn’t try the intermittent fasting diet.
It can interfere with your sleep.
Receiving adequate sleep each night is crucial for healing and repairing muscles from exercise, supporting brain function, and even maintaining emotional well-being. Going to bed hungry can make it challenging for the body to relax and fall asleep, as this causes your brain to be alert, and, as a result, your body feels restless. Sass weighs in: “I’ve had clients struggle with falling asleep or staying asleep if the end of their IF eating window is too early in the day. Inadequate sleep carries a number of health risks, and sleep is when your body does a lot of healing and repair work.”
Not to mention that when you haven’t eaten in several hours, your blood sugar levels naturally drop, which can cause you to abruptly wake up in the middle of the night, feeling anxious. Disruptions during sleep can be harmful to your health, especially when they occur during the most pivotal stage of sleep, known as the rapid eye movement (REM) cycle. This stage is important for retaining information that you learned during the day and storing it into memory, and it repeats several times throughout the course of your sleep. Of course, not having enough sleep can cause other complications to arise, in addition to not being able to remember things.
“Too little sleep can also interfere with weight management and can pose a safety risk, in terms of cognitive function and driving,” says Sass.
You’ve had a history of disordered eating or eating disorders.
According to the Academy of Nutrition and Dietetics, “Disordered eating is used to describe a range of irregular eating behaviors that may or may not warrant a diagnosis of a specific eating disorder.” Disordered eating is described as a descriptive phrase rather than a diagnosis. However, if irregular eating patterns and habits are not addressed, it can turn into an eating disorder such as anorexia nervosa, bulimia nervosa, or binge eating. Sass says that intermittent fasting may not be a wise choice for anyone that has experienced either disordered eating or an eating disorder.
“Any strategy that encourages restriction can trigger a disordered pattern in people with this history. For anyone, but particularly for someone with this history, it’s so important to listen to your body, and be mindful of what allows you to feel well both physically and emotionally. If limiting your eating window doesn’t support this, it’s not the right path for you,” she says.
Calder concurs: “Fasting can worsen an already unhealthy relationship with food. It can also make symptoms of an eating disorder worse or reoccur.”
Bannan also adds that “this dietary approach is not appropriate for people with a history of an eating disorder since it plays into the ‘binge eating’ and ‘restrictive’ mindset.”
You’re engaging in intensive training or trying to build muscle mass.
As one would likely assume, attempting to do intermittent fasting while engaging in an intense training cycle is not an ideal—or safe—combination. If you’re training for a marathon or regularly do CrossFit, you may want to reconsider doing IF. Oftentimes, you will need to eat something before you exercise to help you power through your workout. It’s also extremely important to eat something after you’ve finished exercising, too. “During a hard workout, you will put little tears in your muscle and deplete your glycogen stores,” says Kacie Vavrek, a sports medicine registered dietitian at The Ohio State University Wexner Medical Center. “A recovery meal within 1-2 hours plus regular meals every 3-4 hours after will help to replace glycogen stores and repair and rebuild muscle throughout the day.” Vavrek says skipping this post-workout meal could prolong your recovery and even inhibit essential muscle-building and repair.
Similarly, if you’re trying to gain muscle, Sass says it’s important that you consume protein at different times throughout the whole day, rather than trying to jam-pack it all into a specific eating window. In fact, many experts say that your body cannot properly metabolize more than 30-35 grams of protein per sitting. As a result, any excess protein that’s consumed and not utilized (ex: working out, weight lifting) during the day typically stores as fat in the body, not muscle.
Sass says that “spreading protein throughout the day and eating a protein-rich snack about one hour before bed” are two research-based strategies that will help you achieve the best muscle-building results. “Narrowing your eating window to just eight hours counters this approach,” she adds.
You have digestion issues.
As if digestion issues weren’t cumbersome enough to deal with on their own, adding a wonky eating schedule into the mix can only cause more gastrointestinal distress. “If you already have problems with digestion (e.g. IBS), intermittent fasting may worsen your symptoms,” Calder says.
IF can even spur digestive issues due to the prolonged fasting bouts. “Periods of fasting can disrupt the normal activities of the digestive system, causing constipation, indigestion, and bloating,” she says.
Bannan also says that eating large meals—which is often required for the types of IF that call for a long fast—can cause gastrointestinal stress. “This is particularly concerning for people with IBS, who already have a more sensitive gut,” she explains.
Energy, focus, and concentration are critical for your daily activities.
Food provides sustenance and energy, and it enables you to focus. When you’re extremely hungry, all you can think about is food, which diverts your attention away from immediate tasks at hand. Of course, everyone responds to IF differently—it’s dependent on the person—but know that it may initially hinder your ability to concentrate if you’re not already used to going long periods of time without eating.
“Although some people report increases in energy with intermittent fasting, others may experience fatigue, reduced concentration, and low energy levels,” says Calder. “This can affect your productivity in your daily work. If you have the type of career or engage in activities where energy and concentration are needed, intermittent fasting may not be right for you.”
You have diabetes.
People who live with diabetes already deal with frequent spikes and drops in blood sugar throughout the day, so the last thing they need is to heighten those blood glucose responses via fasting. This is especially concerning for those who have type 1 diabetes, because the pancreas cannot produce insulin—the hormone that takes sugar from the bloodstream and transfers it to various cells in the body such as muscle tissue, adipose (fat) tissue, and even your liver. People who have type 1 diabetes often need to take insulin injections so that they can eat food without going into a state of hyperglycemia, in which there is too much sugar in the bloodstream.
“If you’re a diabetic and currently on medications for diabetes, especially insulin, you should never do intermittent fasting without consulting with a doctor first and being monitored closely,” says Calder. “Intermittent fasting combined with diabetes medications can cause your blood sugar to get dangerously low.”
Calder even says that anyone who experiences issues with low blood sugar should avoid partaking in IF because it’s critical for them to consume food periodically to maintain adequate blood sugar levels.
You’re pregnant or breastfeeding.
Engaging in IF while pregnant or breastfeeding can pose a threat to a child’s development. Calder says, “Pregnancy and breastfeeding require adequate intake of calories for proper development of the baby and milk production. Periods of fasting will interfere with your caloric intake, so pregnant and breastfeeding women should not do intermittent fasting.”
If you’re trying to get pregnant, IF may also not be the diet of choice for you. Bannan points out that IF may also be linked to fertility issues, causing changes in menstruation, disruptions in metabolism, and even prompting early menopause in women.
You’re on medication that must be taken with food.
There are some medications that must be taken in the presence of food because without it, they can make you feel nauseated or light-headed, among many other side effects. IF fasting periods could even affect people who take a handful of vitamins or supplements every day. For example, those who have a low iron count in their blood or have anemia may have to take a daily iron supplement (or several) to help restore iron levels. Iron supplements are notorious for causing nausea, and taking it with food can help suppress that feeling. The time at which you take an iron supplement may be flexible, but what if you’re on a medication that must be taken at a very specific time of day and with food? That’s when things get a little sticky, and ultimately, it’s just not a good idea to dive into this diet if it doesn’t work with your medication.
You have a weak immune system or cancer.
Those who have recently experienced a major illness or are currently facing one should not engage in IF without clearing it with a doctor first. Here’s why: “In most cases, adequate caloric intake is needed in order to maintain lean body mass and a healthy immune system, which is necessary for individuals with cancer or weakened immune systems,” says Calder. “These individuals should speak with a doctor before trying intermittent fasting.”
Your lifestyle can’t accommodate the eating hours.
Your work schedule can greatly influence your ability to engage in IF successfully. For example, if you work the night shift and have to sleep during the day, but one of your eating periods falls within the daytime, what do you do? Or worse, what if the majority of your fast occurs while you’re hard at work. Or, what if you work different shifts each day and never have a consistent schedule? Bannan says that fasting intervals can make you feel cold and experience headaches and mood swings. Having to cope with all of those potential side effects could distract you from work and make you less productive.
You don’t want to eat in a designated time frame.
It takes a lot of mental strength to commit to IF. “It takes a lot of willpower to go for extended periods of time without food,” says Calder. “If you’re not mentally ready to do this, you could potentially develop an unhealthy relationship with food. Fasting periods may cause you to obsess over food, leading to overeating and bingeing during non-fasting periods.”