Intermittent fasting is generally safe for healthy adults, but several groups of people should avoid it entirely or approach it with serious caution. These include pregnant and breastfeeding women, anyone with a history of eating disorders, people with type 1 diabetes, children and adolescents, individuals who are underweight, and people taking certain medications that require food.
Pregnant and Breastfeeding Women
Pregnancy increases caloric and nutrient demands significantly, and fasting disrupts the steady supply of glucose and amino acids that a developing fetus needs. A large meta-analysis covering more than 1.3 million pregnancies found that intermittent fasting during pregnancy was associated with a small but statistically significant reduction in birth weight. Animal studies help explain why: fasting in pregnant mice impaired the placenta’s ability to transport amino acids to the fetus, leading to restricted growth in the womb.
Timing matters too. Fasting during the first trimester appears more likely to affect fetal development than fasting later in pregnancy. The biological explanation is that fasting triggers temporary drops in blood sugar, increased fat breakdown, and ketone production, all of which can alter how the placenta delivers nutrients during the critical early weeks of organ formation. There’s also emerging evidence that maternal fasting may cause changes in how fetal genes are expressed, potentially affecting the baby’s metabolic programming long term.
For breastfeeding women, the concern is similar: restricted eating windows can reduce overall caloric intake and hydration at a time when the body needs both to maintain milk supply and nutrient density.
People With Eating Disorder History
This is one of the clearest contraindications. Clinical guidance is straightforward: people with an active eating disorder, a history of an eating disorder, or a pattern of disordered eating should never be encouraged to practice intermittent fasting. The structured restriction of eating windows can trigger restrict-then-binge cycles that worsen symptoms rather than improve health.
Research on adolescents and young adults found that intermittent fasting was independently associated with a higher likelihood of binge eating and food cravings compared to people who didn’t diet. When combined with other restrictive approaches like low-carb diets, the risk of binge eating increased even further. Women and gender-diverse individuals appear especially vulnerable. Social media amplifies this risk by promoting fasting as a weight loss tool to younger audiences who may already have fragile relationships with food.
One study found that people who entered a fasting trial with low baseline levels of disordered eating did fine, while those who started with higher risk factors got worse. In other words, intermittent fasting doesn’t create eating disorders out of nothing in most people, but it reliably aggravates existing vulnerabilities.
People With Type 1 Diabetes
Fasting with type 1 diabetes creates a dangerous push and pull with blood sugar. Without regular food intake, the risk of hypoglycemia (dangerously low blood sugar) increases, especially for people on insulin. At the same time, skipping meals can paradoxically lead to hyperglycemia if insulin doses aren’t carefully adjusted. The International Diabetes Federation lists four specific complications of fasting with diabetes: low blood sugar, high blood sugar, dehydration, and diabetic ketoacidosis, a life-threatening condition where the body produces excessive ketones.
People with type 2 diabetes on blood sugar-lowering medications face similar, though sometimes less acute, risks. The core problem is that fasting changes the timing and magnitude of blood sugar swings, making medication dosing unpredictable. If you take insulin or other diabetes medications, any fasting plan would need close medical supervision and likely medication adjustments.
Children and Adolescents
Growing bodies need consistent energy and nutrients. Children and teenagers are building bone, muscle, and brain tissue at rates that adult bodies simply aren’t, and caloric restriction during these years can interfere with that process. Research on intermittent fasting in youth has flagged the risk of inhibiting growth, particularly in adolescent girls following very low calorie fasting protocols.
Studies in young active women have shown decreases in IGF-1 (a hormone essential for growth) and free testosterone during time-restricted eating. While these shifts may be insignificant for a fully grown adult, they could meaningfully affect a teenager still going through puberty. Beyond the physical risks, there’s also the behavioral concern: adolescents who adopt fasting patterns may be more likely to develop disordered eating habits that persist into adulthood. The short duration of most studies in this age group means researchers still don’t fully understand the long-term consequences for development.
People Who Are Underweight
If your BMI is below 18.5, you’re classified as underweight by the World Health Organization, and fasting could push your body into a genuinely dangerous deficit. A BMI below 17.0 indicates moderate to severe thinness, and below 16.0 is associated with a markedly increased risk of serious illness, poor physical performance, lethargy, and death.
Losing additional weight when you’re already underweight compromises your immune system, weakens bones, and drains energy reserves your body relies on for basic functions. Harvard Health notes that people who are marginal in body weight risk losing too much during fasting, with consequences for bone density, immunity, and daily functioning. Intermittent fasting is designed to create periods of caloric deficit, which is the opposite of what an underweight person needs.
Older Adults at Risk of Muscle Loss
Adults over 65 naturally lose muscle mass with age, a process called sarcopenia. The concern with intermittent fasting in this group is that compressed eating windows make it harder to consume enough protein and total calories to preserve muscle. While direct research on fasting and muscle outcomes in older adults is limited, the existing data on younger adults shows that fasting can reduce lean mass even when combined with resistance training.
For an older person, losing muscle means losing mobility, balance, and independence. The protein needs of aging adults are actually higher per pound of body weight than those of younger adults, and fitting adequate protein into a six- or eight-hour eating window is a practical challenge that many older people won’t meet. People on blood pressure or heart medications also face increased risk of sodium, potassium, and other mineral imbalances during prolonged fasting periods.
People Taking Certain Medications
Several categories of medication interact poorly with fasting. Insulin and other diabetes drugs are the most obvious, since they’re dosed around expected food intake. Blood-thinning medications, including warfarin, can become unpredictable when eating patterns change significantly, because dietary vitamin K intake (which counteracts blood thinners) fluctuates with irregular meals. Antiplatelet drugs that slow blood clotting also interact with fasting in ways that may increase bleeding risk.
Beyond these specific interactions, many common medications simply need to be taken with food to work properly or to avoid stomach damage. If your medication label says “take with food,” a prolonged fasting window forces you to either skip doses or take them on an empty stomach, neither of which is a good option.
Warning Signs That Fasting Isn’t Working for You
Even people who don’t fall into the categories above can run into trouble. Common side effects like headaches, irritability, fatigue, and constipation are typical in the first week or two but should improve. If they persist, your body may be telling you something.
More serious signals include persistent dizziness, hair loss, feeling cold all the time, losing your menstrual period, or noticing significant unintended weight loss. Electrolyte imbalances can cause muscle cramps, heart palpitations, and confusion, particularly during longer fasts. If you’re experiencing any of these, the fasting protocol is doing more harm than good regardless of what category you fall into.