Intermittent fasting has real downsides that don’t get as much attention as its benefits. The most significant cons include muscle loss that can outpace fat loss, a higher risk of binge eating patterns, potential nutrient deficiencies, hormonal disruption (particularly for women), and gallstone formation. Here’s what the evidence actually shows.
Muscle Loss Can Exceed Fat Loss
One of the biggest concerns with fasting is that you don’t just lose fat. During extended fasting periods, your body breaks down lean tissue (muscle, organ mass, and stored water in muscle) alongside fat stores. In a study published in the Journal of Cachexia, Sarcopenia and Muscle, participants who fasted for 10 days lost an average of 3.53 kg of lean soft tissue compared to just 2.34 kg of fat mass. That means roughly 60% of their total weight loss came from lean tissue, while only 40% came from fat.
That ratio matters. Losing muscle slows your resting metabolism, making it harder to keep weight off long term. It also affects strength, balance, and overall physical function, especially as you age. If you’re fasting primarily to lose body fat, a significant portion of what you’re losing may be the tissue you actually want to keep. Resistance training and adequate protein intake during eating windows can help offset this, but the underlying tendency remains a structural weakness of fasting-based approaches compared to steady caloric restriction paired with exercise.
Binge Eating and Disordered Eating Patterns
Restricting when you eat can set up a psychological cycle of deprivation followed by overconsumption. A study of 682 college students found that intermittent fasting was independently associated with a higher likelihood of binge eating and food cravings compared to not dieting at all. When participants combined intermittent fasting with a low-carb diet, the likelihood of binge eating increased even further beyond either approach alone.
The restrict-then-binge cycle is especially concerning for anyone with a history of eating disorders. Fasting can reinforce the kind of rigid, all-or-nothing thinking about food that characterizes conditions like anorexia and bulimia. That said, the picture isn’t entirely one-sided. In controlled settings, healthy adults following a 5:2 pattern (eating normally five days, restricting calories two days) did not show increases in disordered eating and actually reported fewer food cravings and mood problems during a four-week trial. The risk appears to depend heavily on your relationship with food going in.
Menstrual Cycle Disruption in Women
Women’s reproductive hormones are particularly sensitive to energy restriction. When the body detects a significant calorie deficit, it can suppress the hormonal signals that drive ovulation and menstruation. Research published in the Proceedings of the Nutrition Society found that healthy-weight women aged 18 to 30 experienced shortened luteal phases, missed ovulation, and irregular cycles when exposed to daily energy deficits of 470 to 810 calories (a 22% to 42% reduction) over the course of four menstrual cycles.
The mechanism is straightforward: energy restriction disrupts the pulsing release of the hormone that triggers the entire reproductive cascade. One study comparing a 5:2 intermittent fasting diet to standard daily calorie restriction in premenopausal women with overweight found that average cycle length was slightly longer with intermittent fasting (29.7 days versus 27.4 days), suggesting a mild but measurable impact on cycle regularity. For women trying to conceive or those who already have irregular periods, this is a meaningful drawback.
Nutrient Gaps From Compressed Eating Windows
When you shrink your eating window to six or eight hours, it becomes harder to fit in all the nutrients your body needs. Multiple randomized controlled trials have found that people practicing time-restricted eating and alternate-day fasting tend to fall short on calcium, magnesium, potassium, folate, vitamin C, and several B vitamins. These aren’t obscure micronutrients. Calcium and magnesium support bone and muscle function. Potassium regulates blood pressure. Folate is critical for cell division and is especially important during pregnancy. B vitamins drive energy metabolism.
The issue isn’t that fasting itself depletes these nutrients. It’s that eating fewer meals means fewer opportunities to get them, and most people don’t compensate by making their remaining meals more nutrient-dense. Over weeks and months, these small daily shortfalls can add up.
Gallstone Risk
Fasting increases the risk of gallstones through two distinct pathways. First, when you go long stretches without eating, your gallbladder doesn’t contract as often. Bile sits in the gallbladder longer and becomes overconcentrated with cholesterol, which can crystallize into stones. Second, if fasting leads to rapid weight loss, your liver secretes extra cholesterol into bile as it metabolizes stored fat, compounding the problem. Johns Hopkins Medicine lists both fasting and rapid weight loss as independent risk factors for gallstone formation. People who have had gallbladder issues before or who are losing weight quickly on an intermittent fasting plan should be aware that the combination of these two mechanisms makes gallstones more likely.
Cardiovascular Concerns Remain Unclear
A widely discussed study presented at an American Heart Association conference suggested that people eating within an 8-hour window had higher cardiovascular death risk. But a closer analysis published in JAMA found significant limitations. The group eating in fewer than 8 hours was very small (just 414 people), and those individuals had higher BMIs on average, which could explain the elevated risk on its own. The JAMA review concluded that 8-hour time-restricted eating was not associated with increased all-cause or cardiovascular mortality in people with existing heart disease risk factors, but it also stated the findings “do not support long-term use of 8-hour TRE for preventing cardiovascular death nor for improving longevity.” In short, the cardiovascular risks aren’t confirmed, but the hoped-for heart benefits aren’t confirmed either.
Effects on Sleep Are Unpredictable
You might expect that going to bed hungry (or conversely, eating a large meal in a compressed window) would affect sleep, and the research is genuinely mixed. A review of human trials found that most studies showed no change in sleep quality or duration with either time-restricted eating or alternate-day fasting. Seven of eight time-restricted eating trials reported no change in how long people slept.
However, one 12-week study that tracked participants using sleep-monitoring rings found that sleep latency (how long it takes to fall asleep) and sleep efficiency both worsened in the fasting group compared to controls. Other studies using wrist trackers and sleep questionnaires found no such effect. The bottom line is that most people won’t notice a difference, but a subset of fasters may find it harder to fall or stay asleep, and there’s no reliable way to predict which camp you’ll fall into before trying.
Who Should Avoid It Entirely
Certain groups face outsized risks from intermittent fasting. Pregnant women are a clear example. In surveys of pregnant women, the top concerns about time-restricted eating included safety for the baby (cited by 50.6%), nausea (50.3%), and hunger (42.7%). One respondent with gestational diabetes noted that fasting can raise blood sugar levels, which is the opposite of what someone managing that condition needs. Women who are underweight (BMI below 18.5) are also typically excluded from fasting studies due to the risk of further nutrient depletion and hormonal disruption.
People with a history of eating disorders face a unique danger, since the rigid structure of fasting windows can reinforce harmful restriction patterns and trigger binge episodes. Children, adolescents, older adults at risk of sarcopenia, and anyone taking medications that need to be taken with food at specific times are also poor candidates for fasting regimens. The cons of intermittent fasting aren’t universal, but for these groups, the risks clearly outweigh any potential benefit.