What is the negative side of intermittent fasting? This popular diet trend promises weight loss and better health, but research shows some serious downsides you need to know about before you start.
Intermittent fasting means eating all your food within a set window each day. The most common version is the 16 to 8 method, where you fast for 16 hours and eat during an 8 hour window. Millions of people follow this eating pattern, but the science reveals it comes with real risks.
Does Intermittent Fasting Increase Your Risk of Heart Disease?
A large study of over 20,000 adults found something alarming. People who ate all their food within an 8 hour window had a 91% higher risk of dying from heart disease compared to those who ate over 12 to 16 hours.
This research was presented at the American Heart Association conference in 2024. The lead researcher, Victor Wenze Zhong from Shanghai Jiao Tong University, said the results surprised them. Short term studies had shown intermittent fasting improved blood pressure and cholesterol. But this long term data painted a different picture.
People who already had heart disease faced even greater danger. Those with existing heart conditions who ate within an 8 to 10 hour window had a 66% higher risk of cardiovascular death.
One possible reason for these findings is muscle loss. People who fast for long periods tend to lose more lean muscle mass, and low muscle mass increases heart disease risk.
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Does Intermittent Fasting Make You Lose Muscle Instead of Fat?
Research shows intermittent fasting can cause you to lose more muscle than a normal diet.
A study tracked 116 people doing time restricted eating for three months. Among participants tracked in person, 65% of the weight they lost came from lean mass. This is more than double what doctors consider normal. Weight loss typically should be only 20% to 30% from lean mass.
This matters because muscle burns more calories than fat. When you lose muscle, your metabolism slows down. Your body becomes weaker. You become more likely to regain weight later.
A review published in Frontiers in Nutrition found that intermittent fasting creates long periods where your body breaks down muscle protein. Without regular protein meals throughout the day, your muscles struggle to repair and grow.
The fix is to eat enough protein, at least 1.6 grams per kilogram of body weight daily, and spread it across multiple meals within your eating window. But many people doing intermittent fasting fail to meet these protein targets.
Can Intermittent Fasting Cause Eating Disorders?
Fasting increases the risk of binge eating and eating disorders. This is one of the biggest hidden dangers of intermittent fasting.
A five year study of adolescent girls found that fasting was a strong predictor of developing binge eating and bulimia. When you go long hours without food, your brain responds by craving high calorie foods. Once you start eating, you struggle to stop.
Research on Canadian teenagers and young adults found that people who practiced intermittent fasting had higher rates of all eating disorder behaviors. This included overeating, binge eating, vomiting, laxative use, compulsive exercise, and excessive fasting.
Women face the highest risk. The study found that women who did intermittent fasting in the past 12 months showed higher scores on eating disorder tests across the board.
A study of 682 college students found something even more concerning. People who combined intermittent fasting with low carb dieting had much higher rates of binge eating and food cravings than those who did just one or the other.
Experts at UIC recommend that anyone with a history of eating disorders avoid intermittent fasting completely.
What Side Effects Does Intermittent Fasting Cause?
A meta analysis of 15 studies involving 1,365 adults found several common side effects from intermittent fasting.
- Dizziness affected about 10% of people doing intermittent fasting
- Headaches were common, especially during the first weeks
- Fatigue and low energy levels hit many people
- Feeling cold, irritable, and unwell were reported frequently
- Hunger and food cravings made daily life difficult
The good news is that fatigue and headache rates were similar to control groups in some studies. But the dizziness was notably higher in people doing intermittent fasting, especially those without diabetes.
These side effects often improve after a few weeks as your body adjusts. But for some people, they never fully go away.
Does Intermittent Fasting Affect Women Differently?
Women may experience more problems with intermittent fasting than men. This comes down to hormones.
Research shows fasting can affect the gonadotropin releasing hormone, which controls estrogen and progesterone. When this hormone gets disrupted, your menstrual cycle can become irregular.
A study in rats found that fasting every other day for 12 weeks caused menstrual cycle problems and changes in reproductive hormones. While human studies show mixed results, the concern remains.
Cleveland Clinic dietitian Julia Zumpano explains that fasting can make estrogen and progesterone levels drop quickly. This affects not just fertility but also mood, sleep, and overall health.
One study of pre and postmenopausal women found that 8 weeks of time restricted eating did not change reproductive hormone levels in postmenopausal women. But researchers excluded perimenopausal women, the group most likely to have problems.
For women with polycystic ovary syndrome, intermittent fasting might actually help by lowering androgens. But for most other women, the hormonal effects remain unclear.
Is Intermittent Fasting Better Than Regular Calorie Counting?
Research says no. When calories are equal, intermittent fasting produces the same weight loss as regular dieting.
A University of Sydney study found that while intermittent fasting helped people lose weight, it lacked the health benefits of other weight loss methods. The researchers found no improvement in inflammation markers like C reactive protein. There was only a very small improvement in insulin sensitivity.
Professor Luigi Fontana, who led the study, said the results show that from a metabolic standpoint, not all calories are equal. Regular calorie restriction with good nutrition and exercise training had much bigger effects on insulin sensitivity and glucose control.
Meta analyses comparing low carb, low fat, and time restricted eating diets found they all produce similar weight loss when calories are matched. The diet you can stick with long term beats any specific approach.
As one nutrition researcher put it, you get to pick the tool you want. Pick the one that feels least restrictive to you.
Who Should Not Do Intermittent Fasting?
Certain groups should avoid intermittent fasting entirely.
- People with a history of eating disorders face serious risk of relapse
- Pregnant and breastfeeding women need consistent nutrition for themselves and their babies
- Children and teenagers need regular meals for growth and development
- People with diabetes who take insulin or other blood sugar medications can experience dangerous drops in blood sugar
- Anyone with a history of low blood pressure should be careful
- People who take heart or blood pressure medications may experience mineral imbalances
- Those who are underweight already should not restrict eating times
If you experience any of these warning signs while doing intermittent fasting, stop immediately and talk to a doctor.
- Your menstrual cycle stops or becomes irregular
- You have trouble sleeping
- You notice hair loss, dry skin, or acne
- Wounds heal slowly
- You feel stressed, anxious, or depressed
- You start binge eating during your eating window
Does Intermittent Fasting Work Long Term?
Most weight loss research on intermittent fasting covers only short periods, usually less than 12 weeks. We lack good data on what happens over years.
The worrying cardiovascular study followed people for years and found increased death risk. This suggests the short term benefits might not last.
Research on weight loss in general shows a troubling pattern. Six out of seven people who lose weight eventually gain it back. The key to keeping weight off is making permanent lifestyle changes, not temporary diet rules.
People who maintain weight loss long term tend to exercise regularly, weigh themselves often, and make sustainable food choices. Whether intermittent fasting fits into that picture depends on the person.
One key finding from long term weight loss research is that people who keep the weight off had to develop a new identity around food and exercise. Temporary diets rarely lead to permanent results.
FAQ
Does intermittent fasting slow your metabolism?
Short term fasting does not significantly slow metabolism. But if you lose muscle mass from fasting, your metabolism will drop because muscle burns more calories than fat. Eating enough protein and doing resistance training can prevent this.
How long does it take for intermittent fasting side effects to go away?
Most people adjust within 2 to 4 weeks. Headaches, fatigue, and irritability usually fade as your body gets used to the new eating schedule. But some people never fully adjust and continue to feel worse.
Can intermittent fasting cause hair loss?
Yes. If you do not get enough nutrients during your eating window, your body prioritizes vital functions over hair growth. Hair loss is a sign you may not be eating enough overall or missing key nutrients.
Is the 16 to 8 method safer than other fasting schedules?
The 16 to 8 method appears to be the most studied and may be easier to stick with than longer fasts. But the cardiovascular study showed increased risk even with 8 hour eating windows. Eating within 12 to 16 hours may be safer.
Should I try intermittent fasting for weight loss?
Intermittent fasting is one option among many. It works no better than counting calories when total intake is the same. Choose the approach you can maintain long term. If fasting makes you binge eat, crave food constantly, or feel terrible, try a different method.
Does intermittent fasting affect sleep?
Eating close to bedtime can reduce melatonin levels and disrupt sleep quality. Some forms of intermittent fasting, especially those where you eat late, may interfere with your sleep cycle.
Is intermittent fasting safe for older adults?
Older adults face higher risk of muscle loss, which already accelerates with age. Losing additional muscle from fasting can increase fall risk and reduce quality of life. Older adults should prioritize protein intake and consult a doctor before trying intermittent fasting.
What happens if I break my fast early?
Nothing bad happens. Your body does not have a timer that resets. If you need to eat, eat. Listening to hunger signals is more important than hitting an exact fasting window.
While intermittent fasting has its drawbacks, understanding which fat deposits are hardest to eliminate can help you set realistic expectations. If you’re looking to build lean mass instead, learning about the fastest muscles to develop might shift your focus. A Southbank personal trainer can help you find a sustainable nutrition and training balance.
