Fasting can be helpful for some people when it’s planned and supervised, but it also carries real risks and is not automatically healthy for everyone.

Quick Scoop

Fasting (including intermittent fasting) means going for set periods with little or no food, which can stress your body and mind if it’s too long, too frequent, or not suited to your health. Below are the main ways fasting can be bad for you, especially when done without medical guidance.

1. Immediate Side Effects People Notice

Common short‑term problems show up in the first days or weeks of fasting.

  • Strong hunger and food cravings.
  • Headaches and even migraines in some people.
  • Dizziness or feeling faint, especially when standing up.
  • Fatigue, low energy, “brain fog,” and trouble concentrating.
  • Irritability, mood swings, and feeling unusually emotional.
  • Bad breath and an odd taste in the mouth.
  • Insomnia or poor‑quality sleep.

Forum users often describe needing “a lot of mental strength” to get through hunger, boredom from not eating, and nights of poor sleep during longer fasts.

2. Digestive Problems and Dehydration

Changing when and how much you eat can upset digestion and fluid balance.

  • Constipation, bloating, or indigestion when your eating window suddenly shrinks.
  • Diarrhea or urgent bathroom trips, especially during extended fasts or when electrolytes are off.
  • Nausea or stomach discomfort after you break a fast with a big or heavy meal.
  • Dehydration and increased urination (polyuria) if you’re not drinking enough water or managing electrolytes.

Some fasters on forums mention needing to stay “close to a bathroom at all times” because of diarrhea during multi‑day fasts.

3. Blood Sugar Swings and Serious Risks

Fasting changes how your body handles blood sugar and energy.

  • Low blood sugar (hypoglycemia) can cause shakiness, sweating, confusion, weakness, and fainting.
  • People with diabetes or on blood sugar–lowering medications have a higher risk of dangerous lows if they fast without medical supervision.
  • Prolonged or very strict fasting can trigger extreme fatigue and make everyday tasks unsafe (driving, operating machinery, intense workouts).

In untreated cases, severe hypoglycemia can lead to seizures or brain damage, which is why experts strongly warn certain groups against fasting on their own.

4. Nutrient Deficiencies and Malnutrition

If fasting causes you to skip balanced meals, you can slowly slide into under‑nutrition.

  • Not getting enough protein, vitamins, and minerals increases the risk of malnutrition over time.
  • Symptoms may include unintentional weight loss, always feeling cold, weakness, frequent infections, and trouble concentrating.
  • Very restrictive patterns or repeated long fasts can worsen these risks, especially if you break your fast with highly processed, low‑nutrient foods.

One review notes that the long‑term safety and effectiveness of some fasting styles (like strict intermittent fasting) are still not clearly proven in humans.

5. Mental Health and Eating Disorder Concerns

Fasting is, by definition, a form of restriction, and that can be psychologically risky.

  • Rigid rules around when you “can” and “can’t” eat may increase anxiety about food and weight.
  • People vulnerable to eating disorders (past or present) may find fasting triggers obsessive thoughts, guilt, or cycles of bingeing and restricting.
  • Some treatment programs for eating disorders explicitly caution against intermittent fasting because it can worsen symptoms.

Professionals highlight that what looks like “disciplined fasting” online can sometimes hide an underlying eating disorder that needs proper care, not another diet trend.

6. Who Fasting May Be Especially Bad For

Health organizations and clinicians generally advise avoiding fasting or using extreme caution if you are in any of these groups:

  • People with diabetes, especially on insulin or other glucose‑lowering drugs.
  • Anyone with a history of eating disorders.
  • Pregnant or breastfeeding people.
  • Children, teens, and very active athletes who need steady energy and nutrients.
  • People with certain chronic illnesses, underweight individuals, or those recovering from surgery or serious illness.

For these groups, fasting can do more harm than good and should only be considered, if at all, with close medical supervision.

7. What Forums and “Latest Discussions” Say

Online discussions show a split: some people feel great on fasting, while others report clear downsides.

  • Users mention migraines that don’t go away, constant diarrhea, and no sleep during long fasts, even after trying electrolyte tricks.
  • Moderators on fasting communities often remind people to read health and electrolyte guides and to be wary of extreme claims or anti‑doctor rhetoric.
  • There’s pushback when commenters dismiss doctors completely, with others pointing out that online advice can be wrong or unsafe.

This mix of experiences is a reminder that “it worked for someone on a forum” does not mean it’s safe or right for you personally.

8. Key Takeaways: Why Fasting Can Be Bad

Fasting isn’t automatically dangerous, but it becomes risky when it is extreme, unsupervised, or mismatched to your health situation.

  • Side effects: hunger, headaches, fatigue, mood changes, digestive issues, poor sleep, dehydration.
  • Health risks: blood sugar crashes, nutrient deficiencies, possible impacts on reproductive health, and strain on mental health or eating patterns.
  • Not for everyone: many people—especially those with medical conditions—are advised to avoid fasting unless a clinician is actively monitoring them.

If you’re thinking about fasting or already doing it and feel unwell (extreme hunger, faintness, severe headaches, persistent fatigue), that’s a strong sign to stop and talk to a healthcare professional rather than just pushing through.

Information gathered from public forums or data available on the internet and portrayed here.