Expert Q&A: A Nutritionist Answers Common Fasting Questions

Everyone has a strong opinion about intermittent fasting. Some call it life-changing. Others say it’s dangerous. Social media shows transformation photos. Your doctor raises concerns. You’re stuck sorting through conflicting advice about eating windows and whether coffee “breaks your fast.”

Should you try it? Will it work? Is it safe? We asked registered nutritionists who counsel patients daily to answer the questions they hear most.

We gathered the most common questions people ask about intermittent fasting and got answers from nutritionists who counsel patients daily.

Question 1: Does Intermittent Fasting Actually Work for Weight Loss?

Julia Zumpano, RD, LD, Registered Dietitian with Cleveland Clinic Preventive Cardiology, answers:

Yes, but it’s not magic. “Intermittent fasting is just when you restrict your eating for a period of time,” Zumpano explains. “It can be done in many ways”. The weight comes off because you’re eating fewer hours each day. Most people naturally consume less when they shrink their eating window.

A 2025 University of Colorado study found people lost 7.6% of body weight with 4:3 fasting (three fasting days per week) versus 5% with daily calorie restriction over one year. That’s the real weight that stayed off.

But some people compensate by eating more on non-fasting days. If you consume the same total calories, your weight stays the same.

Track your progress with our fasting tracker to see your actual results over time.

Question 2: What Side Effects Should I Expect When I Start?

Brandy Sarrette, RDN, LDN, Clinical Dietitian with Baystate Health, answers:

You’ll feel rough at first. That’s your body adapting.

“When done day after day, week to week, or month to month, fasting can work to decrease your metabolism,” Sarrette warns. But in the beginning, side effects are normal.

In a study of 147 people, 61.3% experienced headaches, 68% had lethargy, 57.8% dealt with mood swings, and 55.8% felt dizzy. The good news is that these symptoms usually disappear after two to four weeks once your body adjusts to the new pattern.

What’s normal:

• Hunger pangs (especially days 1-3)
• Mild headaches
• Feeling tired or foggy
• Irritability

What’s NOT normal:

• Extreme weakness where you can’t walk
• Severe headaches that won’t stop
• Constant nausea
• Feeling faint when standing

If severe symptoms last more than three weeks, stop fasting and talk to your doctor.

Question 3: Can I Drink Coffee and Water While Fasting?

Julia Zumpano, RD, LD, answers:

Yes. Black coffee is allowed.

“Generally, fasting means that you should not be eating any solid food and also not consuming any beverages that have calories,” Zumpano says. “Coffee is an exception and so is tea. They technically have five calories or less per serving”.

What you CAN drink:

• Water (drink lots)
• Black coffee (no sugar, no cream)
• Plain tea (no sweeteners)
• Sparkling water (unsweetened)

What you CANNOT drink:

• Coffee with milk or cream
• Juice
• Smoothies
• Protein shakes
• Anything with calories

Anything above 5-10 calories generally breaks the fast.

Water is critical. Dehydration makes hunger worse and triggers headaches. Drink more than you think you need.

Question 4: Will I Lose Muscle If I Fast?

A dietitian answers:

You can lose muscle with any diet. But you can prevent it. The key is what you eat during your eating window and whether you lift weights. Nutrition experts recommend high-protein, high-fiber, and high-quality fats during your eating window.

Research shows there is minimal protein loss with a 16:8 regimen or even after a couple of days of fasting. Your body burns fat for fuel during fasting rather than burning muscle.

Protect your muscles by:

1. Eating enough protein (aim for your body weight in grams)
2. Doing resistance training 2-3 times per week
3. Not cutting calories too drastically on eating days

Elite athletes train fast. Your workout might feel harder, but your muscles will be fine if you eat right and lift weights.

Question 5: How Long Should I Fast Each Day?

Julia Zumpano, RD, LD, answers:

Start with 14-16 hours. Most people find this sustainable. “Intermittent fasting isn’t about starving yourself,” Zumpano explains. “It’s about cutting back on calories for short time periods”.

The 16/8 method is most popular. You fast for 16 hours and eat during an 8-hour window.

Here’s how 16/8 looks:

• Stop eating at 8 PM
• Skip breakfast
• First meal at noon
• Last meal before 8 PM

A 2025 study found that people who ate earlier in the day showed better insulin sensitivity and blood sugar control. This is because our bodies are more insulin sensitive in the morning.

Try different windows. Some people skip dinner instead of breakfast. Experiment with 8 AM to 4 PM or noon to 8 PM. Find what fits your life.

Use our AI assistant to find the best fasting schedule based on your work hours and lifestyle.

Question 6: Is Fasting Safe for Everyone?

Brandy Sarrette, RDN, LDN, answers:

No. Some people should never fast. “If you have a chronic medical condition like diabetes, heart disease, high blood pressure, or kidney stones it’s important to consult with your doctor or a registered dietitian before beginning any restricted eating plan,” Sarrette stresses.

Who should NOT fast:

• Children and teenagers
• Pregnant or breastfeeding women
• People with eating disorders (past or present)
• People with Type 1 diabetes
• People on blood pressure medications
• People with kidney disease
• Those with heart conditions

People who take medications for blood pressure or heart disease may be more prone to imbalances of sodium, potassium, and other minerals during fasting.

Women’s health note: Some women report their menstrual periods stopping when they started fasting. The periods returned to normal when they stopped fasting. Always talk to your doctor before your first fast if you take medications or have health conditions.

For a comprehensive breakdown, read our full guide: Who Should Avoid Intermittent Fasting?  Your Ultimate Safety Guide

Question 7: What Health Benefits Can I Actually Expect?

Heather Mangieri, MS, RD, CSSD, Nutrition CheckUp, answers:

Blood pressure improvements are the most consistent benefit across studies.

The most significant improvement seen across multiple studies is in both systolic and diastolic blood pressure. These reductions happen even without weight loss.

Other proven benefits:

• Reduced insulin resistance (helps prevent Type 2 diabetes)
• Lower LDL cholesterol
• Reduced inflammation markers
• Better blood sugar control

What about thinking clearly while fasting? A 2025 analysis of 71 studies with 3,484 participants found no meaningful difference in cognitive performance between fasted and fed states. You won’t think worse while fasting.

But Mangieri cautions: “It’s ultimately their choice whether to take our advice,” she says about clients who want to try fasting. She recommends working with a registered dietitian to guarantee proper nutrition.

Question 8: What Should I Eat During My Eating Window?

Julia Zumpano, RD, LD, answers:

Quality matters more when you eat less often. “We can encourage a low-carbohydrate diet under a dietitian supervision with lean sources of protein, mainly carbohydrates coming from vegetables,” Zumpano advises for her cardiac patients.

Focus on:

• High-protein foods (chicken, fish, eggs, Greek yogurt)
• High-fiber vegetables and fruits
• Healthy fats (avocado, nuts, olive oil)
• Whole grains in moderation

Avoid:

• Binge eating during your window
• Processed foods and sugar
• Skipping protein
• Not eating enough overall

Registered dietitians recommend three balanced meals spread evenly throughout your designated eating window. Don’t save all your calories for one giant meal.

Ready to Start?

Here’s what experts want you to remember:

Fasting works for weight loss. Expect to lose 3-8% of your body weight if you stick with it. The 4:3 method showed the best results in 2025 research, with 7.6% weight loss over one year.

Side effects are normal for 2-4 weeks. Headaches and hunger mean your body is adapting, not that you’re doing something wrong.

Start with 14-16 hours. The 16/8 method is most popular and easiest to maintain long-term.

Not everyone should fast. Check with your doctor first if you have any health conditions, take medications, or are pregnant.

Work with a registered dietitian. They can help you choose the right foods during your eating window and guarantee you’re meeting your nutritional needs.

As Sarrette notes, “Intermittent fasting may be the answer for those with insulin resistance, but it may put others with heart disease and other conditions at risk. It’s important to weigh your options carefully”. Intermittent fasting isn’t magic. It’s a tool. Some people thrive with it. Others don’t. Start small. Listen to your body. Adjust as you go.