Intermittent fasting FAQ
Who should NOT fast?
You should not do intermittent fasting if you are:
- Underweight (BMI < 18.5) or have an eating disorder like anorexia.
- Pregnant – you need extra nutrients for your child.
- Breastfeeding – you need extra nutrients for your child.
- A child under 18 – you need extra nutrients to grow.
You can probably fast, but may need medical supervision, under these conditions:
- If you have diabetes mellitus type 1 or type 2.
- If you take prescription medication.
- If you have gout or high uric acid.
- If you have any serious medical conditions, such as liver disease, kidney disease, or heart disease.
Won’t intermittent fasting put me into starvation mode?
Not likely. This is the most common myth about intermittent fasting, and generally it’s not true. In fact, some studies indicate that intermittent fasting may even increase the basal metabolic rate (at least initially) and might potentially improve overall body composition.Learn more
Can I exercise during fasting?
Yes. You can continue all your usual activities, including exercise, while fasting. You do not need to eat before exercising to provide energy. Instead, your body can burn stored energy (like body fat) for fuel.
However, for long-duration aerobic exercise, eating before exercise may increase performance. It’s also important to drink fluids and replenish sodium (salt) around exercise when fasting. This is good to know if you’re competing. Learn more
What are the possible side effects?
There can be a number of possible side effects of intermittent fasting. Here’s what to do if you encounter them:
- Hunger is the most common side effect of intermittent fasting. This may be less of an issue if you’re already on a keto or low-carb, higher-fat diet.Learn more
- Constipation is common. Less going in means less going out. However, keep in mind this is a normal response to eating less. It is not a concern and shouldn’t require treatment unless you experience significant bloating or abdominal discomfort. Standard laxatives or magnesium supplements can be used, if needed.
- Headaches are common and tend to disappear after the first few times on fasts. Taking some extra salt often helps mitigate such headaches.
- Mineral water may help if your stomach tends to gurgle.
- Other possible side effects include dizziness, heartburn and muscle cramps. Learn more
A more serious side effect is the refeeding syndrome. Fortunately, this is rare and generally only happens with extended fasts (5-10 days or more) when one is undernourished.
Since most of these side effects are manageable, they do not mean you have to stop your fast. However, if you truly feel unwell, are excessively dizzy, profoundly weak or have other severe symptoms, then you should break your fast.
Just remember to go slowly when you break it and prioritize fluids and salt (bone broth is a great way to start). And of course, if the symptoms persist, see your doctor immediately.
Fortunately, severe side effects are very rare, especially if you remain hydrated and supplement with electrolytes.
Learn more about common side effects of fasting
Why does my blood sugar go up during fasting?
While this does not happen with everyone, it can occur due to hormonal changes that occur during intermittent fasting. Your body is producing sugar in order to provide energy for your system. This is a variation of the dawn phenomenon and in general is not a concern as long as blood sugars are not elevated the rest of the day.
How do I manage hunger?
The most important thing to realize is that hunger usually passes like a wave. Many people worry that hunger during intermittent fasting will continue to build until it is intolerable, but this does not normally happen. Instead, hunger comes in a wave. If you simply ignore it and drink a cup of tea or coffee, it will often pass.
During extended fasts, hunger will often increase into the second day. After that, it gradually recedes, and many people report a complete loss of hunger sensation by day 3 or 4. Your body is now being powered by fat. In essence, your body is ‘eating’ its own fat for breakfast, lunch and dinner and therefore is no longer hungry. Learn more
Won’t intermittent fasting burn muscle?
That depends on the person and the duration of the fast. During fasting, the body first breaks down glycogen into glucose for energy. After that, the body increases body fat breakdown to provide energy.
Excess amino acids (the building blocks of proteins) are also used for energy, but the body does not burn its own muscle for fuel unless it has to.
Some studies, however, suggest that leaner individuals are at higher risk of lean body mass loss, and even reduced metabolic rate. Yet it appears this is less of a concern with overweight subjects.
In my experience with over 1,000 patients on various intermittent fasting regimens, I have not yet seen a single case of significant muscle loss.Learn more
What are your top tips for intermittent fasting?
Here are the seven top tips, briefly:
- Drink water.
- Stay busy.
- Drink coffee or tea.
- Ride out the hunger waves.
- Give yourself one month to see if intermittent fasting (such as 16:8) is a good fit for you.
- Follow a low-carb diet between fasting periods. This reduces hunger and makes intermittent fasting easier. It may also increase the effect on weight loss and type 2 diabetes reversal, etc.
- Don’t binge after fasting
Learn more practical intermittent fasting tips
How do I break a fast?
Gently. The longer the fast, the more gentle you might have to be.
Eating too large a meal after fasting (a mistake that we have ALL done, myself included) can give you a stomach ache. While this is hardly serious, people usually learn quickly to eat as normally as possible after a fast.
Learn more about breaking a fast safely
Isn’t it important to have breakfast every morning?
Not necessarily. This appears to be an old misconception, based on speculation and statistics, and it does not hold up when it’s tested. Skipping your morning meal gives your body more time to burn fat for energy. Since hunger is lowest in the morning, it may be easiest to skip it and break your fast later in the day.
Can women fast?
Yes, but there are exceptions. Women who are underweight, pregnant or breastfeeding should not fast.
Furthermore, for women trying to conceive, be aware that – perhaps especially for athletic women with low body fat percentage – intermittent fasting might increase the risk of irregular menses, and lower the chance of conception.
Other than that, there is no special reason why women should not fast. Women can have problems during intermittent fasting, but so can men. Sometimes women do not get the results they want, but that happens to men, too.
Studies show that the average weight loss for women and men who fast is similar.
Learn more about women and intermittent fasting
Isn’t fasting the same as reducing calories?
No, not necessarily. Fasting can reduce the time you spend eating and primarily addresses the question of “when to eat.” Calorie reduction addresses the question of “what and how much to eat.” They are separate issues and should not be confused with each other.
Fasting may reduce calories but its benefits extend far beyond that.Learn more
Will I lose weight?
Most likely. If you have weight to lose, it is extremely likely that you will lose weight if you do not eat.
In theory, it’s possible to eat more after fasting, of course, cancelling out the weight lost. But studies generally show that most people tend to eat significantly less overall.
I call intermittent fasting “the ancient secret of weight loss” because it might be one of the most powerful dietary interventions for weight loss, yet it has been mostly ignored by doctors and dietitians for a long time.
Many more questions and answers about intermittent fasting