Fasting, or going without food for an extended period of time, has become a popular way to lose weight quickly. Proponents claim that fasting helps speed weight loss in several ways:
Does fasting lead to quick weight loss?
Yes, fasting can lead to relatively quick weight loss. When you fast, you restrict your calorie intake dramatically or completely eliminate calories during the fasting period. This creates a large calorie deficit that forces your body to burn through glucose and fat stores for energy.
For example, alternate-day fasting involves fasting or dramatically cutting calories one day and eating normally the next. This can stimulate up to a 20–32% decrease in calorie intake over the course of a week, leading to 0.5–1.0 kg of weight loss per week, on average.
Similarly, complete water fasts that provide zero to very few calories for multiple days in a row have been shown to stimulate up to 0.9–2.3 kg of weight loss per week on average.
Does fasting burn a lot of fat?
Yes, significant fat burning occurs during fasts. After glucose stores are depleted during a fast, your body starts ramping up fat burning to provide energy for your cells.
Complete or alternate-day fasting may cause your metabolism to adapt and start burning 57–70% more calories from fat, on average. Significant fat burning has been observed between 12–48 hours of fasting.
Ketones, molecules produced during fat burning, may also suppress appetite. This allows you to go longer without feeling hungry, further boosting fat burning.
Can fasting help you lose weight by lowering calorie intake?
Yes, fasting leads to an automatic reduction in calorie intake that almost always results in weight loss. The more days per week or hours per day you fast, the lower your calorie intake and the more rapidly you’ll lose weight.
During feeding periods, fasts shorter than 24 hours also help reduce calorie intake because they allow less time to eat. Intermittent fasting and alternate-day fasting studies show participants spontaneously decrease their calorie intake by up to 38% on average.
Does fasting slow down your metabolic rate?
Major calorie reduction always causes a slight metabolic slowdown. However, fasts lasting 24–48 hours only cause minor decreases in metabolism, allowing an an acceptable rate of weight loss.
Longer fasts last past the point where your body has burned through its glucose stores andketosis kicks in. At this point, fasting may actually boost metabolism by up to 14%. This effect may persist even after you resume a normal diet.
Is fasting muscle wasting a concern with fasting?
Loss of a small amount of muscle mass is almost unavoidable with any weight loss strategy. However, moderate fasts do not appear to cause major muscle loss.
Muscle loss during short fasts is minimal. During alternate-day fasting, 24-hour fasts and 5:2 diet fasting protocols, participants appear to lose mostly fat while retaining 90–97% of their muscle mass.
In fact, several studies indicate that fasting may actually help preserve lean muscle mass if adequate protein is consumed during feeding periods. Muscle loss may also be minimized by doing light exercise while fasting.
Does fasting slow down your metabolism long-term?
The most concerning side effect of a very low calorie intake is metabolic slowdown. Drastically reducing calories lowers the number of calories your body burns daily.
Minor metabolic slowdowns of about 5% can persist even after you resume normal calorie intake following a fast. However, these metabolism decreases are temporary and unlikely to prevent weight loss.
In fact, fasting intervals followed by periods of healthy refeeding may boost metabolism by up to 3.6–14% for up to 1.5 weeks at a time. Periodic fasting cycles could provide metabolic benefits that aid long-term weight loss.
Can fasting help lower blood sugar and insulin levels?
Yes, fasting causes several positive metabolic changes related to blood sugar control and insulin sensitivity. This includes:
- Lower fasting blood sugar and insulin levels
- Decreased insulin production and increased insulin sensitivity
- Improved control of blood sugar after meals
These beneficial effects may help protect against type 2 diabetes, especially when paired with healthy diet and lifestyle habits.
Does fasting produce belly fat loss specifically?
There is some evidence that fasting may help target abdominal, or belly fat. This type of fat is particularly harmful and linked to health issues like type 2 diabetes and heart disease.
In one small study, people who fasted every other day for 22 days lost significantly more abdominal fat than those who ate normally every day. Animal research confirms intermittent fasting reduces abdominal fat.
Other fasting animal studies note preferential decreases in abdominal fat even when overall weight loss is similar between groups. This suggests there’s something unique about fasting that helps strip away unhealthy belly fat.
Does fasting help weight loss more than traditional calorie restriction?
Intermittent and alternate-day fasting don’t seem to provide dramatic benefits for weight loss compared to basic calorie restriction. Studies show similar amounts of weight loss with intermittent fasting and daily calorie restriction.
However, fasting methods may be slightly more effective at retaining lean muscle during dieting. Fasting may also be more convenient for some people because calorie counting is not required.
What are the cons of fasting for weight loss?
Potential downsides of fasting for weight loss include:
- Hunger, weakness and headaches are common side effects, especially when just starting.
- Longer fasts may cause nutrient deficiencies over time.
- Binging is a risk during feeding periods.
- Fasting may slow metabolism somewhat long-term.
- Not suitable for those with a history of disordered eating.
Proper hydration, nutrition and planning feeding periods are key to feeling your best while fasting and maximizing fat burning during fasts.
How much weight will you lose fasting 1 day per week?
Fasting just 1 day per week can provide weight loss benefits. However, the amount of weight lost will vary based on the fasting method and length of your fast.
Typical projections indicate fasting 1 day per week may cause:
- 24-hour fast: 0.5 lbs per week
- 36-hour fast: 0.7 lbs per week
- 42-hour fast: 0.8 lbs per week
However, longer fasts of 2–4 days done periodically may cause up to 2 lbs of weight loss per week.
How much weight will you lose fasting 2 days per week?
Similar to the 1 day per week fast, fasting for 2 days per week can also provide weight loss benefits:
- 24-hour fasts: 0.7 lbs per week
- 36-hour fasts: 1.0 lbs per week
- 42-hour fasts: 1.2 lbs per week
Again, longer 2–4 day fasts done periodically may ramp up weight loss even more and cause up to 3 lbs of weight loss per week.
How quickly will you lose weight fasting 5 days per week?
Fasting 5 days per week falls into the alternate-day fasting plan. This involves fasting every other day, with either normal eating or restricted calorie intake on fasting days:
- Normal eating on feed days: 0.5-1 lbs per week
- 500 calorie fast days: 1.5-2 lbs per week
- Water fast days: 2-3 lbs per week
This frequency of fasting can quickly create a large calorie deficit but may be more difficult to adhere to long-term.
Is fasting safe long-term?
Evidence suggests intermittent and alternate-day fasting are generally safe for most healthy adults in the long-term. However, research is limited and more studies are needed.
There are some concerns with very frequent long-term fasting, such as:
- Increased side effects like headache, hunger and constipation.
- Greater risk of nutrient deficiencies.
- Unknown impact on bone health and reproductive health.
- Increased risk of disordered eating patterns.
People with health conditions should consult their doctor before fasting. More research is needed on long-term impacts.
Should you exercise while fasting?
Light exercise is possible for shorter fasts under 24 hours. However, most people choose to avoid vigorous exercise during longer fasting periods.
Exercising in a fasted state may take some getting used to but is unlikely to impair performance. In fact, working out while fasted may even help boost fat burning.
That said, studies indicate strength training is better supported by eating protein before and after workouts. Reduce weights and intensity if you feel low on energy when training while fasted.
How can you reduce side effects from fasting?
Practical tips to reduce side effects and make fasting comfortable include:
- Staying well hydrated between fasting periods.
- Minimizing physical discomforts by resting.
- Trying shorter fasts and working up gradually.
- Keeping busy and avoiding constant thoughts of food.
- Choosing low-calorie fluids like water, coffee, tea and broth during fasts.
- Splitting fasts into multiple shorter periods if one long fast is too difficult.
Should you take supplements while fasting?
No supplements are required during short fasts under 24-48 hours. For longer fasts, the following may help prevent nutrient deficiencies:
- Multivitamin
- Calcium, vitamin D, magnesium and zinc
- Omega-3 fatty acids
- B vitamins
- Electrolytes like sodium, potassium and magnesium
BCAAs are often marketed for use while fasted during workouts but don’t seem necessary for most people.
Can fasting help you live longer?
Emerging evidence from animal studies suggests fasting may slow aging and extend lifespan by:
- Reducing metabolic rate and oxidative stress.
- Preserving learning and memory function.
- Increasing resistance to age-related diseases.
- Triggering adaptive cellular stress responses.
However, human data is currently very limited. More research is needed to confirm whether fasting helps longevity in people.
What types of fasting are best for weight loss?
All major fasting methods can be effective for weight loss. The best plan depends on your lifestyle and personal preferences.
Some of the most popular fasting styles include:
- 16/8 method: Fast for 16 hours per day, eat during the remaining 8 hours.
- Eat stop eat: 24-hour fasts 1–2 days per week.
- Alternate-day fasting: Fast every other day.
- The 5:2 diet: Very low calorie fasts 2 days per week.
- Periodic fasts: Fasts lasting 24–48 hours done routinely.
Start slowly and opt for shorter, easier fasts to minimize side effects, hunger and increase compliance.
How much weight will you lose per week fasting?
Weight loss rates while fasting depend on the frequency and length of your fasts:
- 16/8 method: 0.5–1 lb per week
- 24-hour fasts 1–2 days per week: 0.5–2 lbs per week
- Alternate-day fasting: 0.5–2 lbs per week
- 5:2 diet: 1–2 lbs per week
- 48-hour fasts 1–2 times per month: 2–3 lbs per week
Faster weight loss can be achieved with longer fasts of 3–5+ days every 2–4 weeks. However, these extended fasts may be harder to stick to long-term.
Can fasting help break a weight loss plateau?
Yes, fasting may help jumpstart weight loss during a plateau. Reasons fasting can help you break through a plateau include:
- Lower calorie intake to create a large calorie deficit.
- Boosts fat burning hormones like norepinephrine.
- Enhances insulin sensitivity for reduced fat storage.
- Increases metabolic rate via ketosis.
Try a 24-72 hour fast 1–2 times per month, ensuring to eat healthy nutrient-dense foods during feed days.
Should you count calories on feed days?
Calorie counting during feed days is optional but may boost weight loss results.
Monitoring calories can prevent overeating and ensures you create a calorie deficit needed for weight loss. However, calorie tracking may not be necessary if you naturally eat moderate portions.
What should you eat on non-fasting days?
Focus your non-fasting days around whole, minimally processed foods:
- Vegetables
- Fruit
- Lean proteins like fish, poultry, tofu, beans, eggs and dairy
- Whole grains like oats, brown rice, quinoa and barley
- Nuts and seeds
- Healthy fats like olive oil, avocado and coconut
Limit added sugars, fried foods and highly processed snacks and convenience meals.
Conclusion
Fasting can be an effective tool to stimulate fat loss and manage body weight. It may also provide health benefits like reducing inflammation, improving blood sugar control and extending lifespan.
That said, fasting is just one piece of the puzzle. Combine fasting with an overall healthy lifestyle – including nourishing whole foods, physical activity and good sleep – to reach your weight goals and optimize wellbeing.