Fasting to Lose Weight, Upsides and Downsides

Fasting to Lose Weight, Upsides and Downsides

Fad diets come and go, but fasting has been around for thousands of years. It’s been used for political protest, spiritual preparation, and as a treatment designed to rid the body of harmful substances, but have you ever considered using it as a strategy for losing weight?

What is the Definition of Fasting?

Fasting is the act of willingly foregoing food for a period of time. Before the advent of agriculture, our ancestors had no choice but to abstain from food when food was scarce. During times of feast, excess calories are stored to tide the body over during times of severe food shortage.

Tell Me a Reason Why I Should Fast

Fasting gives your body a much-needed break from the stress of digestion. Today, about half of all Americans are overweight. This implies that a great number of people are consuming too many calories every day. These are mostly calories without nutrients. Overconsumption of empty calories stresses the liver and kidneys, which are responsible for filtering out toxic substances, and creates an accumulation of food waiting to be processed. Toxins are stashed in fatty tissue, to be dealt with later. When you stop eating for a period of time, you give your body a chance to catch up, to heal and repair the damage.

The Physiology of Fasting: Ketosis

Your body needs energy in the form of glucose in order to survive. Glucose is produced by the break down of glycogen, which is made by and stored in the liver and muscles. During the first day of a fast, almost all glycogen reserves are used up. At that point, fat is used in a process called ketosis. For women, it begins on the second day of fasting. For men, it’s the third day. During ketosis, the body breaks down fat molecules for energy. Until the body enters ketosis you may feel hungry and energy depleted. Once ketosis is established, you will feel that your hunger pangs are minimized, mental alertness is restored and energy levels increase. 

Am I Going to Starve?

The amount of time a person can fast varies depending on the needs and goals. You should never fast to the point were starvation kicks in. Starvation occurs when the body has to use vital tissues to sustain life, because all of the stored energy sources (carbs and fat) have been depleted.

Will I Suffer Muscle Loss?

Some muscle loss takes place mainly on the first day of a fast. When you stop eating, the body converts muscle tissue into glycogen. This process does not continue, though. The brain alone would require the conversion of a pound of muscle tissue per day in order to function. By shifting to ketosis, the body preserves muscle mass and taps into its fat reserves.

How Much Fat Will I Burn?

In the initial phase of a fast, it may seem like you’re losing weight quickly, but that’s because most of the weight you have lost is water. In fact, it’s important to drink plenty of water while fasting, to avoid dehydration. Once your body enters ketosis, you can expect to lose about 3-4 pounds of fat per week.

How Long Does a Fast Last?

Typically, a fast does not take more than just 1-3 days. Generally, it is not safe to fast for an extended period of time unless you receive medical guidance. It is not uncommon for advocates of non-conventional medicine to fast for half or even a whole month in an attempt to treat long-lasting ailments.

Is Fasting to Lose Weight Considered Healthy?

Experts’ opinions differ on this issue. Some say that fasting should not be attempted at all. Others point to the multiple health benefits they have witnessed as a result of fasting. Each side of the issue has credible arguments.

One expert in favor of fasting is Joel Fuhrman, M.D. He wrote a comprehensive book about the subject called Fasting And Eating For Health. He and other fasting enthusiasts list these upsides:

  • Since the American diet is deficient in nutrients, the body cannot free itself from toxins on its own and needs assistance. When you eat empty calories, but also through normal metabolism, your cells accumulate oxidizing substances. This condition leads to premature aging, as well as vascular disease, diabetes, and vital organ deterioration. By fasting, you give your body the opportunity to purge waste products.
  • Overweight and obese individuals have difficulty ‘just saying no’ to poor food choices. Fasting can change your relationship with food, and gradually enhance discipline.
  • Some persistent conditions associated with obesity do not respond to conventional treatments. Fasting for just four days per month can have beneficial effects.
  • Scientific research has proven that periodic fasting combined with a calorie-restricted diet lengthens longevity.

Those who oppose fasting for weight loss list these downsides:

  • When you go without food, your resting energy expenditure slows down by as much as one fifth. When you begin eating again, the slower metabolism makes your body more vulnerable to weight gain.
  • The ultimate goal of a weight loss program should be a healthier lifestyle that includes reduced calories and increased activity, not drastic measures such as going without food.
  • Calcium and protein are two essential nutrients that your body cannot live without. If you’re not getting protein in your diet, your body resorts to muscle mass. If you’re not consuming calcium, your body will take it from your skeletal system, which may cause osteoporosis.
  • Most people don’t drink enough fluids during a fast and wind up becoming dehydrated.
  • Building up muscle mass is a desired outcome because muscle boosts your basic metabolism, which means that your body burns more calories while at rest. Fasting goes against this goal, since it leads to muscle mass loss, to significant degree sometimes.

So with equally persuasive arguments on both sides of the issue, should you make fasting part of your plan of action to lose weight? One thing to remember is that all of that extra fat on your body is storing toxic material that needs to be dealt with sooner or later. If you’ve noticed that you often seem to develop a collection of unpleasant symptoms while on a diet – such as a headache, rash, runny nose, and aches that feel like the flu – that could be your body catching up on its detoxification every chance it gets.

Fasting allows you to plan for that ‘healing crisis’ by designating a day to forego food, drink plenty of liquids, and get lots of rest. Once the fast is over, although your metabolism will be slower, your digestive tract will be cleaner and will use fuel much more efficiently.

How Do I Fast Safely to Lose Weight?

It is advised that you do not try fasting for longer than three days without a doctor’s supervision. For short fasts, it’s still a good idea to have someone monitor you. If you have serious health problems or if you’re taking medication, consult your doctor before embarking on a fast.

It is recommended that as you approach the day of fasting you progressively decrease the amount of food you consume. As a first step, stop eating all meat and dairy. The next day, eliminate beans, grains, and nuts. On the eve of the fast, eat only light foods – soup, salad, and fruit. Reverse this process after you finish fasting by incorporating heavier foods incrementally into your diet. And of course, drink lots of pure water during the fast, to avoid dehydration and flush out toxins.

If traditional fasting for weight loss has both pros and cons, then how can one decide whether fasting, as a means to shed pounds, is good or bad? Is there a way of fasting that eliminates the cons and multiplies the pros? Sure there is! It is called Intermittent Fasting—a clinically studied and well-research type of fasting, which I am going to talk about in my next guest post (coming tomorrow).

About Author: Biologist Matthew Papa, PhD, worked as a postdoctoral research fellow at Washington University in Saint Louis, MO, from 2004 to 2010. He enjoys blogging about obesity-treatments and best weight loss programs.

Image credit:  Alan Cleaver

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About the Author

Biologist Matthew Papa, PhD, worked as a postdoctoral research fellow at Washington University in Saint Louis, MO, from 2004 to 2010. He enjoys blogging about obesity-treatments and best weight loss programs.

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  1. Amy Upton says:

    Fasting is absolutely not the way to lose weight. All of that weight will come right back on as soon as you go back to eating like normal and then you mess up your metabolism making it even harder to lose weight the right way later on down the road. To lose weight permanently it must be a healthy lifestyle change not a drastic change we can’t keep up with that shocks our bodies. Great post!

  2. Kevin says:

    I actually did a 24 hour fast yesterday. I’m personally not big on fasting more than that because the “other” benefits of fasting (other than very low amount of calories) diminish greatly. If you haven’t picked up Brad Pilon’s Eat Stop Eat book, it,s a good one.

  3. Jay says:

    I had read your fasting blog post last time and it was very informative as am also in the field of diet weight loss. All your weight loss tips share perfect information to users.


  4. Is fasting a really good choice for weight loss. Is it not better to reduce food intake, rather than completely stop taking food for a period of time?
    There are positives and negatives in this. But reduced food intake with adequate proteins, minerals and vitamins is better option, in my opinion.

  5. John McNally says:

    Very interesting Evelyn, I hadn’t heard of ketosis before. It looks like men should fast for 3 days in order for the fat burning process to start. Does that mean that out of the 3 days fasting, fat is only lost on the third day?

    Leamington Spa, England

  6. Daniel Zupella - Weight Loss Coach says:

    I think one key thing that everyone should always keep in mind when it comes to weight loss is that it’s not an overnight process and it’s going to take time and you have to be patient.

    I write about and I talk about it every day and it is truely something that needs to be a complete life change rather than just a diet.

    • Hi Daniel,

      I totally agree with your point of view. There are no quick fixes.

      It is definitely a lifestyle change and it must be ongoing in order for one to maintain their ideal weight.

      Welcome to this blog and thanks for your comment!


  7. rex gloria says:

    There are people I know who fast more than three days and they do it without medical guidance. I believe they should read this article. Thanks for the info. Regards.

  8. Alex says:

    I try to fast at least two days every month, but i don’t do it to get down weight, i do it to cleanse my body. And i have to tell you, i feel so much better after those 2 days! I would recommend this to everyone.

  9. This is a great way to inform your viewers – an unbiased presentation of fasting. I’m a regular faster. I tried fasting by taking just water, juice and coconut water and I tell you the results are amazing. However, you have to be very disciplined and persevere the first 3 days if you want to continue 7-15 days of fasting.

    I suggest to do fasting at least 4 times a year to cleanse your body from toxins.

  10. Bryan says:

    Matthew, what a huge load of info on fasting. I like your point about supporting and opposing fasting. Personally I would support it but not for long periods of time but I never knew how long it would take until I came across your post. 3 days sounds alright to me.

  11. Susan says:

    I’m a huge fan of fasting and I have very little negative to say about it. Research shows you can go at least 3 days without food before your metabolism adjusts down so short term intermittent fasting will not affect your metabolism negatively. Research also shows that people who went several weeks eating only 500 calories per day only “slowed” their metabolisms by about 250 calories per day. That’s not a significant slow down at all when you consider that a drink from Starbucks is far more costly then that. This is all great news because the myth that you are damaging your metabolism if you don’t eat every 3-4 hours is now busted. The best rules for weight loss is eat less – eat when you’re hungry – stop when your satisfied.

  12. Justin Dupre says:

    Fasting method sounds like an awesome way to go about dieting. I really enjoyed your post and tips.. you pretty much covered it all! Great stuff!

  13. MatthewPapa says:

    Hi Evelyn,

    thanks for your comment.

    Certainly, fasting for 1-3 days has great health benefits.

    When you fast for only 1 day you have an advantage. Your metabolism does not have the chance to slow down. There is a lag phase between the time we stop eating and the time our body reacts to fasting by dropping the metabolic rate. It takes our body to respond to the “no food” signal around 24 hours.

    This is why using a fasting method where you stop and start (intermittent fasting) is, according to many, the best way to fast.

    You get all the benefits of fast with no risks!

  14. Matthew Papa says:

    True Patricia,

    while fasting has potentially many health benefits, gaining your weight back after prolonged fasting is the major downside to fasting for weight loss, in my opinion.


  15. Patricia says:

    Hi Evelyn
    Look forward to your next post as I feel that the only danger with this topic is there are those who may not be sensible about it and go too far. I have fasted for spiritual reasons but done it in moderation (no more than 3 days but usually only 1).
    One of the problems with using it for weight loss is that usually as soon as the person starts eating “normally” again, it is too easy to put the weight back on. As I have never been overweight; I can say I wouldn’t use it to lose weight but be interesting to hear the feedback from those who have been and have used this method. Did the weight stay off or not? Well-researched quality post as always Evelyn. Thanks
    Patricia Perth Australia

    • Hi Patricia,
      Thanks for chiming in!

      You have a good question here.

      The author of this article is Matthew and he should be able to answer you question better than I. 😉

      But for me(experience and not according to research), I have found the weight stays off. I do fast maybe 1 day a week, but I will usually do a 3 day fast on quarterly basis. I don’t do fasting for weight loss, I fast because I believe it is good practice and because it does have positive outcomes on the my overall health.

      Thanks for sharing you input, great points made!

      Take care,


      P.S. I agree with Mattehw’s response. I find that many people who fast for weight loss only, do gain the weight back. Because the person tends to go back to the same way of eating that packed on the weight in the first place. The key is eating a better and healthier diet after doing a fast.

      Okay, I’m done!