Is Fasting Bad for You? No. Here’s Why.

By Jay Richards Published on February 20, 2018

This is the fifth piece in a series on fasting. Read the entire series here.

You may think fasting is bad for you. That’s what I used to think. Sure, once in a while I’d eat a bit less as a time-honored sacrifice — on Ash Wednesday and Good Friday. And I’d abstain from land animal meat on Fridays during Lent. But I was glad I didn’t have to endure the fast of Orthodox Christians — just the vestigial fasts common among Western Catholics.

This isn’t only because I’m a spiritual light weight. I thought real fasting — voluntarily going without food for many hours or days — undid what I was trying to do with exercise and a healthy diet. Fasting would kick me into “starvation mode.” My metabolism would shut down, and my body would shed muscle and store fat.


For years, and for the same reasons, I believed that we should eat lots of small meals throughout the day. Like most people who read the news, I knew that insulin and blood sugar have something to do with body composition. I also believed that grazing helps level out blood sugar so it doesn’t have jarring spikes and drops. In addition, it gives your muscles a nice stable protein feed, so they don’t flee from your body like rats leaving a sinking ship.

By the same logic, I thought, I should never skip breakfast. And I never did, except under extreme duress. When I woke up every morning, I ate a feast, to fire up the ole metabolism and save my hard-gained lean tissue.

Hence my fear of fasting.

Now, if you’re trapped in a box and go two months without food, you will lose lean mass — both bone and muscle. But that’s not fasting. It’s torture.

And if you run a calorie deficit for days or weeks at a time, you will be hangry, and your metabolism will down-shift. But again, that’s not fasting. It’s bad dieting.

Fasting Isn’t Starving.

In the Minnesota Starvation Study near the end of World War II, subjects were put on low calorie, carb heavy diets. They didn’t just lose lean weight. Many became depressed and psychotic. Their body temperature dropped. They couldn’t think straight. They felt weak and cold all the time. And they fixated on food — even in their sleep.

Fasting for Body and Soul Jay Richards 2 - 600

When we think of fasting, we imagine these scenarios because we extrapolate from those bad days when we miss a meal, or bad weeks or months when we go on a diet. That’s a mistake. The Minnesota Starvation Study focused on the effects of long term calorie reduction. It also, unwittingly, tested a low-calorie diet made up almost entirely of carbs. It tells us little or nothing about the effects of fasting.

Proper Fasting is Not a “Diet.”

Fasting isn’t the same as running a calorie deficit, let alone starvation. It’s not what most people refer to as a “diet.” A diet is when you try to get more calories out than you take in, with the hope of losing weight and keeping it off.

Proper fasting is not like that. Done right — for the right length of time and when your body is adapted to use fat for energy (more on that later) — none of these bad things happen.

Your Metabolism Won’t Shut Down.

Prolonged diets drag down your metabolism. Most of them work at first, but then plateau and peter out. That’s one reason most dieters regain all their unwanted weight within one to three years.

During a fast, however, your REE (resting energy expenditure) goes up, not down. In one study, subjects’ REEs went up for three straight days, and only started to slow on the fourth. Even on day four, their metabolism was much higher than it was at the start of the fast.

This makes sense biologically. The human body is well-designed, even this side of the fall. Think of how humans needed to be adapted to the environment for most of our history (not just the last couple of centuries). It would be a bad design if, at the first sign of food shortage, the whole system started to shut down. If our ancestors had such metabolisms, we probably wouldn’t be here now.

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In a good design, the body, at least at first, would kick into a higher gear to help with the search for more food. And when it’s working as it should, that’s just what the body does. It secretes energy-boosting hormones like norepinephrine to help you chase down a gazelle or at least a tasty rabbit.

You Won’t Hoard Fat.

It’s the same story when it comes to fat. A metabolism that winds down at the first sign of hunger would be a really bad design for a body with a lot of reserve energy in the form of fat, which describes pretty much everyone reading this. And guess what? The same hormones that boost energy during a fast also signal the body to draw down the energy reserves on your hips and belly.

You Won’t Shed Muscle.

Now let’s apply the “good design” idea to muscle. To catch that gazelle, our ancestors needed clear heads (the brain uses a lot of energy), fast muscles, and energy to fuel them. Let’s assume they’ve eaten well and have plenty of fat and muscle, and then the pantry runs dry. Does it make sense that their bodies would start burning muscle for fuel right off the bat? Muscle is mostly zero-calorie water plus protein. Your body needs either glucose or ketones (made from fat) for fuel. So, it would use a good bit of energy just converting muscle protein to fuel. And for the trouble, it could get about 700 calories per pound of muscle burned. Compare that to fat, which packs 3,500 calories per pound.

So, if fasting mimics that need-muscle-plus-more-energy-to-catch-a-gazelle scenario, what do you think happens?

Just what you’d expect. During a fast, the body spares lean mass such as skeletal muscle and burns fat.

Dr. Jason Fung, an obesity expert, puts it well:

Let’s imagine that we are living in Paleolithic times. During the summer of plenty, we eat lots of food and store some of that as fat on our body. Now it is winter, and there is nothing to eat. What do you suppose our body does? Should we start burning our precious muscle while preserving our stored food (fat)? Doesn’t that sound pretty idiotic?

It’s as if you store firewood for a wood-burning oven. You pack lots of firewood away in your storage unit. In fact, you have so much, it is spilling out all over your house and you don’t even have enough room for all the wood you’ve stored. But when the time comes to start up the oven, you immediately chop up your sofa and throw that into the oven. Pretty stupid right? Why would we assume our body is also so stupid?

The logical thing to do is to start burning the stored wood. In the case of the body, we start to burn the stored food (fat stores) instead of burning precious muscle.

How does this work? During a fast, the pituitary gland boosts human growth hormone, the stuff many bodybuilders inject to help them grow muscle. This not only helps our body burn fat. It also helps us spare muscle.

That said, if you’ve been on a more or less constant sugar feed for the last few decades, switching to a fasting lifestyle in one fell swoop can be … unpleasant. We discuss why in the next installment of this series.


Jay Richards is the Executive Editor of The Stream and an Assistant Research Professor in the Busch School of Business and Economics at the Catholic University of America. Follow him on Twitter.

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  • Jennifer Hartline

    Really enjoying this series, Jay. Can’t wait for the next installment!

  • Craig Roberts

    This is a great article about burning fat and preserving muscle. About the spiritual exercise of fasting…er…not so much. If your motivation for fasting is health related, it’s not really focused on God. In fact it would be taking something that is supposed to be sacred (set apart for God alone) and using it for a self centered reason. The word for this is ‘desecration’, that is to ruin something sacred. Not the sort of activity God fearing people would intentionally indulge in. The word for explaining words to people that should already know them is ‘condescension’.

    There is nothing wrong with dieting or fasting for health reasons. But to do that and flatter yourself that this is somehow ‘spiritual’ would be worse than not fasting at all. It would be offering up a sacrifice to God with obviously impure motives. Again, for a Christian, not a good thing!

    • Jennifer Hartline

      Craig, you should probably go read the first four parts of this series, and then get back to this one.

      • Craig Roberts

        I enjoyed them also when they were first published in The Stream. I remember a lot of interesting stuff about the early Christians, other spiritual traditions, the dangers of Gnosticism. Perusing them again I see that the author even admits that his research into fasting was motivated by a medical condition. Again, I don’t see anything wrong with that per se. The problem is that the focus has shifted from making a sacrifice for God to getting in shape.

        I also noticed that many commenters see this as a “win-win” and an opportunity to get something “more” out of fasting. These are mixed motives at best and if there is one thing Jesus warned against over and over again it’s splitting our spiritual practices between God and our personal temporal ambitions.

        Thank you for the response and if I missed something in the prior articles that addressed this concern please point it out to me.

        • Kathy Freshour

          I think the point with this last installment was to address those who would like to fast for spiritual reasons but don’t for fear that it is unhealthy. Dispelling that myth was the objective, not convincing us to fast for health reasons.

          We need to contantly work on purifying our motives, but finding out that fasting isn’t bad for your health does not automatically mean that it isn’t spiritually beneficial or that on will have mixed motives.

          • Craig Roberts

            Good point. Thank you. I guess there is always the danger of self deception when it comes to our motives but we can’t let that keep us from striving.

    • Jay W. Richards

      I’m quite intentionally focusing on *all* the benefits of fasting, because virtually all the literature, especially in Catholic circles, focuses on fasting as a sacrifice. And many people avoid it because they also think it’s bad for you. Sure, if the only reason one fasts is for health or to lose weight, then it wouldn’t be a spiritual fast. But I dispute the implication that if something good for you mentally and physically, and you know that, then it can’t also be a spiritual discipline. We don’t use that logic with anything else. Prayer and worship isn’t tainted by the fact that many of us find it fulfilling and beneficial to our family life. My assumption is if the Christian view of the human person is correct, then regular spiritual practices (as opposed to, say martyrdom) will help align the person, body, mind, and soul.

      • Craig Roberts

        Thanks for the response. I understand that you were “forced” to do some serious diet research and practice due to a medical condition. Congratulations for successfully overcoming the barriers to healthy living that many people suffer. If others can benefit from this information and do the same, God bless you.

        I just think that you should realize that if a normal relatively healthy person were to do the same thing as you it would be bordering on food obsession. Again, if someone is suffering from a diet related malady, they should seek advice and try to change their eating habits, perhaps even radically. But regular people should not be obsessed with what and when they eat.

        The Jewish people were famous for their laws governing kosher eating and Jesus confronted them many times regarding these laws. Every time his advice boiled down to: stop worrying about what and when you eat all the time! (Mark 7:15-19, Matt 15:11, Luke 12:22, Matt 6:25, etc.)

        Thanks again for the response. Your thoughts on the matter are appreciated. Not everyone has your experience and insights, so I’m sure that many can learn and benefit from your example. I myself have learned some very interesting facts about the way our bodies handle and process food. Unfortunately there always remains the danger that others with dissimilar circumstances may be led astray.

  • Sarah Pierzchala

    Intermittent fasting according to Dr.Fung’s guidelines is very doable for me this Lent, as I became fat-adapted over the last year. However, my understanding of the traditional Orthodox approach to fasting doesn’t allow much (or any) fats. It seems to me this restriction would lead to a lot of hunger and blood-sugar swings, to say the least. Thoughts?

    • Ron Robson

      As a recent convert to Orthodoxy, I can tell you that during the fast I add nuts to my diet to provide fat. Also, although most days oil is not allowed, it seems that most Orthodox read that as ‘olive oil’ so other vegetable oils are allowed. (Conversely, when the fast says ‘no wine’, most read that as ‘no alcohol’.)

      Personally, I have grown to really appreciate the fasts. Prior to joining the concept concerned me, but the Church is very forgiving and these are not looked on as rules that determine how ‘good’ you are. The fast is there to help us grow and indeed that’s what it does.

      • Sarah Pierzchala

        Thanks, that’s helpful! Have a blessed Great Fast!

    • Jay W. Richards

      This is a good point. I’m still researching this question.

  • Lisa

    Fasting would be a win-win proposition if it won’t ruin my metabolism and I can draw closer to God as a result of it. Now, to overcome my ingrained fear of losing muscle and slowing metabolism…Keep the installments in this series coming, Mr. Richards! You have years of biased misinformation and confusion on this issue to overcome. I’m so glad this wasn’t just one short article.

  • Michael G. Siddle

    I have been on a low carb ( 20- 40 grams a day on average) diet for two years and have maintained significant weight loss. Interestingly I eat far more calories per day than I ever did when I put on the weight so it is not a matter of restricting calorie intake. I eat large quantities of eggs, cheese, chicken, fish, bacon, avocado, water chestnuts,leafy green vegetables and any vegetables that grow above ground and cheesecake made with Philadelphia cream cheese and diet jelly and I am in a permanent state of ketosis with no ill effects. I have no trouble fasting for up to five days as my body is constantly burning fat stores which have been kept in balance by a high calorie low carb diet.

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