Why You Need Magnesium on a Fast

Fasting and Magnesium

If there’s one piece of advice you’ve probably heard over and over again for fasting it’s this: take magnesium. But if you’re like most people, you probably have no idea what the magnesium’s doing in your body or why it’s important. 

Magnesium is one of several essential minerals called electrolytes, which again… we all know to be important, but we don’t all know why they’re important. 

You may know that electrolytes have something to do with the way water behaves in your body, but what exactly is an electrolyte? Simply put, it’s a mineral that, when dissolved in a solution, is electrically charged—electrolytes are “electric.” They’re critical for a bunch of essential functions. Yes, they help maintain fluid balance, but they also aid in muscle contractions, keep your heart beating normally, and help your body send signals from your nervous system to cells throughout your body.

Since you aren’t eating during a fast, you aren’t getting electrolytes through food. Even if you’re time-restricted-feeding (doing a 16:8 for example), there are 16 hours of the day when you aren’t getting a consistent stream of electrolytes into your body. Even non-fasters are at risk for electrolyte deficiency. Supplementation may be critical to help keep your body functioning properly, especially on a longer fast. 

Supplementing with two key electrolytes—magnesium and sodium—may also help you avoid some of the nasty side-effects of electrolyte depletion. 

For this article, we’ll focus on magnesium.


Magnesium is an essential mineral, meaning that we cannot make it in our bodies; we have to get it from food and beverages or supplements. Our bodies use magnesium to aid enzymatic reactions, energy production, membrane function, regulation of calcium and potassium, plus cardiac and brain function, just to name a few. 

Magnesium also plays a role in transporting calcium and potassium ions across cell membranes. Without those two minerals, your muscles may have difficulty contracting properly, which is why a common symptom of low magnesium levels is muscle cramping—something you might experience when fasting. 

Other symptoms of low magnesium

How can you tell if you might have a magnesium deficiency? Your body’s pretty good at telling you when the magnesium tanks are nearing the danger zone. Aside from the muscle cramping mentioned above, other symptoms of low magnesium include fatigue, weakness, irritability, and vertigo. 

Recommended supplementation dose

Most people are at risk for sub clinical deficiency in magnesium, even when they’re not fasting, so you likely need more than the suggested RDA (300-420mg a day).  A few studies have shown that even when consuming close to the recommended amount, participants were still in a negative magnesium balance, potentially setting them up for higher risk of chronic disease and other long term health issues.

In addition, during a fast, your body actually starts getting rid of magnesium in order to preserve other electrolytes (it has to do with the electric charges of the molecules). 

During a prolonged fast, your kidneys can excrete magnesium at a shockingly high rate. In one study, the kidneys of participants on a mutli-week fast (seriously, mutli-WEEK), were excreting magnesium at a rate 4-5 times higher, than normal. It seems the deeper into your fast you go, the more magnesium you’re prone to lose.  Across a few different studies of prolonged fasting in obese males, the average magnesium loss per day ranged from 83 mg to 203 mg.

So, while fasting, it’s reasonable to say that a range from about 400-600 mg per day would be appropriate to provide the body with the essential magnesium it needs to function optimally and to prevent the nasty symptoms of magnesium deficiency. 

Types of magnesium

The type of magnesium you choose will impact how, when, and where your body absorbs it. This is important for maintaining a steady drip of magnesium throughout the day and avoiding any gastrointestinal issues.

1.   Magnesium oxide is one of the most common forms of magnesium. In all likelihood, if you purchased a magnesium supplement in the past, this is the one sitting on your shelf. The downside of magnesium oxide is that if taken in large quantities (the quantities you’d need on a fast), it can have laxative effects. Not all magnesium oxide is absorbed, so once it reaches the small intestine, it draws in additional fluid and stimulates bowel movements. The other problem with magnesium oxide is that your body absorbs what it can, and excretes the rest pretty quickly, so if you just take it once or twice a day, you won’t maintain a steady level of magnesium in your bloodstream.

2.   Magnesium L-threonate is a novel and proprietary form of magnesium developed by scientists at MIT. It’s better absorbed than magnesium oxide and has been used to support brain health and sleep. In this form, magnesium is bound to L-threonate, a magnesium transporter that helps it cross the blood/brain barrier. Magnesium L-threonate has been shown to have a calming effect, which is why people often take it in the evening.

3.   Magnesium glycinate is another highly bioavailable form of magnesium and has been used in the optimization of sleep, memory, bone health, blood pressure and blood sugar control, headaches and leg cramps.

4.   Magnesium malate is considered a magnesium salt with high bioavailability that helps support cellular energy production.

5.   Slow release magnesium is also becoming increasingly popular. It tends to be more expensive since it relies on cutting-edge technology to encapsulate the magnesium – usually in a lipid or fiber outer later – so your body has to break down the capsule before it can absorb the magnesium itself. 

In an ideal scenario, you’d supplement with multiple types of magnesium whenever you’re not eating — whether it’s a shorter TRF window like a 16:8, or a multi-day fast — and your magnesium intake should include at least one or two slow-release forms to optimize a steady uptake throughout the day.

Read PART TWO for the second of the BIG 2 electrolytes you’ll need to make your fast a little happier: sodium.


Posted in Health & Science


  • Please suggest what to take for magnesium & sodium deficiency if one is on a 16:8 fast? Are supplements enough?

    • I’m 62 & take 500 mg. Initially, fasting causes insomnia. Magnesium intake while eating your evening meal helps!

  • Skin is also a very good absorber of magnesium and applying magnesium oil ( magnesium chloride solution) immediately relieves cramps
    A lot is available on its effectiveness in the web but if ZERO can have an authentic article on this it’ll be of great help

    • I use a topical spray under my arms and lotion on my back and legs. They’re a life saver. Helps relax muscles and nervous system.

      • A magnesium supplement will not break your fast, but start out with a lower dose to check your tolerance. If you have never taken magnesium before you may experience some GI issues the first few days that you take it. And always check with your doctor first!

    • Check the ingredients. If there is only Magnesium, it is fine during a fast. My fasting coach recommends putting trace minerals in our water during our fasting window. You can find them at health food stores. One brand is TMSPORT.

    • If there’s no digestion taking place, you’re ok. Ie Drinking black coffee doesn’t break your fast but adding cream will technically break it.

    • Pepitos/shelled pumpkin seeds are very high in magnesium! But this is a Fasting app/article so you must supplement while not eating! I put Epsom salts(magnesium salt), baking soda, salt, and a little boron in a foot bath or bathtub and soak in it as well!

    • As this is an article about fasting with magnesium, what help would knowing foods with magnesium for your fast? Also, perhaps Google for “high magnesium foods” would answer your question, rendering such information redundant.

      • Dude… their wondering what to eat when NOT fasting, but in their TRE window… To make sure they’re doing all they can to get those levels up before going back to fasting mode

    • I take magnesium citrate powder form 1tsp am and 1tsp pm. Works like a charm for cramps and bowel. My Rule, looser stool, decrease dosage, harder stool or less frequent than daily, increase.

      • Also helps break up stones! My liver Doc recommenced switching to the citrate form of Mag and Potassium when I had a Bole Duct Obstruction from an Oxalate dump while Fasting! She also said to make sure to consume some oxalates while Fasting to avoid another dump/stones! I drink Earl Grey tea as I read a study that the Bergamot in it increases ketosis.

  • Most of the magnesium supplements mention to take while eating. But I read here that you can take it on an empty stomach??

  • What about Hymalayan salt? I heard that when you’re exercising a lot, 2 tsp‘s will be sufficient as a supplement. What’s you opinion about that?

    • It’s already been disproven that pink Himalayan salt has any discernible benefits over regular sea salt. Yes, it does contain extra minerals, but not enough (very little) to make any impact to our diets. Just stick to leafy greens and legumes, lots of vegetables in general, and you’ll hit adequate electrolyte levels.

    • We generally have three or four authors per article (from both our writing and science teams). To give them all credit we have to make some changes to WordPress which we are working on. All previous articles will be updated with full author details in the coming weeks.

      • Nick …. It sounds like you’re from Zero …. Why aren’t any of the other questions being answered?

  • I’ve been taking magnesium before I started fasting because it helped with my anxiety also my sleep. Sleep is so good I wake up startled.

  • Magnesium deficiency can also cause heart irregularities and palpitations or worse predispose your heart to succumb into a lethal rhythm due to magnesium deficiency

  • Well written article. Everything you need to know and why. Being an athlete I’ve been chasing optimum electrolytes doses especially magnesium for quite a while. As a result, I take a steady and consistent intake of magnesium and sodium (non table salt of course) throughout the day and have been for the past 10 years. I use my body’s feedback based on the exercise efforts and adjust accordingly. I use a powdered electrolyte in my drinking water and sip it throughout the day and evening, everyday. This works for me. My blood work reports my magnesium and sodium is optimal.

  • I am an ER nurse. Yes, low mag can cause seizures. We give mag sulfate IV frequently for cramping, dehydration and during a cardiac arrest.

    • I’m also an RN and I have a pacemaker which paces 24/7 for electrical issues and I can’t take mag supplements. It can interfere with electrical *stuff* in the heart so definitely make sure you check with your doctor with ANY supplements. BUT, wondering if there’s anything someone who couldn’t supplement with mag could do? Eating more magnesium rich foods during feeding window?

  • Great information, but how about some foods to eat which would help with that? Also what is a good supplement to take and when?

    • I was thinking of the same question Amy. Since it can be absorbed though the skin which would have a slower absorption than ingestion and the calming effects of a warm bath seems to make sense but a definitive yes or no from someone would be great!

  • Is there a blood test one can take to find if they are deficient on Magnesium?
    @Pat, I’m 58, not an athlete but I do exercise regularly – do you mind if I ask which powder you add to your water and you mentioned you drink it throughout the day and evening, am I to assume this is during your eating window? TIA!

  • Where are the studies and results of magnesium deficiency on women? Male and female bodies require different doses and most likely have different symptoms. It would be more helpful if these studies were done on every kind of body instead of just males. This is not surprising though. Most medicine relies on studies done on male bodies, not female or intersex bodies so we only get one piece of the puzzle.

    • Agree. Thanks for mentioning this long-standing, ubiquitous problem, Maureen. There was a brief mention of it in the article, but I always figure there’s more that hasn’t been uncovered or studied. Who knows?

  • Because magnesium malate is much more bioavailable would you recommend supplementing less than 400-600mg? I currently use the Thorne CitraMate and each serving is only 55mg Citrate & 80mg Malate (135mg overall).

  • Which of these forms of magnesium is best when fasting? From reading the brief descriptions of each, I’m guessing the malate. Is this correct?

    • Adult female 320mg. Teenage female close to 400mg. If possible, better break it into two to three times a day. So in powder form seems a better choice than in capsule. Take it an hour before sleep to help insomnia.

  • I purchased a product from GNC call ’Beyond Raw Chemistry Electrolytes’ and my question is: Can this product be used during my intermittent fasting or will it break my fast?

  • I currently take magnesium, potassium and calcium all in one tablet but I do not take it while fasting. The instructions day take with food. Is that ok?

  • Hmmm … looks like so many of us have the same questions …
    1. What brand to take?
    2. How much to take?
    3. When to take?
    Can you please answer? Would be very grateful!

  • Can we get these questions answered? I lost once & appeared to be a seizure, & they said I had low magnesium & low potassium after fasting.

  • I take Multi Vitamin for women. It has 50 mg magnesium. It sounds like this is enough. I have sleep issue and sometime have foot cramps. It could be because of lack of magnesium…. Should I take the vitamin during non-fasting time?

  • Hi!
    I am 37 years old, I exercise 5 times a week. I am fasting for almost two months now, the 16:8 fast.

    Can someone tell me what supplements should I take? The amount recommended? Any tips?

    Thanks 😊

  • Hi, can I take my supplements during my fasting hours? B-12, magnesium etc…
    or should I take them once I break the fast?

  • Foods rich in calcium can be roughly classified as follows: dairy products, soy products, kelp and shrimp skins, animal solids, etc. However, at the same time as calcium supplementation, it is necessary to bask in the sun to help the skin synthesize vitamin D and promote calcium absorption. In addition, there is sour horn, which is known as “the king of calcium”. Many people think that the most abundant calcium is milk. In fact, the most abundant calcium in daily food is sesame sauce. The calcium content per 100 grams of sesame sauce is 1057 mg.

  • I can’t see the logic of magnesium supplementation during a fast : your body will excrete it just as soon and that’s for a reason. If it needed it it wouldn’t excrete it but use it.

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