Intermittent Fasting: A Physician’s Perspective

I started intermittent fasting (IF) five months ago while researching the topic for a lecture that I was to give. I must say, as someone who grew up in a poor country where voluntarily turning food away is a blasphemy, I was impressed by the numerous benefits reported in scientific literature on IF. I gave time-restricted feeding a try and was surprised to find how quickly my body adapted to the torture of not eating for 16-18 hours. In this article, I hope to give you some background and guidance on IF, and how to do it safely especially if you have diabetes.

How Is Intermittent Fasting (IF) Different From Calorie Reduction (CR)?

Calorie reduction is reducing the total daily caloric intake. Studies have shown that CR without malnutrition leads to reduction in body weight and extending lifespan across animal models (both primate and non-primate). Studies in humans have also shown that calorie reduction lead to better cardiovascular health, insulin sensitivity, mitochondrial function, and cognition. But the same studies have shown that people have a difficult time sustaining continuous calorie reduction over time.

Intermittent fasting (IF) has gained popularity as an alternate to continuous calorie reduction. IF is more user-friendly since there are many ways to go about doing this without having to reduce total calories. The goal of IF is to fast long enough to make ketones (more on that later) which are considered a “super fuel” with multiple benefits on longevity, inflammation, cognition, cardiovascular health, obesity, etc.

How does IF work?

Most of us fast every night for at least 10-12 hours if we truly stop eating at least 3-4 hours before bedtime. Unlike the hunter-gatherer times, we are both blessed and cursed with electricity and easy access to food, which keeps us up longer, eating longer. This causes a number of disadvantages to our health. After a meal, we have plenty of sugars (glucose/carbohydrates) to use as fuel. But as we fast, we run out of these sugars and then start breaking down the stored sugars (i.e. glycogen). Once these glucose stores are depleted, our bodies turn to fat breakdown (as opposed to breaking down proteins, which our bodies need to save).  Some of these fats are also sent to the liver to make ketones, which have many downstream benefits that I will describe below. In general, it takes about 12-36 hours of fasting to make ketones. The time is variable since it depends on the time from your last meal, how large that meal was (how much glucose you have floating around), and how much stored glucose is in your liver.

What Types of IF Regimens Could You Try?
  1. Time-Restricted Feeding (or fasting): Fasting for various periods of time, typically 12 hours or longer and eating normally during the rest of the time. For example, fasting for 16-17 hours and eating for 7-8 hours
  2. Alternate-Day Modified Fasting: 5:2 intermittent fasting is commonly used when you have 5 days of normal eating with 2 days of 500-1000 calories/day
  3. Alternate-Day Fasting: Alternating a 24-hour fasting day with a normal feeding day
  4. Periodic Fasting: Fasting 1-2 days/week and eating as usual 5-6 days/week
What Are the Benefits?

The obvious benefit that I noticed was a decrease in my appetite. At first, I felt compelled to eat my own arm. But after few weeks, my body got used to this routine and now most days I do not feel the need to eat until around noon. I also do not experience night-time cravings and my sleep has improved.

Other Benefits Seen in Literature, in No Particular Order:
  • IF, in contrast to CR, helps preserve muscle mass while losing fat mass. This is important for any athletes out there.
  • Better regulation of circadian rhythm (sleep, wake cycle) especially when eating is restricted to earlier parts of the day.
  • Reduction in inflammation (studies show markers associated with inflammation decrease and anti-inflammatory markers increase).
  • Improvement in insulin sensitivity.
  • Improved memory in patients at risk for Alzheimer’s disease and memory problems.
  • A 2019 UCSD pilot study of patients with metabolic syndrome (diabetes, high cholesterol, high blood pressure) on 10 hours/day time-restricted eating showed reduced blood pressure, belly fat, weight, cholesterol and blood sugars.
What Are Some Side Effects?

There are not many side effects with IF except some irritability from being “hangry.” This disappears with time. Extended fasts can be unhealthy and can lead to nausea/vomiting and nutritional deficiencies. Also, data is mixed on the effects of IF on patients with active cancer.

What Are the Implications for T1D and T2D with IF?

The topic of ketones and their association with ketoacidosis can bring up unpleasant memories to folks with T1D. But, with IF, we are talking about a lot less ketone body production than what you would get with diabetic ketoacidosis (DKA). For people on medications called SGLT2 inhibitors (Empagliflozin, Canagliflozin, or Dapagliflozin), discuss with your endocrinologist before any attempt at IF since there could be increased risk of ketoacidosis. You might also notice that adjustment of your basal insulin is needed during fasting.


Intermittent fasting is best summarized by a quote from Mark Twain, who wrote, “A little starvation can really do more for the average sick man than can the best medicines and the best doctors. I do not mean a restricted diet; I mean total abstention from food for one or two days.”


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    Thanks for this. I had to change my work schedule for Covid at first. Worked 10 weeks of 3:00-1130 to relieve usage on my healthcare related secure network during the day. It had a hand in Covid weight gain & all the side effects. I want to try this & so will ask my doctor.

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      If you do decide to try it, let us know what you think. Changing work schedules is tough – good luck and be well!

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    I love IF! I switch it up almost every day to keep my metabolism “guessing”, if you would. The only downside is, I drink more water…aka more trips to the bathroom! lol!

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    What do you think of PROLON

  4. We’re fasting from 3pm to 7am. My blood sugar seems to drop every evening no matter what I eat or don’t eat. I’ve dropped my basel as low as my pump will let it. If I exercise it drops even lower. Any thoughts on how to keep my sugars at an acceptable level?

    • It sounds like you’re on too much insulin. I would check the accuracy of your lowest basal rate and if you still get low on the lowest basal rate, then you need to switch to subcutaneous insulin and give the lowest dose that you don’t get low on when you’re fasting.

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    So if you happen to drop low during a fasting period, let’s say because you’re exercising, then you have to have sugar. Does this nullify the positive effects of the fasting?

    Many thanks.

    (Loved the video)

    • You want to take in as few carbohydrates as you can to treat that low, and it really could be something like juice or regular soda…for sure nothing with protein and fat like a candy bar. So if you just take a minimum amount of something sweet to drink, that should be okay.

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    Not quite sure how intermittent fasting works if I get up at 4:30 AM, eat, walk 2.5 miles to my first requirement, than 4 to my 7 hour volunteer activity, basically arriving back home at around 5 PM, still needing to do house work, preparation for the next day, taking care of the dog, and finally getting to bed around 9:30 to 10:30 at night. This is a 6-day a week schedule, with only Saturday as a late day sleep in until 6:30. I have cut back (eating less food), but if I cut back too much, I get tired. I’m 70, mostly blind (legally) and my wife died a couple years ago, so I’m on my own. How does this intermittent fasting work?

    • Hi Howard, intermittent fasting isn’t necessarily for everyone. It sounds like your schedule is perfect for your lifestyle. If you do want to learn more about it though, one of our dietitians did a video that includes more info here:
      The intermittent fasting part starts at about 33 minutes in. We’re also doing a podcast on intermittent fasting soon as well. The fact that you get symptomatic when you don’t eat for an extended period of time tells me it may not be for you, and that’s totally okay. It’s not for everyone, and you have to do what works best for you and for your lifestyle.

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    Patricia, I am wondering the same. Will the fast still work to burn keytones if you have to eat in order to treat a low sugar episode?

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    What about doing a 72 hour fast? I’m on a pump and a CGM so I can control insulin and closely monitor my BS. Besides getting too high or too low, is there anything else to watch out for? Do you think there are benefits to doing a 72 hour fast?

    • A 72 hour fast is really a long time and cannot be achieved on a regular basis like 6pm to 12 noon every day. But…if you want to get going with weight loss and your blood sugar stays danger in trying. You need to ask your doctor first.

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    I’m into intermittent fasting (and low carb diet, around 50 g total carbs/day), but after my eu-DKA (was taking jardiance), the emergency doc took me off jardiance and advised that I have at least 80 g of total carbs/day AND stop intermittent fasting. He seems to think that IF and low carb diets are fads. When I pointed to the research on these topics, especially IF, he was unconvinced. I was testing my ketone levels using a urine ketone tester and they were as high as possible (and I had the ‘other’ symptoms of it), and lab tests found the ‘usual’ markers for eu-dka. Since then, my urine ketone sticks say that I’m at a moderate level of ketosis (still adapting my diet), but my family doctor says that this is due to my diet. I am “backing off” a bit on IF, fasting just from 9 pm to 11 am or so. I’m wondering if I should be measuring my blood ketone levels? Would this be more accurate than urine strips in measuring my risk for eu-dka? Or, is that not an issue now that I’m no longer on jardiance?

    • It sounds like you’re someone who produces keytones at a certain level, which may just be part of your own metabolism. I would suggest getting a keytone meter that measures beta-hydroxybutyrate which is a much more accurate marker for ketoacidosis. The urine tests don’t measure beta-hydroxybutyrate, rather they measure acetoacetate which is not that specific of a marker.

      Secondly, because you may just be a keytone producer anyway, being on a low carb diet may throw you into Euglycemic DKA easier, however, I’m guessing this was the first time in your life. It may have been an unusual situation that occurred because you were starting on an SGLT-2 inhibitor. If you have kidney dysfunction or a propensity for congestive heart failure, I would go back on it, but with a better detection plan.

      Lastly, a lot of people don’t know how to treat Euglycemia DKA at home. In addition to fluids and insulin, it’s ingesting 30-60g of carbs immediately with appropriate insulin in order to turn off the brain’s drive to produce keytones. Please google STICH protocol for early recognition and treatment of DKA.

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