What’s the deal with Intermittent Fasting?


Intermittent fasting (IF), also known as therapeutic fasting (TF) or intermittent energy restriction (IER) has been getting a lot of attention lately in the news media, internet and also in the doctor’s office because it has been shown to help people lose weight and also help those with diabetes (both type 1 and type 2) lower their blood sugar and insulin demands.

Disclosures:  I have no conflicts of interest from any pharmaceutical company or research organization.  I do not make any money from sharing this information.  The purpose is for informational value for patients or others who are interested in learning what I have discovered about how to lose weight and also help improve the health for patients with diabetes or prediabetes.  I believe it’s very important to work with a doctor familiar with your diet, especially if you take medications to treat diabetes or heart related conditions so that together, you can ensure you remain safe whenever you change your diet.

Weight loss is a popular topic, both in social media and in the doctor’s office.  I would suggest that a majority of major healthcare problems facing Americans today are related to being overweight or obese.  In fact, 75% of Americans are currently either overweight or obese and this statistic is rapidly worsening.

Complications from the obesity/overweight epidemic:

  • Metabolic syndrome (high blood pressure, high cholesterol, obesity and diabetes)
  • Increase risk of heart attack
  • Increased risk of stroke
  • Increased risk for some forms of cancer
  • Arthritis/joint pain due to the extra stress from the weight
  • Back pain
  • Low testosterone/sex drive
  • Low energy levels
  • Difficult to exercise because of the extra weight
  • Heat intolerance
  • Obstructive Sleep Apnea
  • Lower self confidence
  • Difficulty with bucket list items on to-do list after retirement such as travel due to decreased endurance, joint/back pain

The problem with traditional “Eat Less Move More” model for weight loss:

The most common model that all of us have heard about (and tried) in order to lose weight is to decrease the amount calories that are consumed and exercise more.  Unfortunately, we all know that this doesn’t work over the long term because whatever weight that is lost (if any) is regained 99% of the time in just a few months.

As an example of the failure of this strategy, consider the popular TV series “The Biggest Loser.”  The show documented over-weight contestants who often successfully lost a surprising amount weight using the eat less, exercise more model that at first seemed to work remarkably well.  Susanne Mendonca, a contestant on The Biggest Loser – season 2 said “the reason why NBC never does a reunion is because we’re all fat again.”  In other words, although the contestants lost weight during the show, it was not sustainable long term.

A study in the New England Journal of Medicine (N Engl J Med 2012; 366:2247-2256) examined the same calorie-deficit approach and found that over 5 years, the Body Mass Index (BMI) of the participants did not improve (an in some cases worsened).  Multiple other well-designed studies, including the Women’s Health Initiative have shown similar results over a 9 year period. This is unfortunate because many medical professionals profess that the eat less, move more approach to weight loss works, but the evidence shows the opposite is true.  

Why doesn’t simply eating less and moving more work for long term for weight loss?

Slowing metabolic rate:

Over years, if you take in more calories than you burn, your body may increase the set-point of what it considers your normal body weight to be and will try to maintain it.   For example, if you are now 230lbs and have been that weight for months or years, your new  normal weight set point may change from the initial set-point of 180lbs to 230lbs .  This can make it much more challenging for you to lose weight, because the body will do all it can to maintain this new body weight.  If you decide to lose weight by taking in less calories, say 1500 calories per day instead of 1800 calories, your body will recognize that you’re not getting enough calories to maintain this weight of 230lbs and will actively try to stop losing weight.  You may feel cold as your metabolism slows down.   Instead of your body burning 1800 calories/day to maintain your weight, your body may decrease its basal metabolic rate to burn only 1300 calories per day, so you feel cold, sluggish, hungry and irritable.  You may start losing weight at first, but as your body changes your basal metabolic rate, you might actually start gaining weight gain (remember you’re taking in 1,500 calories and the body has slowed its rate to burn 1,300 calories/day).

Hunger Signal:  As we lose weight, our body releases hormones which signal us that we’re hungry.  Unfortunately, even over long periods of time, these hormones stay elevated and make it difficult for us to overcome the uncomfortable feeling of hunger.

Energy input/output – Two compartments for energy

The energy that we need for living can either come from stored fat or from food that we eat.  When we eat, insulin levels rise and the energy from the food that we don’t immediately use for energy will be stored in the liver as glycogen or as body fat.   Insulin is the hormone in the body that signals food energy to be stored as fat. Insulin levels rise whenever you eat, and they rise to the highest levels when you eat carbohydrates/sugar. The benefit of a low carbohydrate, high fat diet in addition to intermittent fasting, is that both work to keep your insulin levels low to help prevent storing body fat.

The pathway that your body chooses for energy usage depends on whether calories are coming in or whether you are in a state of fasting.   When you eat, insulin levels rise, and your body will preferentially use calories from the food you eat as energy and store the excess calories from each meal that aren’t immediately needed as fat. At the same time that the body is storing energy as fat, it will not break down fat that is already stored. This is important to remember, because the insulin (which is triggered by eating) turns off lipolysis (breakdown of fat) and turns on the fat storage process.

Intermittent Fasting:  IF is basically described as not eating for a prescribed period of time.  When you take away the food energy coming into the body, insulin levels are decreased and all the energy that we need to maintain our metabolism for living can be mobilized from our fat stores.  Intermittent fasting is synergistic with the low carbohydrate, high fat diet because this treatment works to lower insulin levels and help prevent fat storage and promote the the usage of stored fat for energy.  Intermittent fasting is flexible however and can be used with any type of diet and can be used for short or long term.

If you decrease the frequency of your meals to once or twice a day, do you eat just as many calories as you would normally eat if you weren’t fasting?

Yes, you can in some forms on intermittent fasting.  Some people prefer to eat a large quantity of food all in one meal.  They might for example eat 2,000 calories in one meal rather than eating the same amount spread throughout the day. The idea is to eat until you are full and not more than that.   If you’re trying to lose weight and improve your diabetes, it is still important to pay attention to what foods you eat and the quantity. Lower carbohydrate foods are usually better for weight loss.

How often can you eat with intermittent fasting?

There are different patterns to intermittent fasting – some examples are below:

  1. 16:8 Fast – Skip breakfast and eat lunch and dinner during a 6-8 hour period of time (16-18 hours of fasting each day)
  2. 24 hour fast – aka One Meal A Day Diet (OMAD).   Skip breakfast and lunch and eat dinner (24 hour fast each day)
  3. Alternate day fasting (ADF) – Eat only 500 calories on the fast day alternating with eating whatever you want the next day (feed day)
  4. 5:2 fasting – Normal diet for 5 days, following by 2 days of only 500 calories on each day.

Deciding to fast for longer periods of time than 24 hours is generally considered prolonged fasting which is a different topic.

It should be clear that intermittent fasting is limiting in caloric intake but does not limit water.  In fact, it’s very important to drink even more water throughout the day than you normally would as your body needs extra water to help breakdown fat to use for energy.  Water is essential while fasting.

Is intermittent fasting safe?

Fasting has been used therapeutically since the 5thcentury BC.  Mark Twain said, “A little starvation can really do more for the average sick man than can the best medicines and the best doctors.”  Albert Einstein said, “The best of all medicines are resting and fasting.” Plato said, “I fast for greater physical and mental efficiency.” Even Hippocrates recommended periods of fasting. Nearly every major religions have a period of fasting that they observe.  Christians observe Lent where and other periods of fasting.  In the Muslim religion fasting is observed during Ramadhan.  Fasting is also an integral part of the Hindu religion. Many Buddhist monks and nuns follow the rules of Vinaya and commonly due not eat after noon each day. Traditionally, observant Jews fast and many Native Americans often used fasting before participation in a vision quest.

So yes, I think fasting can be very safe in patients who are overweight.  In patients who are pregnant, have anorexia, trouble gaining weight, or are underweight, fasting is not appropriate.

I recommend working with a doctor who understands your medical history and what you’re trying to accomplish.  If you have a history of diabetes, hypertension or other cardiovascular problems and take medications for these conditions this is especially important.  This is because within a few days of this diet, you will likely require less medication.  If you already use insulin, you can expect to lower your insulin requirements (or better yet,  be able to stop injecting insulin) when you decrease your carbohydrate intake.  If your insulin requirements aren’t monitored closely with this dietary change, you can become sick.  Lowering insulin demands can however be very beneficial because you’re treating the cause of the problem in type 2 diabetes which is taking in too many carbohydrates and insulin resistance.

In my type 1 diabetes patients who have adapted to a LCHF diet, they report more stable blood sugars (less extreme highs or lows) and a lower overall A1c.

When you fast, does your metabolism decrease?

No,  metabolism actually increases slightly with fasting according to a study from the Journals of Clinical Nutrition.  (Zaunder C Am J Clin Nutr 200; 71:1511-5).  As insulin and glucose decrease, the body makes up for this by breaking down fat and using ketones and fatty acids for energy.  Norepinephrine increases and is a key reason why you maintain your metabolic rate. This is why intermittent fasting is more successful for weight loss than the simply eating less and moving more model of dieting which causes your metabolism to decrease.

When you fast, do you just keep getting more and more hungry?

No, we know that ghrelin (the hunger hormone) typically rises and falls over a 24-hour period and usually peaks around breakfast, lunch and dinner times.  If you don’t eat a meal, your hunger hormone levels go back to baseline after a short length of time.  Hunger does not continue to build if you don’t eat because your body is able to provide the energy that it needs from its stored body fat.

If I fast, will I lose muscle mass instead of fat?

Multiple studies have demonstrated the effects of fasting on fat loss verses muscle loss and have found that the body preferentially burns fat rather than muscle in times of fasting.  This makes sense because since the body stores fat when it has more calories than it needs so why would it preferentially decide to burn muscle instead of fat during periods of fasting?

Can women fast?

Yes, there have been studies which show that women and men can lose fat equally when fasting – approximately ½ pound of fat per day of fasting on average (Drenick EJ. Am J Publi Health Nations Health. 1968 ME; 58(3): 477-484)

Advantages of Fasting:

  • Flexibility (you can do it some of the time, and not others)
  • Convenience (save time with shopping/cooking)
  • Free
  • Simplicity
  • Unlimited power – you can keep losing weight until you get to your appropriate weight.

Benefits of Fasting beyond just weight loss:

There are several benefits to fasting which are being studied other than to simply lose weight. Many of these studies have been done in animal models are ongoing studies in humans is currently taking place. Some of these benefits may include:

  • Increased lifespan – this has been seen in animal models.
  • Decrease risk of some cancers.
  • Decrease risk of age related neurogenerative disorders such as dementia (Alzheimer’s or Parkinson’s disease)
  • Improved cognitive function (learning and memory)
  • Improved mood

What is the theory behind fasting causing a boost in brain power?

The idea is that fasting causes challenges to the brain in the form of a mild oxidative stress which causes the brain to adapt by increasing mitochondria and producing neurotrophic factors.  These factors cause the brain to become more active, increasing neurogenesis (creating of more nerve cells), growth and synapse formation (more connections between brain cells) and resistance to degeneration.  This makes sense from a biological perspective because if you’re hungry and haven’t found food, you need to adapt in order to find food or you will not succeed in life.

What are the possible side effects of intermittent fasting?

Some people develop what is called the “keto flu” which really isn’t a flu or infection at all.  It is a constellation of symptoms that can occur while their body is adjusting from using glucose as fuel their fuel source to using ketones for fuel. It is usually caused from a deficiency in one or more of the important minerals.

Symptoms may include:

  • Decrease energy or dizziness
  • Body aches
  • Sugar cravings
  • Difficulty focusing
  • Nausea
  • Irritability
  • Nausea and stomach irritability
  • Constipation or diarrhea

All of these symptoms usually go away within a few days and can be minimized by keeping well hydrated (drink more water) and getting enough sodium (salt).

Why could your salt levels (sodium/potassium) potentially become low with a fasting?

Ketosis (from breaking down fat to use as energy) causes the kidneys to accelerate the excretion of salt.

How much salt do I need per day?

5 grams of sodium per day which equates to about 2.5 level teaspoons of salt per day.  Most naturally healthy, whole food containing diets have about 3 grams of sodium that can come from foods that you would normally eat such as olives, pickles, sauerkraut, kimchi, bacon, etc. For this reason, it is likely that you may need to support a fasting diet with additional salt that can come from broth or salty foods (if you choose a calorie restricting fast).   If you have hypertension or heart failure that you take medications to treat, it is very important to work with a doctor familiar with this diet.

 Adequate intake of minerals is extremely important and those include:

1)  Sodium – if depleted, can cause lightheadedness, dizziness, fatigue when exercising and/or constipation.  With nutritional ketosis your kidneys excrete more salt so you need to increase the salt in your diet as long as you do not have heart failure or severe hypertension.  Some people find may prefer to use bouillon cubes instead of bone broth to help ensure adequate sodium levels.

2)  Potassium important for heart and muscle function.  You can get this from broth

3)  Magnesium – depletion can lead to muscle cramps after exercise or at night.

4)  Calcium – necessary for bones, nerve and muscle function – this is also found in broth.

If you have a medical condition such as diabetes (type 1 or type 2), hypertension, heart failure or take medication, it is very important to work with a doctor familiar with the ketogenic/high fat, low carb diet.  This is because you will likely need to modify your medication regime because you may likely need a decreased dose or may even be able to stop some of your medications.  If you don’t carefully adjust your medications while changing to this diet, you could become sick.

Can I eat or drink anything other than water while I’m fasting?

This is controversial.  You should definitely drink water and I recommend adequate intake of salt from consuming broth.   The goal is to keep your insulin levels from rising so that your body is able to mobilize your stored fat and use that as energy.  If you eat or drink anything with calories, this could jeopardize the entire process.

How to know you are in ketosis with intermittent fasting:

Some people prefer having some sort of physical evidence (lab test) so they can verify that their body is in ketosis (breaking down fats to use for energy).  There are several ways of verifying ketosis.  A method that was popular in the past was to use a urine ketone dipstick which is a chemical test strip that is dipped into a urine collection cup to test the urine for ketones.  That urine test unfortunately isn’t very accurate.  A more reliable test to determine if your body is in ketosis is to check your blood ketone levels using a ketometer. The device is very similar to a glucometer that diabetic patients use to test their blood sugar.   There are multiple brands of ketometers on the market. I cannot recommend one over any others, but they can help you determine whether your body is breaking down fat for energy (in ketosis).

You may also be interested to find out how your blood sugar rises/fall depending on the foods that you eat. Even if you don’t have diabetes, you are able to buy a glucometer and test your blood sugar.  You might be surprised to learn about how your blood sugar changes depending on which foods that you eat.

It would be great to be able to test our insulin levels.  Unfortunately, we don’t have a good over the counter test to check insulin levels, but you can get an insulin level test done at your local laboratory with an order from your doctor.  Usually, if your blood sugar is low, that means your insulin level is also low – unless you have diabetes and are injecting yourself with insulin.

This is written for informational purposes only, and I hope you find it helpful.


Scott Rennie, DO



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