Fasting for Blood Sugar 101: All You Need to Know

Written By


Expert Reviewed By

Dr. Lauryn Lax, OTD, MS

Dr. Lauryn, OTD, MS is a doctor of occupational therapy, clinical nutritionists and functional medicine expert with 25 years of clinical and personal experience in healing from complex chronic health issues and helping others do the same.

Fasting For Blood Sugar - Healthy Breakfast, Diet Food Concept. Organic Meal

In a culture where 88% of Americans are “metabolically unhealthy”—with diabetes, obesity or overweight, pre-diabetes, low energy and thyroid problems, stable blood sugar levels have been touted as a key marker of optimal health.

How to achieve “healthy” blood sugar? Fasting for blood sugar may be one ticket!

Read on for all you need to know about fasting for blood sugar!

Fasting 101: To Fast or Not to Fast?

Fasting For Blood Sugar - Hands Holding Knife And Fork On A Plate With Green Peas

Humans have fasted for most of their history, whether it’s during the typical overnight period, during more extended periods of food scarcity, or for health or religious reasons.

In fact, fasting was a human norm even before people called it “fasting”, considered it a fad or “cool thing to do”. 

Humans did not evolve with access to 24-7 fast food restaurants, grocery stores containing hundreds of millions of calories, and food supplies so ample that we often throw out half of it before we’re able to eat it. Before agriculture, people ate what they could hunt and if there was no game, they returned home empty-handed. At certain times of year, wild berries were nowhere to be found, periods of starvation took place, and weather sometimes prevented foraging. 

Ancient Indians, Greeks and Egyptians used intermittent fasting to both treat and prevent disease. Hippocrates, “the father of modern medicine” often prescribed fasting and the consumption of apple cider vinegar to heal disease, saying, “To eat when you are sick, is to feed your illness”. And throughout religious scripture, fasting was a regular occurrence: Moses fasted for 120 days on Mount Sinai, Jesus fasted for 40 days,  Muslims observe the month-long fast of Ramazan (Ramadan), and during the Jewish holy day Yom Kippur, adherents fast from sun up to sun down.  

Fast forward to the 21st century where fasting is more of a body fat loss and metabolic reset fad, than an intuitive eating or normal practice, making it tough to discern whether or not fasting is the “best” thing to do for you.

Fasting for Blood Sugar: How it Works

There are several types of fasting: 

Intermittent fasting is when a person goes without food for a certain number of hours each day, usually between 12 and 18 hours. Time-restricted fasting is when a person only eats during a certain period of time each day, usually 6-8 hours.  Extended fasting is when a person abstains from food for an extended period of time, usually 24-48 hours.

Fasting for blood sugar is an “art”—meaning there is no one-size-fits all to the perfect length of time between meals or type of fast you do.

The idea behind fasting is to mimic the “optimal human diet”—how our ancestors would have eaten prior to kitchen dining tables, 9 to 5 work schedules and 24/7 food availability—foraging, scavenging and eating as their bodies cued them to eat. And balanced blood sugar overall. 

In practice, fasting is fairly simple—it is alternating periods of fasting and non-fasting:

  1. Take a break from eating for a pre-determined amount of time. 
  2. Consume foods within a pre-determined amount of time. 

Types of Fasting

Fasting For Blood Sugar - Intermittent Fasting Concept

Fasting for blood sugar can be practiced in several different ways, including aforementioned types of fasting and/or specific macronutrient restriction, including: 

  • A full 24-hour fast once per week or month; 
  • Alternate day fasting: Eating one day, fasting the next;
  • Intermittent fasting: 12-18 hour fasting (ex. not eating from 7 p.m. to 7 a.m. would be a 12-hour fast; or slightly longer 8 p.m. to 12 p.m. is a 16-hour fast)
  • Only eating in a 6 to 8 hour window
  • Seasonally (such as integrating fasting or intermittent fasting at certain times throughout the year)
  • Bone broth fasting
  • Water fasting (only drinking water and minerals) 
  • Fasting Mimicking Diet (eating a restricted calorie diet during the day)
  • Carb fasting: Eating a low carb, higher protein diet
  • The “Potato Diet” — eating nothing but cooked and cooled white potatoes for 3 to 5 days, both for rapid weight loss and blood sugar reset (when you cook and cool potatoes most of the starch is converted to resistant starch, which means they will have  little effect on blood sugar) 

Popular juice fasting (other than celery juice in the morning on an empty stomach) is not recommended when fasting for blood sugar stabilization—the fructose in fruit (higher sugar) and the lack of fiber in juice lead to rapid absorption, followed by a blood sugar dip and poor nutrient utilization.

The goal of fasting for blood sugar is not to “starve,” but instead to stabilize extreme highs and lows in blood glucose markers, increase natural energy, reset circadian rhythms and the gut microbiome, and boost the body’s metabolic fire. 

Fasting for Blood Sugar Benefits

Fasting For Blood Sugar -  Woman Measuring Body Weight On Weighing Scale Holding Donut And Apple

There’s tons of evidence—both scientific and empirical—that claim fasting, especially intermittent fasting, is a game-changer for feeling good and improving your health (1). 

A Positive Stressor. Read: “good” stressor. Also known as a “hormetic stressor” or something that contributes to positive adaptation. Intermittent  fasting promotes “autophagy”—a cellular repair process. Studies show that intermittent fasting is an effective tool for blood sugar management, antioxidant and metabolic boosting, cognitive function, and inflammation and oxidative stress reduction. (Read: disease prevention). 

Makes You Less “Hangry” (Helps Regulate Appetite)

Eating more frequently between meals can send the body on a blood sugar roller coaster—up and down, up and down—activating ghrelin (your “hunger hormone”) the more you get into a regular eating schedule. Fasting and intermittent fasting on the other hand increase leptin (fullness hormone)—the opposite of ghrelin. In a study of both lean and obese subjects, over 3 days of fasting, ghrelin gradually decreased (i.e. the patients were far less hungry despite not having eaten for the past 3 days) (2). 

Enhances Energy & Mental Clarity

As long as you are eating enough (not under eating for your energy needs), fasting for blood sugar is like an exercise for your body and brain (3). Similar to how exercise works your muscles to improve your overall health, fasting “works” your metabolic and cognitive functions through the production and release of certain chemicals like glucocorticoids and norepinephrine that boost energy, alertness, and focus. Intermittent fasting also causes an increase in a molecule known as BDNF (brain-derivated neurotrophic factor) which plays a role in important aspects of brain function relating to mood and cognitive function such as regulating serotonin metabolism, improving synaptic plasticity, and increasing the brain’s ability to resist aging. 

Balances Blood Sugar & Insulin Levels

Insulin seems to respond extremely well to intermittent fasting. Although many of the initial studies on the benefits of fasting for blood sugar were done on animals, a recent human study showed improvement in insulin sensitivity. When you eat glucose and insulin levels spike, it triggers a number of actions in your body, such as helping cells in the liver, skeletal muscles, and fat tissue to absorb glucose from the blood.

Once that is fulfilled, insulin signals the liver to take up glucose and store it as glycogen (stored energy) and then fat. If you keep spiking that insulin, you can get insulin resistance (cells get less sensitive to insulin) and in turn get inflammation, increased fat storage. By giving your body a “break” from the spikes, intermittent fasting allows the body to have better insulin sensitivity when you do eat.

Allows Your Gut to “Rest & Digest”

The process of digestion takes work—upwards of 12-24 hours per meal. Optimal digestion occurs in the “rest and digest” state. Fasting for blood sugar allows time between meals to push food through your GI tract and repair the gut lining during the fast. Studies show that fasting also contributes to a more diverse gut microbiome and higher resistance to low stomach acid (4). 

Saves Money on Groceries and Time on Meal Planning & Cooking

Simply put: One meal down during the day means less planning or work in the kitchen. 

Fasting for Blood Sugar: Cons

Fasting For Blood Sugar -  Woman Doing Cross Arms Sign To Refuse Junk Food Or Fast Food

Fasting and intermittent fasting is not for everyone, which is where the “cons” enter in:

May Disrupt Your Hormones

Fasting is a hormetic stressor (positive stressor), until that stress becomes too much for your body to handle. Energy and nourishment keeps your body going. Lack of energy sends stress signals to your HPA Axis (hypothalamus, pituitary and adrenals) (5). If your HPA Axis is already stressed out (read: under-slept, over-caffeinated, overtrained, high screen exposure, exposed to endocrine disruptors, etc.), then your hormone production also gets out of sync—in some women, leading to increased progesterone and estrogen dominance, in other women leading to suppressed progesterone production and low estrogen (6, 7).

There are lots of anecdotal stories of women who have experienced changes to their menstrual cycles after starting intermittent fasting. Such shifts occur because female bodies are extremely sensitive to caloric restriction. When calorie intake is low — such as from fasting for too long or too frequently — the hypothalamus (“mothership hormone”) is affected.

This can disrupt the secretion of gonadotropin-releasing hormone (GnRH), a hormone that helps release two reproductive hormones: luteinizing hormone (LH) and follicle stimulating hormone (FSH), setting you up for irregular periods, infertility, poor bone health and other health effects.

In animal studies, after two weeks of intermittent fasting, female rats stopped having menstrual cycles and their ovaries shrunk while experiencing more insomnia than their male counterparts (though the male rats did experience lower testosterone production) (8). 

Exception: Some women with hormone imbalance—particularly PCOS, insulin resistance, PMS or menopause, all characterized by higher cortisol levels—may actually benefit from fasting—at least therapeutically, in the short term.

A study in women (20-40 years old) with PCOS and no other known medical conditions experienced a significant decrease in cortisol and adrenaline levels after just 26 days of intermittent fasting, despite concurrent sleep deprivation during Ramadan, an Indian holiday (9). This study also displayed the positive effects of connection to others, spirituality and mindfulness in health as a whole. 

Overdosing on Coffee

Bulletproof and butter coffee are staples in the diets of many who practice intermittent fasting regularly—many starting their day off with a cup of Joe and dollop of butter, MCT oil or both, and nothing else. Coffee is not necessarily bad, but just like most things, too much of a good thing is not always a good thing. For one, frequent consumption and daily dependence on coffee may disrupt our HPA Axis—namely cortisol and insulin levels.

One 12 ounce cup of coffee alone contains 200 mg of caffeine—increasing  blood cortisol levels by 30% in one hour; moreover cortisol can remain elevated for up to 18 hours in the blood. Considering intermittent fasting and fasting can be a stressor on the body already (especially if you are not eating enough or maintain a stressful or busy lifestyle), your Bulletproof coffee habit may not do a body good for feeling your best.  

May Cause Accidental Dieting & Nutrient Deficiencies 

Contrary to popular belief, fasting is not necessarily a restrictive or low calorie diet. It is a break from food for a period of time, followed by a period of re-feeding to supply your body with its daily energy needs. Unfortunately many people run into the roadblock of a accidental dieting or under eating. Although some people may benefit from this, many of the health conscious individuals who adopt intermittent fasting as a trend or the cool thing to do do not.

As a baseline, a moderately active relatively healthy woman requires anywhere from 1800 to 2200 calories per day, and for men, this bumps up to 2400 to 2800 calories. Many intermittent fasters get half of that between two meals, and initially, while they may feel great, over time—maybe a month, maybe two months, three or four months—they begin feeling significantly worse. Their sleep quality decreases and energy plummet.

Maybe if they had a pre-existing thyroid condition, metabolic or blood sugar issue, or autoimmune condition, it gets worse. If this happens, many won’t suspect that the fasting is what caused the problem because, after all, they initially felt better with it. In this case, fasting is yet another stressor that the body has to deal with. If you already have multiple stressors in your life, the positive adaptation that can occur with intermittent fasting may not happen. It may actually take you in the wrong direction.

May Cause Binge Eating, Food Obsession or Trigger a Disordered Eating Mentality

The natural response to food deprivation or body hunger is thinking about food—a lot. This is particularly true if you are not eating enough during the day. The Minnesota Starvation Study is a perfect example of this (10). In it, 36 healthy men were fed a “normal” calorie diet for 12 weeks (approximately 3200 calories per day of a varied, balanced diet), followed by half the amount of calories for 24 weeks and finally a recovery phase of their normal diet.

During the 6-month semi-starvation period, each subject’s dietary intake was cut to approximately 1,560 kilocalories per day and their meals and food options were more limited, composed of foods that were expected to typify the diets of people in Europe during the latter stages of the war. Over the course of 24 weeks, researchers observed the men slowly become more and more preoccupied and obsessed with thoughts about food. They dreamed about food and fantasized about high calorie/high fat food items they couldn’t access. They spent much of their time talking about food, recipes, agriculture. They became agitated if the timing of the meal schedule was changed or if a meal was delayed. Some of the men reported experiencing pleasure just by watching others eat or smelling food; others had urges to overeat, extending their eating experiences as long as they could, not wanting the pleasurable experience to end (note: the food served was actually tasteless, cafeteria food).

With unlimited access to coffee and chewing gum between meals, many of the men chewed and drank constantly; up to 40 sticks of gum and 80 plus ounces of coffee each day. Any opportunity to gain access to food, meant that the men would binge eat, consuming thousands of calories in a seating. Once re-feeding began during the last phase of the experiment, the men had extraordinary calorie needs – requiring over 4000 calories per day in order to slowly restore weight.   A subset of the men were allowed to re-feed without a controlled protocol and these men engaged in extreme overeating.

Not Good for Supporting Muscle Growth & Fitness

To some, the thought of working out without a pre-workout protein shake is unfathomable. Won’t my performance suffer? To others, fasted workouts are a must-do for amazing results—fat loss, lean gains and energy. So who’s right? Fasted vs. unfasted workouts really depend on your goals, energy output and body type. In short: Reasonable durations of fasting do not cause muscle loss; you can do a few days of fasting without incurring any significant muscle loss (11). That said, muscles need fuel to grow, tone and develop.

End of story. Exercise is a catabolic process (i.e. it breaks muscle down). Without appropriate fuel, muscle has nothing to fuel it to enhance your body composition—no, not just “bulking” either, but healthy lean muscle. As for stamina, endurance and output, those who train unfasted experience better, longer lasting prolonged exercise output, while those who train fasted have significantly less stamina and strength to push through (12).

But what about fat loss?! The consensus is still out on whether fasted workouts are really best or not, and the current answers don’t show evidence. A meta analysis of five studies (12) on fasted found that, in both men and women, weight loss and fat loss from exercise is not influenced by fasted or fed states, but rather the quality and quantity of fuel over time (ie. fasted cardio is kind of a bust).

While those who have significant amount of extra body fat and weight, who maintain a moderately active (but not too intense) fitness routine, may feel good going for an early morning walk or workout before breakfast, it is not necessarily a deal breaker. The bottom line: If you want to enhance your performance, train for the game, strengthen or tone your muscle or shed body fat, fasted exercise or not eating enough will not support your goals. 

Fasting for Blood Sugar: Exceptions

Fasting for blood sugar optimization is not the only strategy for balancing your blood sugar and supporting healthier blood sugar levels. 

People thinking about fasting for blood sugar also need to consider: 

  • Meal timing
  • Sleep duration and quality 
  • Total protein intake exercise
  • Stress levels 
  • Hormone balance
  • Gut microbiome health

All of these factors can impact blood sugar without necessarily fasting for blood sugar at all and they are hacks you can tap into as well, especially if you lead a stressful life or your energy stores are in high demand. 

For example, meal timing: the individual who wakes up at 5 a.m. for a bootcamp workout followed by a stressful job as a nurse in the hospital or high level CEO at a tech company may not be the best candidate for an 18 hour fast! They may feel best eating breakfast shortly after their workout and eating balanced meals every 4 to 6 hours throughout the day. 

Sleep duration also greatly impacts blood sugar levels. Sleeping less than 6 hours or simply missing out on quality sleep can alter blood glucose stability by 40% for the entire day—meaning, you may feel hungrier, more tired and your body may demand more energy, even if you eat regular meals throughout the day.

Protein intake also greatly impacts blood sugar levels. Ideally you want 0.8-1 gram per pound of healthy bodyweight of quality protein per day. Fasting or intermittent fasting may actually inhibit this goal if you accidentally under-eat and therefore, balanced meals (versus fasting) may be a great strategy for you.

And we cannot discuss fasting for blood sugar without discussing the microbiome. If you have SIBO, dysbiosis, candida or leaky gut, then simply put, you are not going to metabolize your food well and blood sugar dysregulation is often inevitable. Heal your gut, and, again, fasting for blood sugar stability may not be necessary at all. 

Is Fasting for Blood Sugar Best for You? 4 Considerations

Fasting For Blood Sugar -  Preparation For Blood Test With Pretty Young Blond Woman By Female Doctor

Although fasting and intermittent fasting has many potential benefits for autoimmunity and other chronic illness, IF is not for everyone. Whether or not you should try intermittent fasting depends on several factors. 

Consideration 1: Your Health Conditions

Intermittent fasting (IF) is generally contraindicated (not recommended) for the following people:

  • Pregnancy
  • Subclinical Hypothyroidism (Poor T4 to T3 conversion, since low stress, energy and calories help with this conversion)
  • HPA Axis Dysregulation
  • Eating Disorders/History of Eating Disorders
  • Children & Teens
  • Hypoglycemic tendencies
  • Weaker constitution (“Vata” body types)

Note: If you are a woman or yo are dealing with hormonal or hypoglycemic imbalances, thyroid issues, eating disorder issues, or HPA Axis Dysregulation, IF may not be the best option for you. Fasting can throw your hormones out of balance and mess with your menstrual cycle and sleep patterns, resulting in amenorrhea and insomnia. That’s why women, particularly those with a lean body type, need to be careful when trying IF because their bodies are much more sensitive to signals of starvation than men. This is also true if you already have hormonal imbalances or are in perimenopause.

Caution is also warranted f you have thyroid dysfunction or HPA Axis Dysregulation. Added stress from a fast can exacerbate thyroid disorders and chronic fatigue, especially if you are already under a great amount of stress in your daily life. Ketosis brought about by fasting is a major strain on your adrenal glands, which are already out of whack if you have one of these conditions.

Intermittent fasting has been shown to help these individuals:

  • Sickness (such as stomach flu, cold or flu)
  • Weak immune system
  • Chronic infections (i.e. parasites, Lyme, etc.)
  • Weight loss
  • Type 2 Diabetes
  • Metabolic problems 
  • Neurological issues
  • Generally healthy individuals for optimizing longevity, mental acuity and digestion (practiced occasionally) 

Remember: Every body is different. Just like no one diet is best for everyone, the same thing goes for intermittent fasting—particularly if you have multiple conditions (such as being overweight with hypothyroidism and HPA Axis Dysregulation; in this case, you’d want to first address the HPA Axis Dysregulation to then benefit from the intermittent fasting positives for your there conditions). 

Consideration 2: Your Personal Relationship with Food

Determining whether fasting is or is not for you greatly comes down to your relationship with food—food rules, diet mentality, “shoulds” and “shouldn’ts.”

Are you a person who gets caught up in legalism with food and guilt if you don’t stick to the rules? Or do you listen to your body? Do you tend to use food as a distraction or emotional coping mechanism, or eat to fuel your life?

For those both with histories of disordered eating or simply the diet mentality, intermittent fasting may become just one more food rule to add to your list—leading to more stress and boxes than freedom and connection to your body. 

Consideration 3: Your Daily Needs

Simply put: Undereating can contribute to HPA Axis Dysregulation, “adrenal fatigue”, metabolic, hormone or blood sugar imbalances over time. You may feel great at first, but if you are unable to meet your caloric needs, then say goodbye to the benefits fasting. 

Note: Your needs may change depending on your season of life, age and lifestyle.

For instance, if you’re a 34 year-old mom barely running on 6 hours of sleep, waking up early to feed three hungry mouths and eating whenever you can, chances are IF is not for you. But if you’re a 56-year-old newly retiree with a relatively stress-free lifestyle, wanting to optimize your longevity, IF may be worth a spin. A 29 year-old CrossFit enthusiast? Perhaps workout days you are mindful to eat after your morning workout, but on your off days, you can go until 11 a.m. or noon until your first meal. 

Consideration 4: Listening to Your Hunger-Fullness Cues

Just like our ancestors ate based on their body’s cues, seasons and food availability, you have complete permission to not overthink or overplan IF. Listen to your body. If it’s 9 or 10 a.m. and your stomach is growling…listen. If you’re sick, and can’t stomach a full meal…listen—perhaps sipping some meat broth or homemade Ginger Ale instad. IF is not rocket science, so don’t overthink it. 

Note: If appetite suppression (not feeling hungry) is a regular feeling (i.e. not feeling hungry often), this could be a sign of elevated cortisol and suppressed hunger cues. Mindful eating is eating to fuel your body to meet your daily energy needs—even at times when you don’t feel hunger. 

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