Intermittent fasting and blood sugar can work together or against one another. Balance depends on how the fasting and feasting is planned out.

To “fast” is to go without food for a period of time. In a country where we struggle with an epidemic of obesity, the problem may not be as simple as eating too much. Maybe we also eat too often.

Humans have been fasting for as long as we have recorded human history. It is a part of religious and spiritual practice for many, and technically most of us fast daily, through the night while we sleep. In the simplest terms, to eat “breakfast” means to “break” the “fast” of not eating since last night’s dinner. 

There are many reasons that intermittent fasting has gained in popularity over the last few years. For many, it is an effective tool for reaching and maintaining a healthy weight [1,2]. Scientific evidence indicates that intermittent fasting may boost mental clarity and memory [3], and benefit cardiovascular health [2]. 

For those who struggle with insulin and glucose balance, intermittent fasting may also be a natural way to support healthy, balanced blood sugar levels [1,4,5].

What is Intermittent Fasting?

Intermittent fasting is a system of planning when you will go without food, and taking longer breaks from eating based on that schedule. 

A few popular choices are alternating days (fasting vs. unrestricted eating), the “5:2 diet” (2 days per week of fast: 5 days unrestricted eating), and the “8-hour diet” (unrestricted eating in an 8-hour window every day, followed by fasting for 16 hours). On “fasting” days many people will choose to consume 1 low-calorie meal (500-600 calories) at the midpoint, which makes it more of a partial fast.

During the planned breaks from eating, your body takes a break from the work of digestion and absorption of food molecules, and biochemical changes occur.

What is Blood Sugar/ Blood Glucose Balance?

When we use the term “blood sugar” we are typically referring to the levels of glucose in the circulation.

Glucose is the basic sugar molecule that all of our cells prefer to use for making ATP energy. It is found in foods containing carbohydrates (which is almost everything besides pure meat or fat). 

When we eat, our pancreas releases a hormone called insulin. Insulin circulates throughout the body telling cells “there’s glucose in the blood! Time to grab what you can!”. Some cell types passively let glucose in. Some cell types require insulin to work like a key (glucose transporters), opening a channel to allow glucose in.

When someone has low blood sugar, insulin levels are also typically low, and cells aren’t absorbing new glucose. Without their preferred fuel for energy production, cells have to resort to using stored glucose (glycogen) or fat for energy production.

Having “high blood sugar” means cells aren’t responding to insulin’s signal: either there isn’t enough insulin or cells are “insulin resistant”. High blood sugar can be both the cause and/or a symptom of disease pathologies, most commonly type II diabetes.

Balanced blood sugar means that glucose levels rise steadily in the blood after a healthy meal, and then soon steadily decline (due to cellular uptake), following the release of insulin by the pancreas. Cells are getting the fuel for energy they need without being bombarded or starved.

What is the Relationship between Intermittent Fasting and Blood Sugar?

Studies show that intermittent fasting can reduce your risk for developing type II diabetes and heart disease because it improves the regulation of your blood sugar, increases your resistance to stress, and suppresses inflammation [4].

A case study published in the US National Library of Medicine reported that patients with type II diabetes who participated in an intermittent fasting protocol lost weight and no longer required insulin therapy [5]. In studies where weight loss did not occur, fasting still improved the glucose levels of participants [6]. This means blood glucose balance occurred as a direct result of intermittent fasting, not just as a secondary benefit of weight loss. 

It has been proposed that this reduction in blood sugar occurs in response to changes in signaling pathways and metabolic rhythm caused by fasting. An increase in ketones may also play a role. 

Unfortunately, (or fortunately, depending on your outlook) the effects of fasting on blood sugar appear to last as long as you continue to practice intermittent fasting. It typically takes 2-4 weeks of compliance with a fasting protocol to see significant improvement, but if you return to eating regularly the effects will fade quickly. 

If you are practicing intermittent fasting for weight loss, it may be effective as a temporary diet. If you are practicing intermittent fasting for blood sugar balance, you really need to plan on practicing this protocol indefinitely.

How should I go about Intermittent Fasting if my goal is balanced blood sugar?

Firstly, the best plan is one that doesn’t cause you stress. 

Intermittent fasting is a great way to eat fewer calories, overall, as opposed to willing yourself to make better food choices. In fact, improvements to blood sugar balance may come more from the caloric restriction than from the schedule of fasting, so the plan you choose may not be of much consequence [7].

In the US National Library of Medicine case study, participants had success with 3 days a week “fasting” where they only ate one full meal on fasting days [5]. For you, this may mean every other day only eating lunch if you know you need more energy to get through the day. Or, maybe you would prefer to have just dinner to look forward to at the end of the day. 

Do pick the meal that works best for you, but don’t snack if you intend to keep glucose levels low during fasting periods. Non-fasting days are not a free-for-all junk food binge… you should still stick to healthy, low carbohydrate meals and avoid snacking when possible [5].

There is also evidence to show that a six-hour window of unrestricted eating followed by a 16-hour fast, daily, can be effective for blood sugar balance [4]. This may look like waiting until noon to have your first meal and finishing dinner by 6pm. Or it may look like having breakfast at 7am and fasting after a 1pm lunch. 

Choose a window that best suits your lifestyle and what you know about your hunger patterns. The more realistic it is, the better the chances are you will be able to stick to it. Remember that this is a permanent change, not a diet, so the choice must be a sustainable one.

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  1. Templeman, Iain, et al. “The role of intermittent fasting and meal timing in weight management and metabolic health.” Proceedings of the Nutrition Society 79.1 (2020): 76-87.
  2. Zuo, Li, et al. “Comparison of high-protein, intermittent fasting low-calorie diet and heart healthy diet for vascular health of the obese.” Frontiers in physiology 7 (2016): 350.
  3. Ooi, Theng Choon, et al. “Intermittent fasting enhanced the cognitive function in older adults with mild cognitive impairment by inducing biochemical and metabolic changes: a 3-year progressive study.” Nutrients 12.9 (2020): 2644.
  4. de Cabo, Rafael, and Mark P. Mattson. “Effects of intermittent fasting on health, aging, and disease.” New England Journal of Medicine 381.26 (2019): 2541-2551.
  5. Furmli, Suleiman, et al. “Therapeutic use of intermittent fasting for people with type 2 diabetes as an alternative to insulin.” Case Reports 2018 (2018): bcr-2017.
  6. Swoap, Steven J., et al. “Alternate-day feeding leads to improved glucose regulation on fasting days without significant weight loss in genetically obese mice.” American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 317.3 (2019): R461-R469.
  7. Trepanowski, John F., et al. “Effect of alternate-day fasting on weight loss, weight maintenance, and cardioprotection among metabolically healthy obese adults: a randomized clinical trial.” JAMA internal medicine 177.7 (2017): 930-938.