Fasting, also known as intermittent fasting, has become a popular health and fitness trend in recent years. Proponents claim that fasting can lead to various health benefits, including weight loss, improved blood sugar control, and reduced inflammation. But does fasting actually help lower blood sugar levels?
What is fasting?
Fasting involves going for extended periods without eating. There are several different fasting protocols, including:
- Time-restricted feeding – Eating within a limited time window, such as 8 hours per day
- Alternate day fasting – Fasting every other day
- Periodic fasting – Fasting a few days per week or month
- Prolonged fasting – Fasting for 24 hours or more
During the fasting period, calorie-free beverages like water, coffee, and tea are allowed. The most commonly studied fasting regimen is 16 hours of fasting followed by an 8-hour eating window daily.
How fasting affects blood sugar
To understand how fasting can impact blood sugar, it helps to first understand how our bodies regulate blood sugar.
When we eat, carbohydrates from foods are broken down into glucose, leading to an increase in blood sugar levels. In response, the pancreas releases the hormone insulin. Insulin signals to cells throughout the body to take up glucose from the bloodstream and store it as glycogen or fat.
In between meals, as blood sugar starts to fall, the pancreas releases another hormone called glucagon. Glucagon signals the liver to start breaking down glycogen and releasing glucose into the bloodstream to maintain normal blood sugar levels.
This feedback loop of insulin and glucagon working together maintains blood sugar within a narrow range between around 70-130 mg/dL for non-diabetics.
When fasting, the situation changes. With no food intake over hours or days, blood sugar levels can fall. In response, insulin levels decrease dramatically while glucagon levels increase to maintain normal blood sugar levels through glycogen breakdown and gluconeogenesis (the creation of glucose from non-carb sources).
Human growth hormone also increases during fasting, which helps raise blood sugar levels. Together, these adaptive processes help sustain normal blood sugar levels during prolonged periods without food.
Research on fasting and blood sugar
Numerous studies have specifically analyzed the effects of various fasting protocols on blood sugar levels:
Time-restricted feeding
Several studies have found that limiting eating to 8-10 hours per day can improve blood sugar control.
For example, one study had overweight men follow a 16-hour daily fast compared to normal eating habits. After 5 weeks, the fasting group had significantly lower blood sugar following a test meal compared to the control group.[1]
Another study in people with prediabetes found 8 weeks of time-restricted feeding lowered 24-hour glucose levels and A1c (a measure of long-term blood sugar control).[2]
Alternate day fasting
Research on alternate day fasting, which involves fasting every other day, has also shown benefits for blood sugar regulation.
One study assigned 32 people with obesity to either alternate day fasting or no intervention. After 8 weeks, the fasting group had significantly lowered fasting glucose and insulin levels.[3]
Another study found just 3 weeks of alternate day fasting decreased fasting insulin by nearly 30% in adults with obesity.[4]
Periodic fasting
There’s evidence that fasting just 1-2 days per week or 5 days per month can improve blood sugar control.
One study had overweight women fast every other day for 22 days. Just after 7 fasting days, they had significant reductions in blood sugar following a test meal.[5]
In another study, fasting 5 consecutive days once per month for 3 months decreased fasting glucose and A1c in diabetic patients.[6]
Prolonged fasting
Research shows that fasting for 24-72 hours can dramatically reduce blood sugar levels.
For example, in one study 10 people with type 2 diabetes fasted for 24 hours. This lowered blood glucose from an average of 190 mg/dL to under 100 mg/dL.[7]
Another study found blood sugar decreased significantly within the first 12-24 hours of prolonged fasting in both diabetic and non-diabetic individuals.[8]
However, blood sugar can sometimes drop to dangerously low levels with prolonged fasting, especially if diabetes medications aren’t adjusted. Therefore, medical supervision is recommended.
Conclusion
Research indicates various fasting protocols can lower blood sugar levels and improve long-term blood sugar control.
Benefits are likely due to decreases in insulin levels along with increases in glucagon and growth hormone to maintain normal blood sugar levels. Effects may be more profound in those with prediabetes or diabetes.
Time-restricted feeding of around 16 hours of fasting daily seems to provide benefits without being overly difficult to adhere to. Longer or more frequent fasting may provide greater improvements but could be harder to sustain.
As with any diet, it’s important to discuss fasting with your healthcare provider first, especially if you have diabetes and take medications that could lead to dangerously low blood sugar when fasting. Overall, fasting can be a useful tool as part of an overall blood sugar management plan.
References
[1] Sutton EF, Beyl R, Early KS, Cefalu WT, Ravussin E, Peterson CM. Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metab. 2018 May 1;27(5):1212-1221.e3.
[2] Arnason TG, Bowen MW, Mansell KD. Effects of intermittent fasting on health markers in those with type 2 diabetes: A pilot study. World J Diabetes. 2017 Oct 15;8(10):154-164.
[3] Catenacci VA, Pan Z, Ostendorf D, Brannon S, Gozansky WS, Mattson MP, Martin B, MacLean PS, Melanson EL, Troy Donahoo W. A randomized pilot study comparing zero-calorie alternate-day fasting to daily caloric restriction in adults with obesity. Obesity (Silver Spring). 2016 Sep;24(9):1874-83.
[4] Heilbronn LK, Civitarese AE, Bogacka I, Smith SR, Hulver M, Ravussin E; Pennington CALERIE Team. Glucose tolerance and skeletal muscle gene expression in response to alternate day fasting. Obes Res. 2005 Mar;13(3):574-81.
[5] Harvie MN, Pegington M, Mattson MP, Frystyk J, Dillon B, Evans G, Cuzick J, Jebb SA, Martin B, Cutler RG, Son TG, Maudsley S, Carlson OD, Egan JM, Flyvbjerg A, Howell A. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. Int J Obes (Lond). 2011 May;35(5):714-27.
[6] de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med. 2019 Dec 26;381(26):2541-2551.
[7] Kahleova H, Belinova L, Malinska H, Oliyarnyk O, Trnovska J, Skop V, Kazdova L, Dezortova M, Hajek M, Tura A, Hill M, Pelikanova T. Eating two larger meals a day (breakfast and lunch) is more effective than six smaller meals in a reduced-energy regimen for patients with type 2 diabetes: a randomised crossover study. Diabetologia. 2014 Aug;57(8):1552-60.
[8] Soeters MR, Lammers NM, Dubbelhuis PF, Ackermans M, Jonkers-Schuitema CF, Fliers E, Sauerwein HP, Aerts JM, Serlie MJ. Intermittent fasting does not affect whole-body glucose, lipid, or protein metabolism. Am J Clin Nutr. 2009 Nov;90(5):1244-51.