Author: Sheri R. Colberg, PhD, FACSM
With the new year upon us and resolutions made, weight loss may be on your mind, and with it, low-carb eating is one possible way to cut back on calories. Before you decide how to go about losing weight, though, you may want to consider how cutting back on your carbohydrate intake may affect your ability to be physically active.
Although their long-term benefits in managing blood glucose levels are mixed (1), the popularity of low-carbohydrate diets has continued to rise among people with diabetes, especially those with type 1 diabetes (T1D). The same carbohydrate intake that is “low“ is not well defined, but under 130 grams (g) per day or <26% total energy intake or <3 g per kilogram of body weight daily is considered low by most (2, 3). Research on the glycemic impact of low-carbohydrate diets has primarily motivated individuals with T1D to engage in frequent glucose monitoring and insulin adjustments to achieve tight glucose targets.
Adherence to such restricted diets is challenging, and carbohydrate-containing foods like whole grains, fruit, and dairy provide essential nutrients that many low-carbohydrate diets lack (3). The potential for diabetic ketoacidosis, hypoglycemia, altered blood lipids, and depleted glycogen (carbohydrate stores) when following very low-carbohydrate diets remains a concern (4); moreover, adults with T1D consuming less than 50 g per day may not react well to rescue glucagon used to treat hypoglycemia, likely due to reduced liver glycogen (5).
As for exercise performance, your body‘s preferential use of carbohydrate as a metabolic fuel during moderate and intense activities may make it difficult to perform optimally when you severely restrict your carbohydrate intake, especially before, during, and after exercise training and events (6). Prolonged endurance activities are limited by carbohydrates being available, and low-carbohydrate diets have the potential to restrict muscle glycogen stores (7), particularly without a prior period of adaptation (such as a few weeks).
While your blood glucose use typically increases during most activities, the amount available when levels are in a normal range is very limited (only ~4 to 6 g of total glucose, depending on your size). While blood glucose production increases during activity from hepatic glycogen breakdown (glycogenolysis) or de novo (new) glucose formation (gluconeogenesis), the majority of carbohydrates muscles use comes from glucose stored as glycogen (8).
In the body, carbohydrate stores are limited in the skeletal muscles (typically 300 to 400 g) and the liver (more in the range of 80 to 100 g). Given the importance of carbohydrate availability during many sports and activities, pre-exercise levels of muscle glycogen frequently determine how well you perform (9). Moreover, replacement of these stores during recovery depends on the availability of blood glucose, which can come directly from carbohydrates you consume or from new glucose made by your liver from metabolic precursors like lactate, pyruvate, alanine, and glycerol (10).
Sufficient glycogen repletion in people with T1D requires adequate food intake, blood glucose management, and insulin availability during recovery (11, 12). Elevated blood glucose can lead to lower liver glycogen storage (13). If you start exercising with low muscle and liver glycogen stores, you will likely need to take in carbohydrates during extended activities and may not perform as well. Engage in activities that rely largely on muscle glycogen for fuel, such as many power–endurance and power sports. A low-carbohydrate diet may be detrimental to a performance by limiting your ability to rapidly resynthesize adequate amounts of ATP, the energy molecule used for muscle contractions.
Adapted from Colberg SR, Nutrition, and exercise performance in adults with type 1 diabetes. Canadian Journal of Diabetes, 44(8):750-758, 2020 (https://doi.org/10.1016/j.jcjd.2020.05.014)
- Turton JL, Raab R, Rooney KB. Low-carbohydrate diets for type 1 diabetes mellitus: A systematic review. PloS one. 2018;13(3):e0194987-e. doi: 10.1371/journal.pone.0194987. PubMed PMID: 29596460.
- Feinman RD, Pogozelski WK, Astrup A, Bernstein RK, Fine EJ, Westman EC, et al. Dietary carbohydrate restriction as the first approach in diabetes management: critical review and evidence base. Nutrition. 2015;31(1):1-13. doi: 10.1016/j.nut.2014.06.011. PubMed PMID: 25287761.
- Seckold R, Fisher E, de Bock M, King BR, Smart CE. The ups and downs of low-carbohydrate diets in the management of Type 1 diabetes: a review of clinical outcomes. Diabetic Medicine. 2019;36(3):326-34. doi: 10.1111/dme.13845. PubMed PMID: 30362180.
- Leow ZZX, Guelfi KJ, Davis EA, Jones TW, Fournier PA. The glycaemic benefits of a very-low-carbohydrate ketogenic diet in adults with Type 1 diabetes mellitus may be opposed by increased hypoglycemia risk and dyslipidemia. Diabetic medicine: a journal of the British Diabetic Association. 2018:10.1111/dme.13663. doi: 10.1111/dme.13663. PubMed PMID: 29737587.
- Ranjan A, Schmidt S, Damm-Frydenberg C, Steineck I, Clausen TR, Holst JJ, et al. Low-Carbohydrate Diet Impairs the Effect of Glucagon in the Treatment of Insulin-Induced Mild Hypoglycemia: A Randomized Crossover Study. Diabetes Care. 2017;40(1):132-5. doi: 10.2337/dc16-1472. PubMed PMID: 27797928.
- Cermak NM, van Loon LJ. The use of carbohydrates during exercise as an ergogenic aid. Sports Med. 2013;43(11):1139-55. doi: 10.1007/s40279-013-0079-0. PubMed PMID: 23846824.
- Yeo WK, Carey AL, Burke L, Spriet LL, Hawley JA. Fat adaptation in well-trained athletes: effects on cell metabolism. Appl Physiol Nutr Metab. 2011;36(1):12-22. doi: 10.1139/h10-089. PubMed PMID: 21326374.
- Jensen TE, Richter EA. Regulation of glucose and glycogen metabolism during and after exercise. J Physiol. 2012;590(Pt 5):1069-76. doi: 10.1113/jphysiol.2011.224972. PubMed PMID: 22199166.
- Areta JL, Hopkins WG. Skeletal Muscle Glycogen Content at Rest and During Endurance Exercise in Humans: A Meta-Analysis. Sports Med. 2018;48(9):2091-102. doi: 10.1007/s40279-018-0941-1.
- Jensen J, Rustad PI, Kolnes AJ, Lai YC. The role of skeletal muscle glycogen breakdown for regulation of insulin sensitivity by exercise. Frontiers in physiology. 2011;2:112. doi: 10.3389/fphys.2011.00112. PubMed PMID: 22232606.
- Buehler T, Bally L, Dokumaci AS, Stettler C, Boesch C. Methodological and physiological test-retest reliability of (13) C-MRS glycogen measurements in liver and in skeletal muscle of patients with type 1 diabetes and matched healthy controls. NMR in biomedicine. 2016;29(6):796-805. doi: 10.1002/nbm.3531. PubMed PMID: 27074205.
- Bischof MG, Bernroider E, Krssak M, Krebs M, Stingl H, Nowotny P, et al. Hepatic glycogen metabolism in type 1 diabetes after long-term near normoglycemia. Diabetes. 2002;51(1):49-54. doi: 10.2337/diabetes.51.1.49. PubMed PMID: 11756322.
- Hwang JH, Perseghin G, Rothman DL, Cline GW, Magnusson I, Petersen KF, et al. Impaired net hepatic glycogen synthesis in insulin-dependent diabetic subjects during mixed meal ingestion. A 13C nuclear magnetic resonance spectroscopy study. J Clin Invest. 1995;95(2):783-7. doi: 10.1172/JCI117727. PubMed PMID: 7860761.
Sheri R. Colberg, PhD, is the author of The Athlete’s Guide to Diabetes: Expert Advice for 165 Sports and Activities (the newest edition of Diabetic Athlete‘s Handbook). She is also the author of Diabetes & Keeping Fit for Dummies, co-published by Wiley and the ADA. A professor emerita of exercise science from Old Dominion University and an internationally recognized diabetes motion expert, she is the author of 12 books, 30 book chapters, and over 420 articles. She was honored with the 2016 American Diabetes Association Outstanding Educator in Diabetes Award. Contact her via her websites (SheriColberg.com and DiabetesMotion.com).
See more about eating low-carb in our diet and nutrition therapy center.