Sheri Colberg, PhD
Sure, You May Lose Weight, But Will Going Low-Carb Impact Your Performance?
With the new year upon us and resolutions made, weight loss may be on your mind and with it low-carb eating as one potential 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 on 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 exact 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 largely involved highly motivated individuals with T1D engaging 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 a 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 prior to, 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 limit 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 that is 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, carbohydrates 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 in particular 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).
In addition, in people with T1D, effective glycogen repletion 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/or liver glycogen stores, you will likely need to take in carbohydrates during extended activities and may not perform as well. If you 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 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)
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