Replenishing muscle glycogen after endurance exercise Original paper

Carbohydrate provides a versatile fuel for muscular work. Blood glucose and muscle glycogen are the primary source of energy to sustain brief (10–180 seconds) high-intensity exercise and are a major source of energy for events lasting longer than 2 minutes.^[1] There is a strong relationship between muscle glycogen levels and endurance performance,^[2] so adequate muscle glycogen recovery over a 24-hour period is critical for endurance athletes to maintain their daily training quality. How much carbohydrate is needed to fully replenish glycogen stores after prolonged high-intensity endurance exercise?

Adapted from Murray & Rosenbloom, 2018.^[3]

In this randomized crossover trial, 8 male Japanese collegiate endurance athletes (average age of 20, average VO₂max of 56 millimeters/kilogram of body weight/minute) completed an approximately 90-minute incremental (increasing by 20 watts every minute) cycling test until voluntary exhaustion. During the 24-hour recovery period after the test, they consumed one of three diets: 5 grams of carbohydrate per kilogram of body mass (g/kg), 7 g/kg, or 10 g/kg. The three diets provided the same amount of energy and similar amounts of protein, so dietary fat intake substantially differed between conditions. Three meals were consumed after exercise in each condition. There was a washout period of at least one week between conditions, and a three-day weighed dietary record was completed before each condition.

Muscle glycogen levels of the vastus lateralis and vastus intermedius muscles (on the right leg) were measured using ¹³C-magnetic resonance spectroscopy before, immediately after, and 4 hours, 12 hours, and 24 hours after exercise. Blood levels of glucose, insulin, and glucagon were also measured before and immediately after exercise, as well as 30 minutes after breakfast, lunch, and dinner.

Muscle glycogen levels before exercise were not different between conditions, and exercise decreased muscle glycogen similarly in each condition (27–33%). Compared to the 5 g/kg condition, muscle glycogen recovery was greater in the 7 g/kg and 10 g/kg conditions. Specifically, muscle glycogen levels recovered to 81.7%, 97.1%, and 100% of preexercise levels at 24 hours in the 5 g/kg, 7 g/kg, and 10 g/kg conditions, respectively. Muscle glycogen recovery was also greater in the 7 g/kg and 10 g/kg conditions compared to the 5 g/kg condition during hours 4–12.

Compared to the 5 g/kg condition, blood insulin levels were greater in the 10 g/kg condition after lunch, and greater in the 7 g/kg condition after dinner. Also, blood glucagon (a hormone that promotes glycogen breakdown) levels were higher after breakfast in the 7 g/kg group than the 10 g/kg group.

Because the study only included young Japanese men, the results may not be generalizable to other populations, as the metabolic response to carbohydrate intake varies between ethnicities^[4][5] as glucose tolerance decreases with aging.^[6]

The results of this study align with previous evidence, which found greater muscle glycogen levels with higher carbohydrate intakes, up to about 7–10 g/kg/day.^[7] Additionally, the results are in accordance with recent sports nutrition guidelines, which recommend that athletes target a carbohydrate intake of at least 3–5 g/kg/day when they perform low-intensity or skill-based activities, 5–7 g/kg/day for moderate-intensity exercise lasting about one hour, 6–10 g/kg/day for moderate- to high-intensity exercise lasting 1–3 hours, and 8–12 g/kg/day for moderate- to high-intensity exercise lasting 4–5 hours.^[1] The exercise test selected in this study (i.e., about 90 minutes of cycling to exhaustion) falls in the third category, so it’s not surprising that 7–10 g/kg of carbohydrate was better than 5 g/kg of carbohydrate for muscle glycogen recovery.

In order to equate energy content between diets, dietary fat content was manipulated in conjunction with carbohydrate content, so the diet with the least amount of carbohydrate had a substantially greater amount of fat than the diet with the most carbohydrate (174 grams vs. 58 grams of fat in the 5 g/kg and 10 g/kg conditions, respectively). The coingestion of fat with carbohydrate alters the metabolic response to carbohydrate feeding, namely by reducing blood glucose response,^[8] which may be the result of delayed gastric emptying.

This suggests that differences in fat intake (and thus digestion and absorption) between conditions may have influenced muscle glycogen recovery. However, over a 24-hour period, dietary fat and protein don’t appear to affect muscle glycogen recovery.^[9] In other words, the most critical dietary factor in muscle glycogen recovery is the amount of total carbohydrate consumed, with other factors playing a minor role that may increase in importance during shorter periods of recovery.

Muscle glycogen synthesis rates have been reported to be 30–50% higher during the first four hours after exercise with large intakes of carbohydrate (at least 1 g/kg/hour), especially when carbohydrate is consumed in small, frequent feedings, such as every 15–60 minutes.^[10] This is due to the activation of glycogen synthase (a key enzyme in the conversion of glucose to glycogen) caused by glycogen depletion and exercise-induced increases in insulin sensitivity and permeability of the muscle cell membrane to glucose.^[7]

As alluded to above, the timing and frequency of carbohydrate intake increase in importance during short-term recovery (less than 8 hours). In the present study, the carbohydrate intake per hour was 0.6 g/kg, 0.9 g/kg, and 1.2 g/kg in the 5 g/kg, 7 g/kg, and 10 g/kg conditions, respectively, and carbohydrate intake was divided between three meals in each condition.

There were no differences in muscle glycogen recovery between conditions 0–4 hours after exercise, which is likely explained by the low meal frequency and large meal size following exercise. Furthermore, the quantity of carbohydrate provided in the first meal after exercise exceeded 1.5 g/kg in each condition.

The glycemic index (GI, a measure of a food’s effect on blood glucose) of the carbohydrate sources may also influence muscle glycogen recovery, as high-glycemic carbohydrates are digested and absorbed faster than low-glycemic carbohydrates, and some low-glycemic carbohydrates may also be malabsorbed.^[11] As a result, high glycemic index carbohydrates may be optimal for muscle glycogen recovery,^[12] particularly during short recovery periods.^[10]

In the present study, the researchers reported that the participants consumed typical Japanese meals featuring a carbohydrate source (e,g., rice, bread, pasta, fruit), a main dish of meat or fish, and a side of vegetables. As a result, the researchers noted that “many meals with a low GI may have been offered.” Despite the theoretical inferiority of low GI meals compared to high GI meals, the participants were able to more or less fully replenish muscle glycogen in the 7 g/kg and 10 g/kg conditions.

These results fall in line with other evidence demonstrating that the form (i.e., solid or liquid) of carbohydrate,^[13][14] and whether it’s obtained from potato products,^[15] fast food (e.g., hamburger, french fries),^[16] or sports supplements, does not seem to materially influence muscle glycogen recovery.

In summary, details like glycemic index, the timing and frequency of carbohydrate ingestion, and macronutrient coingestion after exercise may be important during short recovery periods (e.g., multiple competitions in the same day), but during longer recovery periods (approximately 24 hours), when the athlete has time to consume an adequate amount of carbohydrate, the type of carbohydrate and the pattern of intake can be selected according to personal preference, as these details don’t meaningfully influence muscle glycogen recovery.^[10]