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5 min readrunning / endurance

10 g per kilo beat 6 and 8: how to carb load before a race

I ran my first 100 km on three nights of pasta and optimism. I never counted a gram. A trial published this year puts numbers on what I was guessing at: endurance-trained athletes who ate 10 g of carbohydrate per kilo of body mass per day stored 26% more muscle glycogen than at 8 g, and 6 g and 8 g produced the same result.

What the research shows.

Karlsson and Saltin sent ten runners over the same 30 km course twice, three weeks apart, once after a carbohydrate-rich diet and once after a mixed one 1. All ten posted their best time in the loaded condition. The gain arrived late in the race: pace held through the closing kilometres instead of decaying.

A 1997 review by Hawley and colleagues drew the boundary 2. Pushing muscle glycogen above resting levels delays fatigue by about 20% in events over 90 minutes. Between 60 and 90 minutes of moderate-intensity running or cycling it does nothing, because the working muscles still hold plenty of glycogen at the finish. Under 5 minutes, nothing again.

The dose stayed an open question until this year. Jones and colleagues fed 11 endurance-trained athletes 6, 8 or 10 g of carbohydrate per kilo of body mass per day across the final 48 hours of a protocol built to mimic race week 3. Glycogen tracked intake in a straight line (r = 0.71, p < 0.001): 460.9 ± 100.7 mmol/kg dry mass at 6 g/kg, 506.1 ± 124.0 at 8 g/kg, and 635.5 ± 78.0 at 10 g/kg. The 6 and 8 conditions did not differ from each other, and no plateau appeared below 10. Body mass did not move (p = 0.70). Fullness was the only gut symptom that rose.

One day covers most of the gain. Bussau and colleagues gave eight endurance-trained men roughly 10 g/kg/day of high-glycaemic carbohydrate while keeping them inactive, with biopsies before loading and after one and three days 4. Glycogen went from 95 to 180 mmol/kg wet mass in 24 hours, then held flat for the next two days. No depletion ride. No glycogen-stripping long run. No three-day diet.

Field data agree. Atkinson and colleagues modelled finishing speed in sub-elite marathon runners 5. Carbohydrate eaten on the day before the race predicted speed independently of sex, body mass index and training distance, worth 0.08 to 0.26 km/h, inside a model that accounted for 56% of the variation between runners. Those above 7 g/kg the day before ran faster and held pace better than those below it.

Following the ritual is not the same as hitting the dose.

Fogelholm and colleagues put six male runners through two versions of the classic depletion-then-load protocol, depleting with either a half marathon or an easier fartlek session 6. Final glycogen came out at 285 ± 25 mmol/kg dry weight after the race and 315 ± 32 after the fartlek, with no statistical difference, and neither was clearly above the value before depletion. Six runners is a thin sample. The title the authors chose says the rest: high glycogen is not certain.

Body size and sex change the arithmetic. Tarnopolsky and colleagues compared a habitual diet, a carbohydrate-rich diet, and a carbohydrate-rich diet with extra energy on top 7. Men raised glycogen on both carbohydrate trials. Women raised it only when total energy went up as well. Shifting the share of your plate toward carbohydrate moves nothing if the absolute grams stay put, and athletes eating a fixed number of normal-sized meals hit that ceiling first.

The protocol.

  1. Use this for races over 90 minutes. For a 10 km or an hour-long tempo, eat your normal diet 2.
  2. Load for one day, not three. Start 36 hours before the gun, finish with dinner the night before 4.
  3. Target 10 g of carbohydrate per kilo of body mass across that day. At 70 kg, 700 g. At 55 kg, 550 g. Aiming at 8 g/kg buys you nothing over 6 3.
  4. Write the grams per meal before the day starts, and weigh your food the first time. 700 g of carbohydrate is around 1.4 kg of cooked white rice plus 1.5 L of sports drink plus four bananas. Eyeballing it lands most people closer to 5 g/kg.
  5. Take 40% to 50% of the grams as liquid: juice, sports drink, a maltodextrin mix. Fullness is the symptom that climbs when you load hard 3, and drinking part of the load is how you get past it.
  6. Drop fiber for those 24 hours. White rice, white bread, peeled potato, ripe banana. Skip legumes and bran. Six days of a low-FODMAP diet cut daily gut symptoms in 11 recreational runners, measured as symptom area under the curve (p = 0.003) 8.
  7. Train close to zero on the loading day. The one-day protocol was validated in athletes who stayed inactive 4.
  8. If you are a woman, or under 60 kg, raise total energy rather than the carbohydrate percentage 7.
  9. Rehearse the whole day once, three weeks out, in front of your longest run. Log morning body mass, gut symptoms hour by hour, and how the run felt after 90 minutes. Keep the plan, or cut the liquid share and raise the rice.

The glycogen you store on Saturday runs out on Sunday, and what you swallow during the race decides the last hour. Loading changes where the decay starts. Take your body mass in kilos, multiply by 10, and that is your gram target for one day.

Sources.

  1. Karlsson, J., & Saltin, B. (1971). Diet, muscle glycogen, and endurance performance. Journal of Applied Physiology, 31(2), 203-206. link
  2. Hawley, J. A., Schabort, E. J., Noakes, T. D., & Dennis, S. C. (1997). Carbohydrate-loading and exercise performance: an update. Sports Medicine, 24(2), 73-81. doi:10.2165/00007256-199724020-00001
  3. Jones, R., Areta, J. L., Bennett, S., Pugh, J., & Louis, J. (2026). Dose-response of dietary carbohydrate intake on skeletal muscle glycogen, gastrointestinal comfort and body composition in endurance-trained individuals in simulated preparation for competition. Scandinavian Journal of Medicine & Science in Sports. doi:10.1111/sms.70312
  4. Bussau, V. A., Fairchild, T. J., Rao, A., Steele, P., & Fournier, P. A. (2002). Carbohydrate loading in human muscle: an improved 1 day protocol. European Journal of Applied Physiology, 87(3), 290-295. doi:10.1007/s00421-002-0621-5
  5. Atkinson, G., Taylor, C. E., Morgan, N., Ormond, L. R., & Wallis, G. A. (2011). Pre-race dietary carbohydrate intake can independently influence sub-elite marathon running performance. International Journal of Sports Medicine, 32(8), 611-617. doi:10.1055/s-0031-1273739
  6. Fogelholm, G. M., Tikkanen, H. O., Naveri, H. K., Naveri, L. S., & Harkonen, M. H. (1991). Carbohydrate loading in practice: high muscle glycogen concentration is not certain. British Journal of Sports Medicine, 25(1), 41-44. link
  7. Tarnopolsky, M. A., Zawada, C., Richmond, L. B., Carter, S., Shearer, J., Graham, T., & Phillips, S. M. (2001). Gender differences in carbohydrate loading are related to energy intake. Journal of Applied Physiology, 91(1), 225-230. doi:10.1152/jappl.2001.91.1.225
  8. Lis, D. M., Stellingwerff, T., Kitic, C. M., Fell, J. W., & Ahuja, K. D. K. (2018). Low FODMAP: a preliminary strategy to reduce gastrointestinal distress in athletes. Medicine & Science in Sports & Exercise, 50(1), 116-123. doi:10.1249/MSS.0000000000001419

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