FondoLab

Neutral service · The bonk

Carbohydrate budget

How much carbohydrate your body stores, how much a ride at your power asks for, and whether eating makes up the difference.

Your numbers
Fine-tuning

Sensible defaults; change them if you know better.

Free, no account, nothing saved. Your numbers stay in the address bar, so you can bookmark or share a result.

Carbohydrate budgetFondoLab

352

g left at the finish

Store
478 g
Burn
85 g/h
IF
0.72

At 180 W (IF 0.72) you burn about 85 g of carbohydrate an hour. Over 5:00:00, with 478 g in your muscles and liver and 60 g an hour eaten, you finish with about 352 g to spare.

The store counts all your muscle, while cycling leans mostly on the legs: read it as an upper limit. We do not use your body fat; muscle mass is the average share of body weight from a large MRI study.

Where your carbohydrate comes from
  • Food 300 g 70%
  • Muscle 105 g 25%
  • Liver 21 g 5%

What moves it

Fuel lasts up to
221 W
The same 5:00:00 at another intensity, eating 60 g an hour
IFPowerg an hourAt the finish
0.5513847+545 g
0.6015056+499 g
0.6516267+445 g
0.7017579+381 g
0.7518895+304 g
0.80200114+210 g
0.85212137+93 g
0.90225167−55 g
1.00250255−498 g

How we got this

The formula with your numbers in it, then every constant we used and where it comes from.

The working

  1. Muscle mass: 72.0 kg × 38.4% = 27.6 kg
  2. Usable muscle glycogen: (150 − 70 mmol/kg) × 0.180 g/mmol × 27.6 kg = 398 g
  3. Store: 398 g in your muscles + 80 g in your liver = 478 g (the liver keeps the 4 g in your blood topped up)
  4. The rider whose threshold (MLSS) is your FTP of 250 W: VO2max 56.4 ml/kg/min at a VLamax of 0.5 mmol/L/s, which puts your FTP at 78% of VO2max
  5. At 180 W you are at 58% of VO2max, where 45% of your energy comes from carbohydrate: 85 g an hour; in 5:00:00 that is 426 g
  6. Balance: 478 g stored + 60 g an hour eaten (300 g) − 426 g burned = +352 g

The numbers and their sources

A number marked FondoLab default is our judgement, not a study's finding; we say so rather than borrow authority.

NumberValueBasis
Carbohydrate store muscle 38.4% of body mass (men) or 30.6% (women), 150 mmol/kg rested or 200 loaded, usable down to 70; liver 80 g; blood 4 g Muscle mass is the average share of body mass that whole-body MRI found in 468 adults, who were not athletes. The glycogen per kg is what a review gives for trained, well-fed endurance athletes, and below 70 mmol/kg muscle function and peak power suffer, so we count the store down to there, not to zero. We count it as glucose, 0.180 g per mmol, the unit the burn and the liver's figure are in; as glycogen it would weigh 10% less. The liver's 80 g is the review's "more than 80"; the blood's 4 g is kept steady by the liver and is not counted. All muscle is counted, while a rider works mostly with the legs, so read the store as an upper limit. [1][2]
The rider from an FTP your FTP taken as the MLSS; VLamax 0.5 mmol/L/s unless you give it or have a fitness test The metabolic model needs VO2max and VLamax. We take the rider whose threshold lands on your FTP at your VLamax: 0.5 unless you give it, a FondoLab default for an endurance road rider (the 13 young men Hauser et al. tested averaged 0.91). At the same FTP, a higher VLamax burns more carbohydrate below the threshold. [3]
Threshold (MLSS) from VO2max and VLamax where glycolysis forms lactate as fast as oxygen can burn it; Ks1 0.0631, Ks2 1.331, 0.02049 mmol lactate per ml O2 over 0.4 of body mass, 11.7 ml O2/min per W Mader and Heck's model as Hauser et al. set it out: calculated from lab-measured VO2max and sprint-lactate VLamax, it came within 12 ± 20 W of the measured MLSS in 13 cyclists (r = 0.92). For their average rider our implementation gives 240 W against their 229 W calculated. This threshold is the FTP your profile uses. [3][4]
Fat and carbohydrate at a power carbohydrate share = (pyruvate glycolysis forms ÷ pyruvate the oxygen can burn)^0.743; fat: the rest of the oxygen; 747 ml O2 per g glucose, 2012 per g fat; rest 3.5 ml/kg/min In Mader's model fat covers the oxygen uptake glycolysis leaves uncovered, the pyruvate deficit, and falls to zero at the threshold; a higher VLamax means more carbohydrate at the same power. Taken as it is, the model gave 10 percentage points less carbohydrate than two INSCYD reports at every zone, so we raise its ratio to the power 0.743, a FondoLab calibration that meets both reports within 1.5 points and puts FatMax within 2%. Measured gas exchange gives more carbohydrate still: riders about as fit as the one INSCYD tested got 46, 51 and 76% of their energy from carbohydrate at 44, 57 and 72% of VO2max, where this split gives 27, 40 and 68%, and most trained men burn less fat at their peak than either. The oxygen per gram follows from the chemistry of burning glucose and palmitate; resting uptake is the conventional 1 MET, a FondoLab default. [4][5][6]

References

  1. Janssen I, Heymsfield SB, Wang ZM, Ross R (2000). Skeletal muscle mass and distribution in 468 men and women aged 18–88 yr. Journal of Applied Physiology 89(1):81–88. doi:10.1152/jappl.2000.89.1.81

    Cross-sectional study (whole-body MRI) · 468 men and women aged 18–88, a broad and mixed sample, not athletes. · Read by us: Abstract

    Measured skeletal muscle mass with whole-body magnetic resonance imaging.

    • Men had 33.0 kg of skeletal muscle on average, 38.4% of body mass; women 21.0 kg, 30.6%.
    • The difference between men and women was larger in the upper body (40%) than the lower body (33%).
    • Muscle as a share of body mass fell from the third decade; absolute muscle mass fell from the end of the fifth, mostly in the lower body.
    • Weight and height explained about half the variation in muscle mass. Muscle made up less of each extra kilogram in heavier people.

  2. Murray B, Rosenbloom C (2018). Fundamentals of glycogen metabolism for coaches and athletes. Nutrition Reviews 76(4):243–259. doi:10.1093/nutrit/nuy001

    Narrative review · Reviews studies of athletes, mostly endurance athletes, written for coaches. The first author runs a sports science consultancy. · Read by us: Full text (open access), the parts on how much glycogen the body holds

    How much carbohydrate the body stores as glycogen, how fast exercise uses it and how fast it comes back.

    • Whole-body glycogen is about 600 g, varying widely with body mass, diet, fitness and recent exercise.
    • The liver holds more than 80 g of glucose as glycogen, which it uses to keep the 4 g of glucose circulating in the blood topped up, at 4.0–5.5 mmol/L.
    • In trained, well-fed endurance athletes, muscle glycogen is about 150 mmol per kg of wet muscle after 8–12 hours of rest, and up to about 200 mmol/kg after a few days on a high-carbohydrate diet. After long, hard exercise it can fall below 50.
    • Below about 70 mmol/kg, calcium release in the muscle and peak power are impaired: a threshold below which muscle function and performance suffer.
    • Above about 60% of VO2max, blood glucose and muscle glycogen are the main fuels.

  3. Hauser T, Adam J, Schulz H (2014). Comparison of calculated and experimental power in maximal lactate-steady state during cycling. Theoretical Biology and Medical Modelling 11:25 (erratum 2016). doi:10.1186/1742-4682-11-25

    Validation study (model against measured MLSS) · 13 men, 24 ± 5 years, 72.9 ± 6.8 kg, VO2max 60.4 ± 8.6 ml/kg/min, VLamax 0.91 ± 0.18 mmol/L/s, on a cycle ergometer in the lab. · Read by us: Full text (open access), and the erratum

    Calculated power at the maximal lactate steady state (MLSS) from VO2max, measured in a ramp test with gas analysis, and VLamax, measured from blood lactate before and after a 15 s all-out sprint, with Mader and Heck's model of oxidative phosphorylation and glycolysis. Compared it with MLSS found by constant-load rides.

    • Calculated MLSS 229 ± 47 W, measured 217 ± 51 W: a mean difference of −12 ± 20 W, r = 0.92.
    • VLamax = (peak lactate after the sprint − lactate before) ÷ (sprint time − alactic time), the alactic time lasting until power falls 3.5% from its peak.
    • Oxidative phosphorylation VO2ss = VO2max ÷ (1 + Ks1 ÷ ADP²), Ks1 = 0.0631; glycolysis VLass = VLamax ÷ (1 + Ks2 ÷ ADP³), Ks2 = 1.331.
    • Lactate elimination is proportional to oxygen uptake: 0.02049 mmol lactate per ml O2, over a distribution volume of 0.4 of body mass.
    • Oxygen uptake at a power: VO2ss = (P × 11.7 + body mass × VO2rest) ÷ body mass. No value for VO2rest is given.
    • MLSS is where glycolytic lactate formation equals its maximal elimination, solved numerically.
    • The 2016 erratum corrects equation 7 (ADP from VO2ss) to use Ks1. Combining equations 3 and 4 gives ADP³ = (Ks1 × VO2ss ÷ (VO2max − VO2ss))^1.5; we use that form.

    The authors put the 12 W gap down to the day-to-day variability of VO2max and VLamax. VO2max and VLamax were measured in the lab, not estimated from power.

  4. Wackerhage H, Gehlert S, Schulz H, Weber S, Ring-Dimitriou S, Heine O (2022). Lactate thresholds and the simulation of human energy metabolism: contributions by the Cologne Sports Medicine Group in the 1970s and 1980s. Frontiers in Physiology 13:899670. doi:10.3389/fphys.2022.899670

    Narrative review (history and method) · Not a study of riders: it reviews the Cologne group's lactate testing and Mader's model of energy metabolism. · Read by us: Full text (open access)

    Explains, in English, the Cologne group's work on lactate thresholds and Alois Mader's mathematical model of energy metabolism during exercise, which is driven by VO2max and the maximal rate of lactate formation, VLamax.

    • VLamax is estimated from the rise in blood lactate after an all-out sprint, divided by the sprint time less the time phosphocreatine covers. It is hard to measure well: the sprint must switch glycolysis fully on while keeping oxidation low.
    • Oxidative ATP resynthesis follows ADP with a second-order Hill equation, glycolysis with a third-order one.
    • The maximal lactate steady state lies where glycolytic pyruvate formation equals the maximal rate of pyruvate oxidation. Below it, fat oxidation covers the "pyruvate deficit"; at and above it fat oxidation falls towards zero.
    • A higher VLamax puts the 4 mmol/L threshold at a lower power at the same VO2max; in the review's simulation about 110 W lower.
    • The simulations shown use 40% of body mass as active muscle; most of the full model's constants are not given in the review.

  5. van Loon LJC, Greenhaff PL, Constantin-Teodosiu D, Saris WHM, Wagenmakers AJM (2001). The effects of increasing exercise intensity on muscle fuel utilisation in humans. Journal of Physiology 536(1):295–304. doi:10.1111/j.1469-7793.2001.00295.x

    Laboratory study (stable isotope tracers, indirect calorimetry and muscle biopsies) · 8 trained male cyclists, 22 years, 74.5 kg, VO2max 5.48 L/min (about 74 ml/kg/min), Wmax 413 W, after an overnight fast and a standard evening meal, on a cycle ergometer. Small sample; no women. · Read by us: Full text

    Measured how much of the energy came from fat and from carbohydrate, and from which stores, at rest and in three consecutive 30 minute stages at 40, 55 and 75% of Wmax, which were 44, 57 and 72% of VO2max.

    • Carbohydrate gave 44% of the energy at rest, 46% at 44% of VO2max (1.44 g/min), 51% at 57% (2.04 g/min) and 76% at 72% (3.91 g/min).
    • Fat oxidation rose to 0.79 g/min at 57% of VO2max and fell to 0.51 g/min at 72%: from 50% of the energy to 24%.
    • Muscle glycogen gave 35%, 38% and 58% of the energy at the three intensities, and blood glucose 10%, 13% and 18%.
    • Plasma lactate rose substantially only at the highest intensity.
    • The stages followed each other without a break; in four riders who repeated each intensity on its own, fat oxidation was much the same.

  6. Maunder E, Plews DJ, Kilding AE (2018). "Contextualising maximal fat oxidation during exercise: determinants and normative values". Frontiers in Physiology 9:599. doi:10.3389/fphys.2018.00599

    Narrative review with pooled normative values · Pooled step tests on a cycle ergometer after an overnight fast, by group; the largest is 201 endurance-trained, lean men. Fewer data for women. · Read by us: Full text (open access)

    Pools measurements of the highest rate at which people burn fat during exercise (MFO) and the intensity where it happens (Fatmax), and what changes them.

    • Endurance-trained, lean men: MFO 0.53 ± 0.16 g/min (20th–80th percentile 0.40–0.67) at 56 ± 8% of VO2max (49–63%). Recreationally active men: 0.46 g/min at 51%; recreationally active women: 0.35 g/min at 50%.
    • Across seven studies, MFO was on average 28% higher in men (0.56 vs 0.44 g/min); Fatmax was a little higher in women.
    • 75 g of glucose 45 minutes before exercise lowered MFO from 0.46 to 0.33 g/min and Fatmax from 60 to 52% of VO2max in trained men.
    • Total carbohydrate storage is typically under 740 g, about 80% of it in muscle and 10–15% in the liver, with about 4 g as blood glucose.
    • Muscle glycogen at the start, diet, training status and the test protocol all change the values, and the test is only moderately repeatable.

Read more: How many carbs per hour on a gran fondo?

60, 90 or 120 grams an hour? What the research says about carbohydrate on long rides, and how to set your own target.

On your race

Fueling planner: carbs, fluid and caffeine stop by stop for your race

More calculators

All calculators