Four things in an athlete’s diet have numbers attached to them: carbohydrate, protein, fluid, and whether you are eating enough overall. Everything else in sports nutrition is either a detail or a supplement claim. This page gives the four numbers, the evidence behind them, and the one that quietly ends more seasons than the other three combined.
The short version
- Carbohydrate: 3–10 g per kg of body weight per day, depending on training load.
- Protein: 1.2–2.0 g/kg/day, with no safety concerns reported up to 2.0.
- Fluid: losing just 2–4% of body weight in water cuts maximal aerobic power by 10–27%.
- Protein timing barely matters once daily total is controlled for — the “anabolic window” is not what the evidence shows.
- Under-fuelling has a clinical name and a documented cost: low energy availability, menstrual disruption, and stress fractures.
Carbohydrate: the fuel number
NIH’s Office of Dietary Supplements puts athlete carbohydrate needs at 3 to 10 g/kg body weight per day. For a 70 kg person that is 210–700 g — a range wide enough that your training volume, not a formula, decides where you sit.
During exercise, the threshold is duration. An Institute of Medicine review states that glucose feedings may be of benefit during prolonged exercise lasting longer than 90 min, while a committee summary sets the practical bar at endurance events that last at least 60 minutes and involve vigorous exercise. Below that, you are fuelled already.
Details on intra-workout and post-workout carbohydrate are in endurance fuelling.
Protein: less than the industry implies
ODS gives 1.2 to 2.0 g/kg body weight per day for athletes, and states there are no safety concerns reported at daily recommended intakes for athletes of up to about 2.0 g/kg body weight. MedlinePlus is blunter still: athletes, even body builders, need only a little bit of extra protein to support muscle growth, and amino acid supplements and eating a lot of protein are not recommended.
Per meal, the useful dose is smaller than most people assume. A review of physique-athlete nutrition reports that a 20 g dose of egg protein stimulated MPS to a greater degree than smaller doses of 5 g or 10 g, with a plateau at a larger dose of 40 g. Spreading it out is the sensible default: the even spread of protein servings over the course of the day is theorized to maximize the 24 hour MPS area under the curve. Our page on protein per meal goes into the dosing.
The anabolic window, honestly
This is where a lot of money is made. The same review reports no significant effect on hypertrophy when controlling for total daily protein intake in longitudinal studies, regardless of timing.
Read that carefully: it does not say timing is irrelevant to everyone in every context. It says that once your daily protein total is held constant, when you eat it stopped predicting muscle growth. Hitting your daily number matters. Sprinting to a shaker inside thirty minutes does not.
And on the shaker itself: protein powders are not inherently more efficacious than any other high-quality protein source and are generally viewed as a convenient means. Convenience is a real benefit. It is just not a metabolic one.
Fluid: the number that degrades performance fastest
The thresholds are low. An Institute of Medicine review found that a water deficit of only 1% of body weight significantly elevates core temperature during exercise, rising a further 0.15°C for each additional percent lost. And small (2%) to moderate (4%) water deficits reduce maximal aerobic power (10%-27%) and physical work capacity (22%-48%).
Practical amounts, from MedlinePlus: about 16 ounces (2 cups) or 480 millilitres of water 2 hours before a workout; during and after, 1/2 to 1 cup (120 to 240 millilitres) of fluid every 15 to 20 minutes; and afterwards, 16 to 24 ounces of fluid within the next 6 hours for every pound lost.
The important caveat: more is not better. CDC’s heat-stress guidance warns that drinking too much water or other fluids (sports drinks, energy drinks, etc.) can cause a medical emergency because the concentration of salt in the blood becomes too low, and sets a ceiling: do not drink more than 48 oz (1½ quarts) per hour. For sessions over two hours the rule shifts — see endurance fuelling.
The fourth number: are you eating enough at all
This is the one that gets skipped. A clinical reference defines energy availability as the energy acquired through oral nutrition less the energy expended during exercise and baseline daily metabolic requirements. When it stays too low, the consequences are documented: low energy (with or without an eating disorder) in combination with a menstrual disorder and altered mineral bone density is known as the female athlete triad.
The chain is mechanical, not vague: menstrual dysfunction in the female athlete directly correlates to decreased estrogen, and the incidence of stress fractures is greater in amenorrheic athletes. Risk concentrates where you would expect: endurance sports such as track and field, swimming, and rowing or those requiring subjective judging like gymnastics and figure skating.
Two more figures worth stating plainly: female athletes are 5 to 10 times more likely than male athletes to have an eating disorder, and prevalence of clinical eating disorders among female elite athletes runs from 16% to 47%.
What we are not claiming
- A 30-minute anabolic window. No source we could reach supports it, and the evidence we did find points the other way once daily protein is controlled.
- That sports drinks are needed for everyone. Carbohydrate-electrolyte drinks are evidenced for endurance over about 60 minutes and heavy-sweat conditions, not for a 40-minute gym session.
- That any supplement burns fat. ODS describes the evidenced supplements in terms of strength, power, endurance and recovery — never fat loss.
- Carbohydrate g/kg figures broken down by training level. The position stands that publish that table were unreachable; ODS gives only the 3–10 g/kg aggregate, so that is what we quote.
- That BCAAs build muscle. We found no source on our approved list addressing it either way, so we say nothing.
FAQ
How much protein do I need if I train? 1.2–2.0 g per kg of body weight per day, per NIH ODS. For 70 kg that is 84–140 g.
How many carbs does an athlete need? 3–10 g per kg per day, scaled to training volume.
Does protein timing matter? Not in the way it is sold. Controlling for daily total, timing showed no significant effect on hypertrophy.
How much should I drink during exercise? Roughly 120–240 ml every 15–20 minutes, and never more than about 1.4 litres in an hour.
Are protein powders better than food? No — they are described as no more efficacious than other high-quality protein, just more convenient.
What to do with this
Work out your two per-kilogram numbers once and keep them. Weigh yourself before and after a hard session occasionally to see what you actually lose in fluid. And if training is going badly across months rather than days, check the fourth number before changing your programme — under-fuelling looks like poor recovery long before it looks like a diagnosis.
Next: what to eat before training, nutrition for strength, caffeine dosing.
Educational content, not medical advice. If you have symptoms of low energy availability — missed periods, repeated stress fractures, persistent fatigue — this needs a clinician, not a nutrition article.
This content is for educational purposes only and is not medical advice. Always consult a qualified healthcare professional for personalised guidance.
