Fueling for the Work Required: The New Science of Sports Nutrition

For decades the dominant message in endurance sport was a quiet form of restriction. Stay light, keep the fat percentage down, and the watts will follow. That belief produced a generation of underfed athletes, fragile bones and stalled careers. The science of sports nutrition has now swung hard in the other direction. The organising principle today is not how little an athlete can eat but how precisely they can match fuel to demand. The phrase that captures it, coined within the research community, is fueling for the work required.

Fueling for the work required

The idea is simple to state and harder to execute. Not every session has the same energy cost, so not every day should look the same on the plate. A long, hard interval session needs a high carbohydrate intake before, during and after. An easy recovery spin needs far less. Periodising carbohydrate to the training calendar, rather than eating the same way every day, lets athletes hit hard sessions with full tanks while occasionally training with lower glycogen to nudge certain adaptations.

This framework owes a great deal to the work of sports scientist Asker Jeukendrup and colleagues, whose research on carbohydrate intake and exercise helped move the field from blanket recommendations toward individualised, demand-matched fueling. The key correction it made was cultural. For years athletes treated carbohydrate as something to fear. The current evidence treats it as the primary fuel for high-intensity endurance work, and treats chronic under-eating as a performance and health liability rather than a discipline.

How much carbohydrate is possible

Nowhere has the change been more visible than in how much endurance athletes now eat during competition. The textbook ceiling for carbohydrate absorption during exercise was long held to be about 60 grams per hour, because the gut transporter for glucose saturates around there. Then researchers showed that combining glucose with fructose, which uses a separate transporter, lets the body oxidise more, pushing the practical ceiling toward 90 grams per hour.

Elite endurance sport has since blown past even that. In professional cycling and ultra-distance triathlon, athletes now train their guts to tolerate 100, 120, and in some reported cases up to 120 to 150 grams of carbohydrate per hour during racing. This is not a free lunch. Tolerating those rates requires deliberate gut training over weeks, and pushing too hard invites the gastrointestinal distress that ends races. But the direction is clear: the limit on race-day fueling turned out to be partly trainable, and the best teams treat the gut as an organ to be conditioned like any other.

Protein timing, in proportion

Carbohydrate gets the headlines, but protein is where a lot of consumer marketing overreaches. The established science is reassuring and fairly boring. Total daily protein intake matters most, somewhere around 1.6 to 2.2 grams per kilogram of body weight per day supports muscle adaptation for most strenuously training athletes, according to position stands from the International Society of Sports Nutrition. Spreading that across several meals of roughly 0.3 grams per kilogram is sensible.

The narrower idea of an anabolic window, the belief that protein must be consumed within a tight window after exercise or the work is wasted, is far weaker than supplement advertising implies. The window, if it exists, is wide, measured in hours rather than minutes, especially when an athlete has eaten normally around training. The practical takeaway separates proof from promise: hitting your daily protein target with decent distribution is well supported, while panicking about a 30-minute post-workout shake is mostly marketing. Recovery nutrition matters, and it connects to the broader recovery toolkit of cold, heat and other modalities, but the fundamentals are unglamorous.

RED-S and the cost of under-fueling

The most important shift in the field is the recognition of Relative Energy Deficiency in Sport, or RED-S. The concept, formalised in an International Olympic Committee consensus statement and updated since, describes what happens when an athlete does not take in enough energy to cover both training and basic physiological function. The body, facing a shortfall, downregulates systems it treats as non-essential. The consequences are wide-ranging: disrupted menstrual function, weakened bones and stress fractures, impaired immunity, suppressed hormones, and, ironically, worse performance.

RED-S grew out of the earlier female athlete triad but applies to men and women alike. Its central metric is energy availability, the energy left over for the body after the cost of exercise is subtracted. Drop that too low for too long and the damage accumulates quietly, often masked at first by short-term improvements as the athlete gets lighter. This is the trap. Under-fueling can look like it is working right up until the bone breaks or the season collapses. Addressing it is now a frontline concern for any well-run performance programme.

The move away from chronic under-fueling

Put together, these threads describe a single cultural correction. The sport is moving away from the romance of the starving climber and toward the idea that fueling is a skill and an obligation. Teams employ dietitians who calculate energy availability, plan carbohydrate around the training week, and watch for the warning signs of RED-S as carefully as they watch power data.

The honest summary is layered. What is firmly established: carbohydrate is the dominant fuel for hard endurance work, total protein intake drives adaptation, and chronic low energy availability harms both health and performance. What is genuinely promising and still being refined: how high race-day carbohydrate rates can safely go, and how best to periodise fuel for individual athletes. What is mostly marketing: narrow anabolic windows, exotic single-ingredient miracle supplements, and the lingering folklore that lighter always means faster. The new science of sports nutrition is, at heart, permission to eat enough to do the work.

FAQ

What does “fueling for the work required” actually mean? It means matching your food, especially carbohydrate, to the energy demands of each specific session rather than eating the same amount every day. Hard interval days call for high carbohydrate intake before, during and after, while easy recovery days need much less. The goal is to enter demanding sessions fully fuelled and to avoid the chronic under-eating that harms both performance and long-term health.

How much carbohydrate can you absorb during exercise? The old ceiling was about 60 grams per hour, limited by the glucose transporter in the gut. Combining glucose with fructose raises the practical limit to around 90 grams per hour, and elite endurance athletes now train their guts to tolerate 100 to 150 grams per hour in racing. Those rates require weeks of deliberate gut training and are not advisable without it.

What is RED-S and why does it matter? RED-S, or Relative Energy Deficiency in Sport, describes the harm caused when an athlete does not eat enough to cover both training and normal bodily function. Defined in IOC consensus statements, it can cause bone loss and stress fractures, hormonal disruption, impaired immunity and worse performance. It affects men and women, and its danger is that short-term gains from getting lighter can hide the accumulating damage.

Sources

1. IOC consensus statement on Relative Energy Deficiency in Sport (REDs) – British Journal of Sports Medicine
2. Carbohydrate intake during prolonged exercise – Asker Jeukendrup, mysportscience
3. ISSN position stand on protein and exercise – Journal of the International Society of Sports Nutrition
4. Nutrition and athletic performance joint position stand – American College of Sports Medicine
5. Carbohydrate periodization and “train low” strategies – Sports Medicine (Springer)
6. The female athlete triad and energy availability – IOC and BJSM