Beyond the Spreadsheet: The Ordinary Logic of Performance Nutrition

Five Grams in the Bloodstream

At any given moment, only a few grams of glucose are circulating in the blood. Against the energy stored elsewhere in the body, it is an almost absurdly small reserve. Human skeletal muscle can hold several hundred grams of carbohydrate as glycogen, while the liver keeps another, smaller supply that helps maintain the glucose concentration of the blood between meals and during exercise. Muscle glycogen largely stays where it is stored: a working muscle can use it, but cannot simply release that glucose into circulation for the rest of the body. [1]

That distinction helps explain why two meals containing the same number of calories can feel quite different. Calories measure energy, but they say little about how rapidly it becomes available or what the body can do with it. Carbohydrate can replenish glycogen and provide glucose quickly. Fat contains more energy per gram and supplies a large proportion of fuel at rest and during lower-intensity activity. Protein can contribute energy too, but its more valuable role for an exercising body is supplying amino acids for the turnover, repair and construction of tissue.

The familiar story that a high-sugar meal inevitably produces a glucose spike followed by a cognitive crash is tidier than the evidence. In a meta-analysis of breakfast studies in adults, high and low glycemic-load meals produced no detectable difference in cognition during the first 5 to 110 minutes after eating. A small advantage for lower glycemic-load breakfasts appeared later in the morning for one measure of memory. Earlier reviews have also found a distinctly mixed picture. Blood glucose matters enormously, particularly when it becomes abnormally low, but ordinary variations in alertness after lunch cannot be read directly from a glucose curve. [2]

Glycogen and the Last Interval

Muscle glycogen is stored with water, at least several grams of water for every gram of glycogen. This is one reason the scale can move quickly when someone sharply reduces carbohydrate intake and then rise again when carbohydrate returns. Much of that early movement is not fat appearing and disappearing but glycogen and its accompanying water leaving and returning. [3]

The usefulness of those stores becomes more obvious as exercise gets harder. As intensity rises, working muscle increasingly depends on carbohydrate. Repeated sprints can drain glycogen rapidly even though the total exercise session is short. During prolonged hard exercise, low carbohydrate availability can become one of the constraints on maintaining pace or power. A person can therefore eat enough calories in the abstract and still arrive at a demanding session poorly fuelled for the work being asked of them.

Sports-nutrition recommendations reflect the size of the task. A short gym session after normal meals does not require elaborate carbohydrate preparation. Before a long or demanding event, recommendations commonly rise to roughly 1 to 4 grams of carbohydrate per kilogram of body mass during the preceding 1 to 4 hours, adjusted for the athlete, the session and what the stomach will comfortably tolerate. During exercise lasting roughly 1 to 2.5 hours, 30 to 60 grams an hour is a common performance range. [4]

There is an even stranger demonstration of carbohydrate's relationship with performance. In experiments, athletes have sometimes improved endurance performance after merely rinsing a carbohydrate solution around the mouth and spitting it out. No meaningful carbohydrate has reached the muscles. Signals from the mouth appear capable of affecting brain regions involved in reward, motor control and effort. The effect is small and inconsistent enough to resist becoming a universal trick, but it is a useful reminder that the sensation of available energy is not generated by muscle chemistry alone. [5]

What Fat Does Well

A long, easy run and a final sprint place very different demands on the machinery that releases energy. At low and moderate exercise intensities, fat can make a substantial contribution. Its enormous storage capacity is part of what makes hours of relatively gentle movement possible without continuously eating. As intensity increases, carbohydrate progressively takes over because it can support energy production at the rates demanded by harder work.

Training changes this mixture. Endurance-trained muscle becomes better at oxidising fat at a given submaximal workload, preserving some carbohydrate for later. Athletes can also deliberately train with reduced carbohydrate availability in selected sessions because low glycogen can amplify some cellular signals involved in endurance adaptation. That does not make low glycogen an advantage everywhere. The same condition can compromise sessions requiring high power or sustained intensity. Nutrition can therefore be periodised with training rather than organised around a permanent allegiance to either carbohydrate or fat.

Protein operates on another timescale. Resistance exercise makes muscle especially responsive to amino acids, but the often advertised "anabolic window" is much wider than a frantic dash from the squat rack to a protein shaker suggests. Sports-nutrition guidance commonly recommends high-quality protein servings of roughly 20 to 40 grams, distributed across the day every few hours. Protein eaten before training can still be contributing amino acids during recovery afterward. The total daily amount and its distribution usually matter more than hitting a particular minute on the clock. [6]

The Bottle and the Scale

Sweat is wonderfully unsophisticated. It moves water to the skin, where evaporation carries heat away. The amount lost, however, can vary dramatically with body size, exercise intensity, temperature, clothing, acclimatisation and the individual. This makes a universal hydration prescription difficult to defend.

A simple field measurement is often more useful than an expensive powder. Weighing before and after a representative training session, while accounting for anything drunk during it, gives an estimate of sweat loss. The National Athletic Trainers' Association recommends individualising fluid strategies and generally avoiding body-mass losses greater than about 2 per cent during exercise when performance is the priority. It also warns against moving too far in the opposite direction. Drinking faster than fluid is being lost can dilute blood sodium and, in extreme endurance events, contribute to exercise-associated hyponatraemia. [7]

Electrolytes become useful in that context rather than because sodium possesses mysterious energising properties. Long, hot sessions, heavy sweating, repeated training bouts and unusually salty sweat can create meaningful sodium losses. A desk-bound afternoon followed by a 40-minute gym session usually presents a less dramatic problem. Food and ordinary fluids cover a great deal of daily electrolyte replacement. Sports drinks become more valuable when they solve a real combination of fluid, carbohydrate and sodium needs.

The Half-Hour After Training

The famous post-workout window changes size according to what happens next.

An athlete who finishes at 10 a.m. and trains hard again at 3 p.m. has a reason to start eating promptly. Glycogen replacement can proceed rapidly after exercise, and carbohydrate intakes around 1.0 to 1.2 grams per kilogram per hour are used when depleted stores need to be restored aggressively before another session. Someone whose next workout is tomorrow has considerably more freedom. Given enough carbohydrate and total energy during the rest of the day, the urgency falls away. [8]

Protein follows a similar pattern without requiring stopwatch precision. Eating a normal meal containing a useful serving of protein within the surrounding hours works perfectly well for most recreational training. Timing becomes more interesting when training volume rises, recovery periods shorten, appetite makes eating difficult or performance tomorrow depends heavily on what is restored today.

Pre-exercise eating is partly physiology and partly logistics. A large meal that looks excellent on paper may become a terrible nutritional strategy if it is still sitting heavily in the stomach halfway through a run. Familiar foods, digestion time and personal tolerance matter. The sophisticated version of performance nutrition often looks less like optimisation by spreadsheet and more like repeatedly discovering which ordinary meal allows someone to train hard without thinking about their stomach.

Hunger After the Session

Hard exercise can briefly make eating less appealing. Recent work on exercise-induced appetite suppression points particularly to changes in the hunger-related hormone acylated ghrelin, alongside other signals originating in the gut and elsewhere. The response varies, and subjective hunger does not always follow the hormonal measurements neatly. An athlete can finish a demanding session having spent a large amount of energy and still have little desire to replace it immediately. [9]

That mismatch matters when it becomes habitual. The International Olympic Committee's work on Relative Energy Deficiency in Sport describes the consequences of prolonged or severe low energy availability, when exercise expenditure repeatedly leaves too little dietary energy for normal physiological functions. The effects can reach metabolism, bone, reproductive health, immunity, glycogen synthesis, psychological functioning and performance in both women and men. Restriction intended to produce a lighter or leaner athlete can eventually interfere with the body expected to perform. [10]

Food also carries its own reward system. Exercise experienced as unpleasant or punitive has been associated in some studies with greater compensatory eating and with using energy-dense food as a reward afterward. The psychology is familiar even without a laboratory: a workout treated as repayment for eating can make the next meal part of the same accounting exercise.

Mindful eating offers something less dramatic than perfect appetite control. A 2026 systematic review and meta-analysis found that mindfulness and mindful-eating interventions produced a modest reduction in food intake in controlled studies but no significant overall change in reported hunger or fullness. The useful skill may therefore be attention rather than appetite suppression: noticing hunger, satisfaction, habit and reward without assuming that every urge requires obedience or that every meal requires negotiation. [11]

The few grams of glucose circulating in the blood still have to be maintained. Glycogen still has to be rebuilt after it is spent. Sweat still needs replacing, protein still has to arrive often enough for tissue to use it, and tomorrow's session still depends partly on what was eaten today. None of those processes requires food to become a permanent performance test. On most days, the job is considerably more ordinary: eat enough, let the meal digest, train, drink what was lost and arrive at the next session ready to work.