Why Triathletes Underfuel — and How to Fix It

By Henry R. Shoemaker

Under-fueling in endurance sports is probably far more common than you realize. I'm seeing a couple of faint signs it may be happening even among our own athletes as training volume increases for our late-summer and fall races, so I want to tackle it now.

The way it usually unfolds: an athlete doesn't quite eat enough whole food day to day, and then doesn't plan to fuel well or supplement before or during sessions. So you deepen the hole and start the next day already behind. It rarely looks dramatic. It looks like flat legs, slow recovery, and progress that stalls for no immediately obvious reason. You have to recognize the problem, take control of your fueling, and plan ahead to break that cascade.

As a team, S.E.T. athletes are a grop that genuinely care about what goes in their bodies. They experiemnt with homemade protein drinks, the rice balls and carb "balls". Some of our athletes go so far as to extract their own almond milk. This is exactly the right instinct — and it's the kind of portable, real-food fueling the sports-nutrition world recommends (Lim & Thomas, Feed Zone Portables). This article is about pointing that care where it helps athlete's training the most.

How much should triathletes eat? Start with total fuel

Before macros, the real question is whether you're eating enough energy overall to support your training. When you're not — and you have low energy availability — recovery, hormonal health, and bone health degrade first, often before performance visibly drops (Mountjoy et al., IOC RED-S consensus). This isn't usually a discipline problem, it's an awareness issue — but once you know about it, solving it takes just as much discipline as getting your workouts done.

Appetite lags training load, which is why so many of you can't face breakfast before an early session. If that's you, don't force whole foods where you can't — drinking your macros is an option. A carb mix or a shake in your bottle is often the easiest first fuel of the day, and liquid carbohydrate empties from the stomach faster and, for many athletes, sits better than solid food when your stomach isn't ready. Getting something in early is what keeps the hole from opening.

Carbohydrate: the fuel that governs your training

A great question came up recently: does the carbohydrate emphasis really hold given all the Z2 volume triathletes do? It does — and here's the part that surprises people: it holds whether or not you're fat adapted. Fat powers your easy aerobic minutes, and consistent training genuinely does sharpen how well you burn it. But fat oxidation has a ceiling — even highly fat-adapted athletes top out around 1.0–1.5 g/min, nowhere near the energy demand of hard work. So the moment intensity climbs, carbohydrate has to fill the gap. That's why your quality sessions run on carbohydrate and can't be done well depleted — no amount of Z2 volume changes that.

Even easy sessions draw glycogen, and long ones draw a lot: a couple of hours of steady Z2 work can burn through a meaningful share of your glycogen stores — often a third or more. Start that session underfueled or skip the refill after, and that's the hole the cascade digs.

So carbohydrate is the fuel most likely to limit your training and most often under-replaced. The practical answer is to bookend your sessions: start them topped up, take carbohydrate on during the longer ones, and refill soon after — especially when your next hard session is inside a day. Refilling after is what makes tomorrow start full instead of already in the hole. That's how the cascade breaks.

Match it to the day rather than eating flat: roughly 3–5 g/kg on rest and light days, 5–7 on normal training days, 7–8+ on long or hard days (Burke et al.). The common pitfall is eating the same modest amount every day, so the big days run on empty.

Carbs taken during a session aren't a separate budget — they're daily carbohydrate you're eating while moving.

This is also where in-workout fueling belongs. On shorter easy sessions, fueling mid-workout is lower priority if the rest of your day is handled — but when in doubt, fuel it; it's cheap insurance against the cascade. On anything longer (roughly 60 minutes or more), take it in: 30–60 g/hr for sessions of one to two-and-a-half hours, building toward 60–90 g/hr on long half- and full-distance-specific work, using mixed carb sources (glucose plus fructose) your gut has practiced. This is also where sprint-to-Ironman comes together: the daily foundation and fueling your quality work matter equally at every distance — the only real difference is that the longer your sessions get, the more in-workout fueling moves from optional to non-negotiable. And if eating on the move is hard, this is the easiest win of all: drink mix and bars go down when food won't, and they train your gut for race day.

Protein needs for triathletes

Protein is having a moment, and while a lot of the noise may seem overblown, there's a real signal underneath — and endurance athletes are included in it, not just the lifters. It repairs tissue and turns training into adaptation. Somewhere around 1.4–2.0 g/kg/day, spread across your meals rather than stacked at dinner, with some after key sessions, covers it (Phillips & Van Loon).

Fat: the floor you don't crash

Fat carries fat-soluble vitamins and supports hormonal health, and it fuels the easy end of your training that carbohydrate doesn't — so it isn't competing with carbs, it's doing a different job. The failure mode is "eating clean" by cutting carbs and fat at the same time, which quietly tanks energy availability and, over time, hormonal and bone health. Keep it around 1 g/kg or ~20% of intake, and don't drive it lower solely for body composition or other purposes without expert guidance.

The bottom line

Don't dig a calorie and macro hole you can't climb out of. Stay ahead of it, fuel during your longer sessions so you finish able to recover rather than deeper in the red, and if eating is hard, use the liquid and portable options to keep the tank full. It's the cheapest performance and recovery gain available other than sleep, and most athletes leave it on the table.

Fuel today like your race-day performance depends on it — because it does.

A note on the numbers: all macro quantities here are starting ranges, not individual prescriptions — your size, sessions, and season shift them. They also come mostly from research on male athletes; sex, and for women the menstrual cycle, move both the targets and the risk of under-fueling, so individual calibration matters more than the numbers.

I coach fueling for performance — I'm not a registered dietitian. For medical nutrition needs or an individualized diet plan, consider a professional sports dietitian.

Henry R. Shoemaker is Co-Founder and Head Coach of Syndicate Endurance Team (S.E.T.). Henry is a USA Triathlon Level 2 Certified High Performance Coach with specialties in Long Course, Draft Legal, Paratriathlon, and U23, and has guided athletes to 100s of podiums including an age-group National Championship, World Championship qualifiers, and a number 5 team ranking. His approach is evidence-based and technology-driven, meeting every athlete — beginner to pro — where they are in their endurance journey.