A client of ours — a surgeon who trains for the Solana Beach Triathlon every June — came into a Saturday session in June describing what happened on a 2-hour bluff-trail run near Torrey Pines: strong for 75 minutes, then a wall. Legs went heavy, pace fell apart, and the last 20 minutes felt like wading through sand. He’d eaten a solid breakfast and drank water the whole way. What he hadn’t done was fuel during the run. That’s the gap intra-workout nutrition closes, and it’s one of the most under-coached pieces of endurance programming we see among busy professionals who train hard but treat fueling as an afterthought.
What Intra-Workout Nutrition Actually Means
Intra-workout nutrition refers to what you consume during a training session — as opposed to pre-workout (fueling up beforehand) or post-workout (recovery). It’s not relevant for every workout. A 40-minute lifting session or a 5K tempo run typically runs fine on the glycogen you walked in with. The threshold where it starts to matter is roughly 60-75 minutes of continuous, moderate-to-hard effort.
Below that window, your muscle and liver glycogen stores — roughly 400-500 grams combined in a well-fed adult — cover the fuel demand without supplementation. Past that window, especially at intensities above 65% of VO2 max, glycogen depletion becomes the limiting factor before cardiovascular fitness does.
This is why the conversation looks different for a client doing 30-40 minute HIIT sessions versus one training for a half-marathon or a century ride. The HIIT client rarely needs intra-workout carbohydrate. The endurance client, especially one stacking a long ride on Saturday and a long run on Sunday, needs a deliberate plan or performance falls off a cliff in the back half of the session.
The practical marker we use with clients: if the session is under an hour, water and pre-workout fueling are enough. If it’s over 90 minutes, plan intra-workout carbohydrate from the start. Between 60 and 90 minutes, it depends on intensity, heat, and how depleted you started.
The Physiology Behind the Wall
Glycogen depletion doesn’t happen evenly. As muscle glycogen drops below roughly 300mmol/kg dry weight, working muscles increasingly rely on fat oxidation, which is slower to mobilize and can’t match the ATP turnover rate that higher-intensity work demands. The result is the pace collapse endurance athletes call bonking — sudden, not gradual.
Dehydration compounds the problem independently. Losing as little as 2% of body weight in fluid reduces plasma volume, which forces the heart to work harder to deliver the same amount of oxygen. Combine a depleted glycogen state with a 2-3% fluid deficit, which is common on a warm North County afternoon, and performance decline accelerates well before either issue would cause trouble on its own.
Sweat rate varies enormously between individuals — from under 0.5 liters per hour to over 2.5 liters per hour depending on genetics, heat acclimatization, and fitness level. A generic “drink when thirsty” rule undershoots for heavy sweaters and works fine for light sweaters, which is why we have clients do a simple pre/post weigh-in on a representative training session before we set their protocol.
The takeaway for programming purposes: fueling and hydration aren’t separate problems layered onto training. They’re part of the training stimulus itself. A periodized approach to what you eat around sessions — not just during them — makes the intra-workout piece more effective, which is a big part of why we built out our periodized nutrition framework around training phases rather than a single fixed diet.
Carbohydrate: How Much, What Kind, When
The research consensus, reflected in the ACSM’s joint position stand on nutrition and athletic performance, sets carbohydrate intake during exercise at roughly 30-60 grams per hour for sessions up to 2.5 hours. Past 2.5 hours, well-trained athletes can push to 80-90 grams per hour, but only using a glucose-fructose blend rather than glucose alone.
The reason for the blend matters. A single carbohydrate source like glucose is absorbed through one intestinal transporter (SGLT1), which saturates at around 60 grams per hour regardless of how much more you consume. Fructose uses a separate transporter (GLUT5), so combining the two roughly doubles the total absorption rate — this is the mechanism behind Jeukendrup’s research on multiple transportable carbohydrates and why most modern sports drinks and gels use a 2:1 glucose-to-fructose ratio.
In practice, that means:
- Sessions 60-90 minutes: 20-30g/hour, primarily as a start-of-fatigue buffer, not a full fuel replacement
- Sessions 90 minutes-2.5 hours: 30-60g/hour, split into 15-20g doses every 15-20 minutes
- Sessions over 2.5 hours: 60-90g/hour using a 2:1 glucose-fructose source, introduced gradually
Timing the intake in small, regular doses rather than one large bolus every hour reduces GI distress and keeps blood glucose steadier. We coach clients to set a recurring watch alert every 15 minutes rather than relying on feel, because by the time you feel like you need carbohydrate, you’re already behind the curve.
Hydration and Electrolyte Strategy
Fluid needs during exercise scale with sweat rate, not with a blanket 16-20oz/hour recommendation, though that’s a reasonable default absent better data. To individualize it, weigh yourself immediately before and after a one-hour training session at the intensity and conditions you’re targeting, without drinking anything mid-session. Each pound lost equals roughly 16oz of sweat loss, which becomes your hourly fluid target going forward.
Sodium is the electrolyte that matters most during exercise, since it’s lost in the greatest quantity and plays the largest role in fluid retention and neuromuscular function. Sweat sodium concentration varies from about 200mg to over 1,500mg per liter between individuals — a five-to-sevenfold range that makes generic electrolyte tablet dosing unreliable for a meaningful share of athletes.
For most clients training in Del Mar’s marine-layer mornings, 300-500mg of sodium per hour during sessions over 90 minutes is a reasonable starting range. For clients who finish sessions with visible white salt rings on their shirts — a sign of heavy sodium loss — we push that to 700-1,000mg/hour and reassess.
Potassium and magnesium losses are smaller in absolute terms and are typically well covered by a normal diet plus the small amounts included in most electrolyte products. Chasing precise potassium replacement during exercise is rarely worth the complexity for anyone outside elite endurance racing.
Building Sessions by Workout Type
Fueling protocols aren’t one-size-fits-all across training modalities. A cyclist grinding a 3-hour ride to Rancho Santa Fe and back has different absorption conditions — lower digestive stress from the seated, lower-impact position — than a trail runner covering the same duration on the Torrey Pines bluffs, where GI tolerance drops due to impact and blood flow redirection away from the gut.
For cycling sessions over 2 hours: liquid carbohydrate (sports drink or diluted juice) works well since hydration and fueling combine into one intake stream, reducing the number of separate decisions during the ride.
For running sessions over 90 minutes: solid or semi-solid options (gels, chews, dates) paired with separate water intake tend to sit better, since large volumes of concentrated liquid carbohydrate can cause sloshing and GI discomfort at higher impact.
For long strength or hybrid sessions — the kind we program for clients working through our strength-for-runners protocols — intra-workout fueling matters less because total session duration at elevated heart rate is usually under an hour, even if the full session runs 75-90 minutes with rest periods included.
The common thread: match the fuel format to what the activity’s movement pattern and digestive demand allow, then keep the format consistent in training so race day or event day isn’t the first time you’ve used it.
Common Mistakes We See in Del Mar Athletes
The most frequent error is waiting too long to start fueling. Clients often wait until they feel depleted before taking in carbohydrate, but by that point blood glucose has already dropped and the recovery lag means 15-20 minutes of continued poor performance even after fueling resumes. Start intra-workout fueling at the 30-45 minute mark for any session expected to run past 90 minutes, not when symptoms appear.
The second is inconsistent hydration in variable coastal conditions. A foggy 6am start along Coast Highway can feel deceptively mild, but the marine layer often burns off by mid-morning, and clients who under-hydrate in the first hour end up fighting a deficit they can’t fully correct in the second.
The third is over-reliance on thirst as a signal. Thirst perception lags actual fluid deficit, particularly during high-intensity efforts where blood is redirected away from areas involved in thirst signaling. By the time you’re thirsty, you’re typically already at a 1-2% fluid deficit.
The fourth, and probably the most avoidable, is testing a new fueling product for the first time on race day or during a benchmark session. GI distress from an unfamiliar gel or drink mix can undo weeks of training in a single afternoon. This connects directly to why we treat fueling like any other trained variable — the same logic that governs how we structure a 90-day benchmark testing cycle applies to nutrition: test what you’ll actually use, under conditions that mimic the real event.
Gut Training: Making the Protocol Tolerable
The gut is trainable. Research on repeated carbohydrate feeding during exercise shows that intestinal carbohydrate absorption capacity improves with consistent exposure, largely through upregulation of the SGLT1 and GLUT5 transporters discussed earlier, along with improved gastric emptying rates.
The practical protocol we use with clients building toward 60-90g/hour intake: start two long sessions per week at your current comfortable intake, typically 15-20g/hour for someone who’s never structured intra-workout fueling before. Increase total hourly intake by 5-10 grams every one to two weeks, always during a session that matches your target race intensity, not an easy recovery day.
Expect some GI discomfort in the early weeks — mild bloating or a heavier stomach feeling is normal and typically resolves within 10-14 days as tolerance builds. Sharp cramping, nausea, or diarrhea are signals to back off the progression rate, not push through it.
This buildup should run parallel to, not separate from, your broader training progression. The same incremental logic that governs a well-built strength program — small, trackable increases rather than large jumps — applies directly here, similar to the approach we outline in our piece on micro-progressions for incremental training gains. Six weeks of consistent, gradual increases produces a gut that can handle race-day fueling without drama.
Whole Food vs. Commercial Products
Commercial gels and drink mixes exist for convenience and consistent dosing, not because they’re metabolically superior to whole food. A medjool date provides about 18 grams of carbohydrate in roughly the same glucose-fructose ratio found in most engineered products, at a fraction of the cost over a training season.
Rice cakes with a thin layer of honey, small portions of dried mango, and diluted fruit juice (roughly one part juice to one part water, which approximates most sports drink carbohydrate concentrations) all work as legitimate intra-workout fuel sources. The main advantage commercial products offer is portability and precise, labeled dosing, which matters more for athletes managing very high carbohydrate targets above 75g/hour.
For most recreational and competitive-amateur endurance athletes training 3-6 hours per week, whole food options perform equivalently in controlled studies comparing glycogen resynthesis and performance outcomes. The deciding factor should be what you tolerate well and what you’ll actually carry and eat consistently, not what a product label promises.
One caveat: concentrated whole foods like dates carry more fiber than engineered gels, which can slow gastric emptying and cause discomfort at high running intensities for some athletes. If that’s you, a hybrid approach — whole food for lower-intensity long rides, commercial gels for high-intensity running efforts — tends to work best.
Putting the Protocol Into a Training Cycle
Intra-workout nutrition shouldn’t be treated as a race-week decision. It’s a trained skill that belongs inside your periodized plan alongside volume and intensity progressions. We typically introduce structured intra-workout fueling in the base-building phase of a program, six to eight weeks before a target event, giving enough time for the gradual gut-training progression described above.
During higher-intensity training blocks, fueling protocols should be rehearsed at race pace, not just during easy long sessions — GI tolerance and absorption behave differently under load. And in deload weeks, when session duration and intensity drop, it’s a good window to dial back and reassess whether your current fueling amounts still match your training demand, since needs shift as fitness and heat acclimatization improve.
By the time race day or a benchmark event arrives, nothing about the fueling plan should be new — not the product, not the timing, not the dose. That’s the difference between the client who finishes strong on the last climb out of Torrey Pines and the one who’s counting steps to the finish.
If you’re training for an endurance event this fall and haven’t built a fueling strategy into your program, that’s worth a conversation before your next long session, not after a rough one. Book a free assessment with our team and we’ll build the intra-workout protocol into your existing plan, whether you’re working with us one-on-one or in a semi-private group.
