5 Things Your Endurance Fuel Must Have (And Most Products Don't)

This is the single most important formulation variable in modern endurance fueling, and most mainstream products still get it wrong.

When you consume carbohydrates during exercise, they are absorbed through protein channels in the small intestine called transporters. The transporter responsible for glucose and maltodextrin (SGLT1) saturates at approximately 60 grams per hour. That is the ceiling. Consume more glucose or maltodextrin beyond this point and your intestine cannot absorb it faster, regardless of how much you take in. The unabsorbed carbohydrate accumulates in the gut, draws water in osmotically, and becomes the reason so many athletes experience bloating, cramping, and GI distress during long events.

Fructose uses a completely separate transporter, GLUT5, with its own independent absorption capacity of roughly 30 grams per hour.

Combining maltodextrin with fructose at a 2:1 ratio allows both transporters to work simultaneously, raising total carbohydrate absorption from 60 grams per hour to 90 grams or more. Jentjens and Jeukendrup (2005) measured this directly: a combined maltodextrin-fructose formula produced a 26% increase in exogenous carbohydrate oxidation compared to maltodextrin alone. Currell and Jeukendrup (2008) showed an 8% improvement in time trial performance from the same formulation change, at the same total carbohydrate dose. Read more in our dual-carb science deep dive.

What to look for: Maltodextrin and fructose listed as separate ingredients, ideally at a 2:1 ratio. If the only carbohydrate source listed is glucose, dextrose, sucrose, or maltodextrin alone, the product has a hard ceiling that will limit you on efforts over 60 minutes.

Why it matters in practice: For marathon runners, triathletes, HYROX athletes finishing in 90-plus minutes, or cyclists on 3-plus-hour rides, this is not a marginal gain. It is the difference between your gut keeping pace with your muscles and falling behind at precisely the moment you need it most.

Ask most athletes how much sodium they lose in an hour of exercise and they will guess somewhere between 100 and 300 milligrams. The reality: trained athletes lose between 400 and 2,000 milligrams of sodium per litre of sweat (Baker, 2017). The average sits around 800 to 1,000 milligrams per litre. For the full breakdown, see our guide on how much sodium endurance athletes actually need.

Most mainstream sports drinks contain 100 to 230 milligrams of sodium per 500 millilitres. At a sweat rate of one litre per hour, that replaces roughly 10 to 25 percent of actual sodium losses.

Why does this matter? Sodium is the primary osmole that regulates plasma volume. When sodium concentration in the blood drops, the kidneys excrete water, plasma volume falls, and cardiac output at the same workload increases. Heart rate climbs, oxygen delivery to working muscles drops, and performance degrades. Sodium also drives the thirst response. Drink only plain water and replace no sodium, and blood osmolality falls, suppressing thirst at the exact moment you most need to be drinking.

What to look for: Sodium content listed in milligrams per serve, ideally 300 milligrams or more per 500 millilitres. For athletes who see visible white salt rings on their kit after training, or who experience late-race cramping, the target should be at the higher end.

Before the session matters too. One often-overlooked application of sodium is the pre-session window. Consuming a high-sodium product 25 to 40 minutes before a hard session (a VO2 max block, a threshold run, a tempo block, or a HYROX simulation) expands plasma volume going into the effort. HYDRAX PRIMER delivers 1,313 milligrams of sodium per serve specifically for this purpose: priming the body before it starts losing fluid, not just replacing what is already gone.

Osmolality describes the concentration of dissolved particles in a solution. Blood plasma sits at approximately 285 to 295 mOsm/kg. A drink is isotonic when its osmolality matches this range.

When you drink a hypertonic solution (one more concentrated than blood plasma), your intestine must first draw fluid from the bloodstream to dilute the drink before absorption can proceed efficiently. This is the mechanism behind gel-induced GI distress when athletes forget to chase gels with water (covered in detail in our drink mix vs gels comparison). The gut is temporarily pulling fluid away from circulation at a moment when you need it there.

An isotonic solution sits in the ideal window for gastric emptying and carbohydrate absorption. Research comparing isotonic and hypertonic sports drinks consistently shows isotonic formulations empty the stomach faster and begin delivering carbohydrates and electrolytes to the bloodstream sooner (Shi et al., 2000).

What to look for: A product that specifies an isotonic formulation at its recommended mixing ratio. Do not mix your drink stronger than recommended. Adding extra scoops increases osmolality, slows gastric emptying, and increases GI distress risk.

Caffeine is one of the most well-supported ergogenic aids in sport. Research supports 3 to 6 milligrams per kilogram of bodyweight, taken 45 to 60 minutes before exercise, as the optimal protocol for most athletes (Ganio et al., 2009). That dose, that timing window, and your individual caffeine sensitivity are highly personal variables.

When caffeine is built into your fuel, you lose control of timing, dose, stacking, and late-race use. A 65kg athlete and a 95kg athlete have very different optimal doses. If you also have a coffee before a race or take a caffeine gel at a specific moment, pre-caffeinated fuel makes your total dose unpredictable.

A stimulant-free fuel lets you design your caffeine strategy independently, at the dose and timing that your specific physiology, body weight, event length, and race day plan require.

What to look for: Caffeine-free fuel as your baseline. Layer in caffeine through a separate, precisely timed source when and if your protocol calls for it.

The fifth criterion is not on any label. It is a principle: the best endurance fuel for you is one your gut can handle at race pace, under race conditions, at race intake rates.

Research on gut adaptation shows that the intestine's capacity to absorb carbohydrates at high rates is trainable. Athletes who practice consuming 60 to 90 grams of carbohydrate per hour in training, consistently over weeks, develop measurably higher exogenous carbohydrate oxidation rates than untrained athletes consuming the same dose (Cox et al., 2010).

This has two practical implications. Never use race day as your first trial of any fuel product. And your fueling volume and timing should be practiced in training with the same product, at the same concentration, at the same intensity, as you plan to use in competition.

What to look for: Ingredients with a research evidence base for GI tolerance in athletes, a dual-carb formulation that lowers individual transporter saturation risk, and a mixing concentration designed for isotonic delivery. Then test it in training.

A fuel that meets all five criteria will have a dual-carbohydrate source at a 2:1 maltodextrin-to-fructose ratio, a sodium content that meaningfully replaces sweat losses, an isotonic formulation at its recommended mixing ratio, no built-in stimulants, and genuine gut tolerability built from athlete testing.

HYDRAX CORE is formulated to meet all five of these criteria. HYDRAX PRIMER meets them as a pre-session formula: the same dual-carb and high-sodium principles, delivered 25 to 40 minutes before the session starts, so you begin every hard effort already primed.