Fuel like you train.

Most triathletes spend months training their legs, lungs and heart. Then they hope their stomach shows up on race day. It rarely works that way. This guide covers what the research says about carbs, water and salt, and how we build it into every MetrIQ plan.

Your gut adapts to fuel the same way your muscles adapt to training: gradually, over weeks.

An untrained gut often tolerates only around 30 to 45 grams of carbohydrate per hour. A trained gut can handle far more. The adaptation takes roughly 6 to 8 weeks of regular practice. The method is simple:

• Fuel only on sessions longer than an hour. Shorter sessions don't need it.
• Start low, where your gut is comfortable.
• Add a little each week, about 5 to 10 grams per hour, on your long sessions.
• Hold steady if your stomach complains. Never force the next step.

Running is much harder on the gut than cycling. The jolting, and reduced blood flow to the gut at higher effort, both play a part. That is why experienced triathletes take in most of their fuel on the bike, and why your MetrIQ run targets sit below your bike targets.

Above about 60 grams per hour, your fuel should mix glucose and fructose, because each uses its own doorway into the body.

Think of your gut as having two separate channels. The glucose channel fills up at roughly 60 grams per hour, and anything extra sits in your stomach, which is where cramps and bloating start. Add fructose and you open the second channel, so you can absorb more fuel with less distress.

When does this matter? For most sessions it doesn't. It matters on long sessions and races, roughly 2.5 hours and up, where you are aiming for more than 60 grams per hour. In one well-known study, cyclists finished a time trial about 8% faster on glucose plus fructose than on glucose alone. Many sports drinks and gels already contain both. Check the label for glucose (or maltodextrin) and fructose.

What you eat during a long session doesn't just help you finish strong. It shapes how you feel in your next one. Fuel during the session means less to refill afterward. After a long or hard session, two things need restoring, and they run on different clocks:

• Carbohydrate (glycogen): early. Your muscles refill fastest in the first few hours. If your next hard session is less than about 8 hours away, start eating carbs soon after you finish and keep going for the next 4 hours. With a full day before your next session, what you eat across the whole day matters more than the first hour.
• Protein: across the day. Your muscles stay responsive to protein for about 24 hours after training. Aim for about 15 to 25 grams after the session, then every 3 to 5 hours at meals.

Skip recovery fuel and you start the next session already behind. That flat, sluggish feeling is often an empty tank, not a lack of fitness.

Drinking far more than you sweat can dilute the sodium in your blood, and in long races that can be dangerous, even fatal.

The condition is called exercise-associated hyponatremia. It is not rare. One review found it in about 11% of Ironman finishers tested. Its main cause is not salt loss but drinking too much: taking in more fluid than you lose through sweat and urine. In 2015, a triathlete died from brain swelling linked to hyponatremia at the Frankfurt Ironman European Championship.

Who is most at risk:

• Athletes on course for a long time, especially beyond 4 hours.
• Recreational athletes, who sweat less than elites but often have more chances, and more belief in the need, to drink.
• Smaller athletes, and women, who on average have lower sweat rates.
• Anyone told to "stay ahead of thirst" or to drink at every aid station no matter what.

Steady sips, don't force it: drink regularly enough that you don't fall far behind, but never more than your body is asking for.

You don't want to lose too much fluid either. Losing more than about 2% of your body weight to sweat can hurt performance, especially in heat. The goal is the middle ground:

• Before: drink normally through the morning. Aim for pale-yellow urine before you start. If it's already pale, you've had enough.
• During: keep a bottle within reach and take a few sips every 15 to 20 minutes alongside your fuel. On hot days, start early and use a drink with electrolytes.
• Salt: on sessions over about 2 hours, in heat, or if you see white salt marks on your clothes, use a drink that contains sodium.
• After: drink with your recovery meal and include something salty. If your next session is soon, drink a little more than you lost.

Want your personal number? Weigh yourself before and after a long session (no clothes, towel off) and note what you drank. Weight before, minus weight after, plus what you drank, equals your sweat loss. Divide by hours for your sweat rate. Once a season, on a race-rehearsal ride, is enough.

For races longer than about 90 minutes, eat more carbohydrate for the 1 to 2 days before while your training tapers. That is all carb loading is.

• No depletion phase. The old method of emptying your tank first is outdated. Trained athletes can fill their muscles in about a day of high-carb eating and rest.
• How much: the guideline is 10 to 12 grams of carbohydrate per kilogram of body weight per day, for 36 to 48 hours. Most of it should come from easy carbs like rice, pasta, bread, potatoes and sports drinks.
• Go low fiber and low fat. Both fill you up and can upset your stomach on race day.
• Women: eat more in total, not just more carbs. Carb loading works for women when total energy goes up too.
• Expect the scale to rise 1 to 2 kg. Stored carbohydrate holds water. That is fuel, not fat.

Shorter races, like most sprint triathlons, don't need a full load. Eat well the day before and have a good breakfast. On race morning, have a carb-rich, low-fiber breakfast 1 to 4 hours before the start. Then use the fuel and drinks you have practiced. Nothing new on race day.