The #1 Nutrition Mistake I See Every Week

Across the many thousands of athletes around the world, there is one nutrition mistake that appears more consistently than any other: underestimating hydration and sodium needs.

It doesn’t matter whether the athlete is preparing for their first half-marathon or training for ultradistance events — the pattern is the same. Fuelling tends to get all the attention, while hydration is usually neglected.  

Hydration is not optional for performance. It is one of the most powerful drivers of performance, recovery, and how you feel during training — and it is also one of the most common things people get wrong.

Underestimating Hydration and Sodium Needs

Hydration is one of the most influential and yet most misunderstood aspects of endurance fuelling. It has a direct impact on performance, digestion, concentration, cardiovascular load, muscle function and recovery.

The effects of dehydration begin at around a 2% loss of body weight, which is only 1.3 L of sweat for a 65 kg athlete. In warm or humid conditions, it’s entirely possible to lose that much through sweat in a single hour. Once you cross that threshold, performance and digestion will decline rapidly because the body is no longer able to maintain the internal environment required for sustained endurance output.

As dehydration develops, several physiological changes occur at the same time. Your heart rate begins to drift upward, meaning the same pace requires more effort. Perceived exertion increases even if your pace doesn’t change. Muscle strength and neuromuscular control drop, making hills, surges and technical terrain feel harder. Many athletes also experience headaches, cramps, and a notable decline in decision-making and mental sharpness.  It feels like low energy, but it’s often low hydration.

A common misconception is that hydration means drinking water. In reality, hydration is a balance between fluids and electrolytes, particularly sodium, which is lost in significant amounts through sweat.

Sodium concentration in the body is tightly regulated. If sodium levels rise too high — for example, if you consume a salty meal — your body retains water to dilute it, leading to bloating and thirst. If sodium levels drop — such as when you drink large amounts of plain water during and after sweating — your body excretes the excess water to restore the correct sodium concentration in your body. You end up with very pale urine and frequent urination, but despite all that drinking, you can remain functionally dehydrated because you haven’t replaced the sodium you lost in your sweat that means you can retain the water you’re drinking.  Without sodium, the fluid simply won’t be retained. 

Digestion and energy absorption also becomes impaired with poor hydration, which is a major issue for athletes trying to fuel. Without adequate hydration, food doesn’t pass through your stomach nearly as quickly, and the gut struggles to absorb both fluid and energy, which increases the likelihood of nausea, sloshing, and gastrointestinal discomfort.

Here’s the other common misconception that most athletes don’t realise: drinking an electrolyte drink doesn’t guarantee you’re getting enough sodium. Many popular electrolyte products contain far too little sodium to replace what is lost through sweat, particularly for salty sweaters or those training in the heat. Athletes often assume “I had electrolytes, so I’m covered,” when in reality they may be replacing only a small fraction of their actual sodium losses.

This is one of the most common hydration misunderstandings I see — and one of the quickest ways an athlete can end up dehydrated without recognising why.  It usually takes far more sodium than you realise to stay hydrated, and even products like Salt Sticks have bugger all sodium in them.  

During and after training, rehydration must include both water and sodium at the same ratio that is lost in sweat, so the fluid can be properly absorbed, retained and used to restore blood volume. When hydration is handled well, digestion improves, blood flow to working muscles increases, and energy delivery becomes more efficient. Athletes who hydrate with purpose — matching their fluid and sodium intake to conditions, sweat rate and session duration — consistently experience better performance, more stable energy and significantly fewer issues with cramps, nausea and mid-session fatigue.

Neglecting hydration isn’t a small oversight; it’s one of the most common reasons racing goes wrong and why sessions feel harder than they should. And in many cases, improving hydration alone is enough to completely transform how an athlete feels and performs.