The pro is at the start line of a 40-kilometre TT. Espresso at the cafe an hour and forty-five minutes ago. Bicarbonate capsule popped at the registration tent twenty minutes ago. He's done the math: caffeine plus bicarb, the legal stack.

The gun goes. By the 5-kilometre mark his stomach is sour. By 15 km, the caffeine has peaked and is now declining. By 35 km, neither effect is doing what the math promised. He finishes inside his planned time but well off what the stack was supposed to deliver.

The math wasn't wrong. The timing was.

This is the most-misused legal ergogenic stack in cycling: two of the four supplements the IOC's expert panel rates as having adequate evidence for performance benefit, deployed wrong. Here's what each actually does, the protocol that gets you the additive effect, and the practising-in-training rule that prevents race-day blow-up.

Section 01What caffeine does, and the 45-minute window

Caffeine's primary mechanism in exercise performance is central, not peripheral. It antagonises adenosine receptors in the CNS, which reduces perceived effort at a given workload, improves vigilance, and delays the recruitment of perceived fatigue (Spriet 2014).

The IOC consensus on dietary supplements rates caffeine as having Group A evidence — adequate support for performance benefits (Maughan et al. 2018). The performance benefit at trained-cyclist TT distances (5–40 km) is reliably ~1.5–3%.

The critical, under-talked-about variable is timing. Plasma caffeine concentration peaks at 45–60 minutes after oral ingestion. By 90 minutes, it's declining. By 2–3 hours, you've lost a meaningful fraction of the effect.

The most common amateur protocol — coffee with breakfast at 6 AM, race starts at 9 AM — produces a caffeine concentration profile that's well past peak by the time the race actually matters. The neurological effect is real but blunted. You paid for the supplement and missed the window.

The dose response is also non-linear. Low doses (≤3 mg/kg, ~200 mg for a 70 kg rider) produce the ergogenic effect with minimal side effects — no jitters, no GI distress, no rebound fatigue. Higher doses (6–9 mg/kg) don't reliably produce a larger effect but do increase the risk of nausea, anxiety, and insomnia, particularly if the race is in the afternoon (Maughan et al. 2018).

Section 02What sodium bicarbonate does, and why it suits longer efforts

Bicarbonate works on the opposite side of the physiology. It raises extracellular pH (mild metabolic alkalosis), which steepens the H+ gradient across the muscle membrane. The result is faster H+ efflux out of working muscle, which delays the intracellular acidosis that contributes to fatigue in high-intensity efforts.

Grgic and colleagues conducted an umbrella review — a review of meta-analyses — of bicarbonate supplementation across 8 meta-analyses (Grgic et al. 2021). Using the GRADE framework, the evidence rated moderate quality for bicarbonate improving peak and mean power in Wingate-style tests and Yo-Yo intermittent recovery performance. For general endurance mean power, repeated-sprint ability, and muscle strength, no significant benefit vs placebo.

Translated: bicarbonate works best for efforts where intracellular H+ accumulation becomes the limiter — supra-threshold intervals, repeated sprints, end-of-race surges. For steady-state riding under LT2, the benefit is small.

The dose is 0.2–0.3 g/kg body mass (~17–21 g for a 70 kg rider), ingested 90–120 minutes before the event for plasma bicarbonate to peak. Timing window is wider than caffeine's but the GI side effects are notorious — cramping, nausea, and acute diarrhoea in untrained users. Split-dose protocols (4 × 0.075 g/kg over 60 minutes) and enteric-coated capsules reduce GI distress meaningfully.

Thestack — done right

Caffeine and bicarbonate work on independent mechanisms (CNS adenosine antagonism vs extracellular H+ buffering). When both are timed correctly, the effects are roughly additive. The combined benefit on a 40 km TT can reach ~2.4% off baseline — not earth-shattering, but in a TT where 30 seconds is the difference between podium and pack, additive percent gains compound.

The timing protocol that actually works:

• T−2 hours: Sodium bicarbonate 0.3 g/kg, split into 4 doses over the next 60 minutes, with small amounts of water (not a single bolus on an empty stomach). Enteric-coated capsules preferred if available.
• T−1 hour: Last bicarbonate dose complete. Light carbohydrate intake (banana, sports drink) to settle stomach.
• T−45 minutes: Caffeine, 3 mg/kg (~200 mg for a 70 kg rider). Coffee, espresso, or pre-workout supplement, ideally consumed in 5–10 minutes rather than sipped slowly.
• T−15 minutes: Final warm-up, no further intake.
• T = 0: Race starts. Caffeine peaking. Plasma bicarbonate elevated. Both ergogenic effects converge on the start gun.

Miss the windows on either side and the math breaks. Sip the coffee over 90 minutes: caffeine never peaks above the threshold dose. Take the bicarbonate as a single bolus at start: GI distress steals the buffering benefit. Take the bicarbonate without timing the caffeine: independent partial effects rather than the stack.

Practisingin training first — not optional

This is the rule that separates riders who get the stack effect from riders who get the stack disaster.

Caffeine. ~30% of athletes are caffeine non-responders — their CYP1A2 enzyme genotype clears caffeine fast enough that the ergogenic window is much shorter, or the effect is meaningfully smaller. If you fall into that group, the stack is just bicarbonate plus discomfort. The test is simple: do two matched TT efforts on a trainer a week apart, one with placebo, one with 3 mg/kg caffeine. Compare. If the gap is < 1.5%, you're likely a non-responder.

Bicarbonate. GI tolerance is highly individual. Untrained bicarbonate users get cramping in the first 10–20 minutes of effort. The dose-response trial: take 0.3 g/kg bicarbonate split-dose 90 minutes before a hard trainer session, and see whether you finish the session with stomach intact. If you don't, drop to 0.2 g/kg, or switch to enteric-coated capsules, or skip bicarbonate entirely.

Do this work in training, three to five times across separate sessions, before you ever try the stack on race day. The IOC consensus is explicit: never debut a supplement protocol at a target event.

Section 05Bottom line

Caffeine works. Bicarbonate works. Stacking them gives roughly additive effects when the timing windows align. Most amateur cyclists deploy both but get the timing wrong, the dose wrong, or the practice-first wrong — and conclude that the supplements don't work.

They work. You're missing the windows.

T−2 hours: bicarbonate split-dose. T−45 minutes: caffeine, 3 mg/kg. Practise both, separately and together, in training before race day. Test for non-responder status on caffeine and GI tolerance on bicarbonate. Then bring them to a target event.