Supplements are the marginal gains of nutrition — mostly noise, a few genuine signals. The industry runs on the same psychology as the aero upgrade aisle: you are training hard, results are slow, and here is a tub promising to do the adaptation for you. Roughly $50 billion a year changes hands on that promise, and the overwhelming majority of it buys nothing a blood test could detect.
But "mostly noise" is not "all noise". A short list of supplements has survived decades of independent, placebo-controlled research, and if you are a cyclist who now runs two or three times a week, a couple of them are worth more to you than they were when you only rode. Impact changes the equation — for your bones, for your iron, and for what your body needs to hold onto muscle as the years stack up.
So here is the full list of what has evidence, with doses, timing, and prices — and then the aisle you can walk straight past.
Caffeine: the one that embarrasses everything else
If every supplement had to justify itself in a lab, caffeine would be the last one standing. It is the most studied ergogenic aid in sport, the effect shows up across cycling, running, and virtually every endurance discipline tested, and it has held up for decades. Improvements in time-trial-type performance of 2-4% are typical in the research — which, for context, is super-shoe territory, from something that costs pennies.
The protocol is well established: 3-6mg per kilogram of body weight, taken 30-60 minutes before the effort. For a 75kg rider that is 225-450mg — and the lower end works. Recent research keeps finding that ~3mg/kg captures most of the benefit with fewer side effects than the heroic doses. A strong coffee runs 80-120mg, a caffeinated gel typically 75-100mg, and caffeine tablets cost about $10 for a bottle that lasts a season.
The mechanism is central, not muscular — caffeine blunts your brain's perception of effort by blocking adenosine receptors, so the same power or pace feels easier. That is why it works identically on the bike and in running shoes.
Two practical notes. Habitual coffee drinkers still get the benefit, so no need for miserable withdrawal weeks before a big event. And caffeine's half-life is around five hours, which means the 4pm gel before an evening interval run is still circulating at midnight. If your training happens after work, weigh the session gain against the sleep cost — sleep is the better drug. There is a longer treatment of the on-bike specifics in the caffeine piece.
Beetroot juice: borrowed from the physiology lab
Dietary nitrate is the most interesting one on the list, because it does something no other legal supplement does: it makes exercise cheaper. Andrew Jones's lab at the University of Exeter — he is known in the field, without much exaggeration, as Professor Beetroot — showed that supplementing nitrate reduces the oxygen cost of submaximal exercise by roughly 3-5%. Same pace, less oxygen. It is one of the few findings in sports nutrition that genuinely surprised physiologists, because oxygen cost for a given workload was long assumed to be fixed.
The pathway: nitrate converts to nitrite in your mouth (via the bacteria on your tongue — mouthwash before dosing actually kills the effect), then to nitric oxide, which improves muscle efficiency and blood flow, particularly in fast-twitch fibres and in low-oxygen conditions.
Here is where it gets interesting for you specifically: the evidence hints that nitrate may benefit running slightly more than cycling. Running carries a higher metabolic demand — more muscle mass working, more of it weight-bearing — and recruits more of the fibre types nitrate acts on. It also seems to help less-trained muscle more, and your running muscles are years behind your cycling ones. A supplement that underwhelms elite cyclists may do noticeably more for a cyclist's 5k.
Dose: 6-8 mmol of nitrate, which in practice means two concentrated beetroot shots (about $3-4 each) taken 2-3 hours before the effort, or daily for the several days leading into an event for the fuller effect. Caveat for honesty: in highly trained athletes the effect shrinks, sometimes to nothing — the fitter your aerobic system, the less headroom nitrate finds. For most amateurs running and riding around a job, there is headroom.
Creatine: no longer just for the gym
Ten years ago creatine in an endurance article would have been a mistake. The evidence has moved, and masters endurance athletes are exactly the population it moved for.
Creatine monohydrate is the most-studied sports supplement in existence — safety and efficacy both — but its classic benefit (more phosphocreatine for repeated short, hard efforts) was always a sprinter's prize. The case for the 40- or 50-year-old endurance athlete rests on newer ground: creatine supports muscle maintenance in exactly the population that loses muscle fastest, appears to support bone density when combined with impact or resistance training, and has a growing evidence base for cognitive function. For a masters cyclist-runner, that trio — muscle, bone, brain — reads like a list of the three things ageing attacks first.
The bone point deserves a highlight. Cyclists carry a well-documented bone density problem — non-impact sport, decades of it — which is half the reason you started running in the first place. Creatine is not a substitute for the impact loading that running provides, but alongside it, the combination is plausibly worth more than either alone. The full picture on cycling and bone is in the bone density article.
Dose: 3-5g per day, every day, timing irrelevant. Skip the loading phase — it just gets you there a couple of weeks faster with more GI complaints. Plain creatine monohydrate powder costs $20-30 for three to four months' supply; every fancier form is the same molecule with better packaging.
And the weight question, because every cyclist asks it: yes, you will gain roughly 1-2kg of water in the first weeks. It is intracellular water — inside the muscle, part of how creatine works — it stabilises, and it reverses if you stop. Unless you are at your race-week climbing weight, a kilo of water is a rounding error against better muscle retention through your forties and fifties. If a kilo genuinely decides your season, drop creatine in your final build; take it the rest of the year. We ran the on-bike numbers in the 30-day creatine experiment.
Carbohydrate and electrolytes: boring, decisive
Filed under supplements because it is sold in the same aisle, but really this is just fuelling — and it dwarfs everything else on this list. No supplement decision you make this year will matter a tenth as much as getting carbohydrate intake right for sessions over about 90 minutes.
The short version: 60-90g of carbohydrate per hour on long rides, trained upward over weeks; less in running, where the gut tolerates less bouncing cargo — most runners work in the 30-60g range, and your long runs at 40-60 minutes may need nothing beyond water. Sodium is the only electrolyte worth active attention, mostly on hot days and long days.
The dose-by-dose detail lives in the fuelling calculator and the running versus cycling fuelling comparison, so I will not repeat it here — but if you are buying beetroot shots while under-fuelling your long ride, you have the list upside down.
Vitamin D: the northern-latitude tax
Not ergogenic — corrective. But the deficiency it corrects is common enough in endurance athletes to earn a place here, and cyclists have a specific stake in it.
Your skin makes vitamin D from sunlight, and above roughly the 35th parallel the winter sun is too weak for it — Dublin, Manchester, Boston, Seattle, all of them. Add indoor trainer season, office hours, and sunscreen, and deficiency rates in athletes at northern latitudes routinely come back high in screening studies. Vitamin D sits on the pathway for calcium absorption and bone remodelling, plus muscle function and immune health.
For a cyclist, this lands on an existing weakness: bone. You are adding running partly because cycling gave your skeleton nothing to push back against for years. Running provides the stimulus, but bone-building runs on calcium and vitamin D as raw material — a deficient athlete is doing the loading without the materials, which is also a fair description of how stress fractures happen.
Get 25-hydroxyvitamin D tested if you can (often $30-50, sometimes covered). Through winter, 1,000-2,000 IU per day is the standard maintenance range and costs about $10 for months. More is not better — this one has a ceiling, and megadosing has gone wrong in studies.
Iron: test, never guess
Iron is where endurance athletes most often go wrong in both directions — deficient and fatigued because nobody tested, or supplementing blindly with something that is genuinely harmful in excess.
Running raises the stakes in a way most cyclists have never had to think about: foot-strike hemolysis. Every ground contact crushes red blood cells in the capillaries of your feet — mechanically destroys them — and over thousands of strides per run this measurably increases red cell turnover. Cyclists lose iron slowly through sweat and the gut; runners lose it through that plus a small demolition operation in each foot. Combine it with the low energy intake many cyclists flirt with, and iron status can slide within a season of adding running.
The symptoms — flat legs, fading power at the same heart rate, poor recovery, the general sense your fitness has quietly left — are also the symptoms of overtraining, poor sleep, and a dozen other things. That is exactly why you test rather than guess: a ferritin check (under about 35 µg/L is the usual flag for athletes) plus full blood count tells you the answer for the price of a couple of beet shots. Supplement only against a confirmed deficiency, ideally with medical guidance, because excess iron is not benign — it accumulates, and men have no route for offloading it. The masters-specific picture is in the iron deficiency piece.
The takeaway for a cyclist adding running: put ferritin on your annual bloods. That is the entire action.
What to skip
The list of things with evidence is short. The shelf is long. The difference is padding, and most of it falls into three groups.
BCAAs. Branched-chain amino acids were everywhere for a decade, and the verdict is now in: if you eat adequate protein — and at 1.6-2g per kilogram per day, an endurance athlete should — BCAAs add nothing. They are three amino acids sold separately from the twenty your food already delivers together. Whole protein wins; save the $40 a month.
Multi-ingredient "endurance blends." The tubs with fifteen ingredients, a name like a fighter jet, and a "proprietary matrix" on the label. The trick is consistent: real ingredients at doses too small to work, hidden inside a blend so you cannot check. If a product will not print per-ingredient doses, it has answered your question.
Anything with an extreme claim. Boosts VO2max 15%, burns fat while you sleep, "trains your mitochondria". The genuinely effective supplements deliver 1-4% and were discovered by scientists, not marketing departments. In supplements, the size of the claim is inversely proportional to the size of the effect.
One flag worth knowing if you race: third-party certification. NSF Certified for Sport and Informed Sport test products for contaminants and undeclared substances — worth looking for on anything you take regularly, since supplement contamination is a real and recurring source of failed doping tests and, more relevantly for the rest of us, of not getting what the label claims.
The priority order
Put the signals in sequence and the shopping list writes itself. Fuelling first — carbohydrate for the long sessions, because nothing else on this page matters if that is wrong. Caffeine for the days you want to be sharp, 3mg/kg, earlier in the day than you think. Creatine daily if you are north of 40 — 3-5g, cheapest per unit of evidence of anything here. Vitamin D through winter if you live somewhere the sun clocks off in October, and ferritin on your annual blood test now that you run.
Total cost, maybe $25 a month. Everything else in the aisle is noise dressed as signal — and you already know how to tell the difference, because you have watched the same movie in cycling for years. The 1% gains that work are boring, cheap, and backed by people with laboratories. The exciting ones are backed by people with ad budgets.