Which supplement course before a sports effort or competition?
Quick summary
A course before a competition delivers a measurable benefit only in case of a confirmed deficiency: iron in a deficient endurance athlete, magnesium where intake is insufficient, or probiotics taken at least 28 days before the event.
Key facts
Key points
- A course delivers a measurable benefit only in deficient athletes; it makes up neither for a training shortfall nor for an unbalanced diet.
- An iron course corrects degraded endurance in deficient female athletes with 100 mg of elemental iron per day for 8 weeks (Pengelly et al., 2024).
- Probiotics taken at least 28 days before the event at 15 billion CFU per day reduce perceived fatigue and digestive disorders in long efforts (Kearns et al., 2024).
- Trying out a supplement for the first time on the day of competition is the most documented mistake: a product never tested in training can trigger acute digestive disorders.
30 to 90% of athletes taking part in a marathon or triathlon report digestive disorders in competition, according to a systematic review published in 2024 (Kearns et al.). This figure illustrates a simple reality: the nutritional preparation for a sporting event is never reduced to a miracle course. In the world of the food supplement, the relevance of a targeted course before a major effort depends on the starting nutritional status, the time available and the type of event. The Swiss Sports Nutrition Society (SSNS) provides a clear framework for distinguishing really useful courses from strategies with no proven benefit.
When is a course of supplements really justified?
In which cases is a course scientifically justified?
A course only makes sense in case of a deficiency confirmed by a biological work-up or by a cluster of clinical symptoms. The International Olympic Committee (IOC) consensus specifies that supplements at best make a minor contribution to a solid nutritional programme built on a healthy diet and good daily hydration[6]. Without a documented deficit in an athlete already taking regular physical activity, the margin for progress on available energy and physical condition remains negligible. Three classic situations justify a course before a sporting goal: an iron deficit in the endurance runner or in long-distance practice — affecting up to 60% of sportswomen according to a systematic review in the Journal of Sport and Health Science[1] — a magnesium and essential mineral intake below the increased requirements of the athlete (10 to 20% above the standard adult benchmark)[2], or recurrent digestive fragility during long events, where carbohydrate intake and food tolerance become critical[3].
How long before competition should the course be started?
The useful timeframe varies from 4 to 8 weeks depending on the targeted micronutrient and the depth of the deficit to correct. The timing of meals and supplementation plays a central role in sports preparation: it fits within the training programme rather than preceding it. An iron course needs at least 8 weeks of oral intake at 100 mg of elemental iron per day to rebuild reserves in deficient female athletes, with an improvement in endurance of between 2 and 20%[1]. Probiotics require a minimum of 28 days of continuous intake at 15 billion CFU per day to durably modify the microbiota and reduce perceived fatigue during long-duration efforts[3]. Magnesium gains from being taken at least 2 hours before effort against a backdrop of prolonged supplementation, in combination with hydration and electrolyte intake suited to the sport[2]. Any course started in the last week before the event has little chance of producing the desired effect.
Which documented courses before a competition?
Iron course: who really benefits?
The iron course improves endurance performance only in deficient athletes, identified by a serum ferritin below 40 µg/L[1]. Women practising running or another endurance sport make up the population most at risk, with iron deficiency documented in up to 60% of them[1]. In this profile, an oral supplementation of 100 mg of elemental iron per day for about 56 days restores maximal aerobic oxygen capacity and endurance performance, with observed gains of 6 to 15% on VO₂max and of 2 to 20% on endurance[1]. Iron acts in synergy with other vitamins and minerals supplied by quality foods (vegetables, wholegrain cereals, lean meats) consumed daily, in a sport-health logic. In the athlete without a deficit, the same course brings no measurable benefit and remains useless given the available data.
Probiotics and magnesium: what do recent studies say?
Probiotics and magnesium act on two distinct dimensions of performance: digestive tolerance to effort for the former, muscle recovery and support of muscle mass for the latter. A systematic review of 13 controlled clinical trials concludes that a probiotic course of at least 28 days at 15 billion CFU per day reduces perceived fatigue and digestive disorders in athletes practising endurance sports, particularly in long-distance events[3]. Magnesium, when it complements an insufficient mineral intake from the diet, eases muscle soreness and improves recovery between strength sessions or intense training[2]. The requirements of athletes engaged in sustained physical activity are 10 to 20% higher than those of sedentary people, meaning a recommended intake of between 330 and 480 mg of elemental magnesium per day — a benchmark to integrate into everyday sports nutrition[2].
Which mistakes to avoid in a pre-competition course?
Why are detox courses and last-minute novelties counterproductive?
Two mistakes consistently stand out in the sports literature: restrictive “detox” courses and introducing a new food, bar, energy drink or supplement never tested on the day of competition. Hypocaloric detox diets deplete the reserves of muscle glycogen built up by the intake of complex carbohydrates (pasta, rice, bread, starches), which form the main fuel of moderate to high-intensity efforts[7]. The ability to maintain sustained energy production depends directly on these stocks. As for antioxidants taken at high doses over a long period, they may even block training adaptations by neutralising the reactive oxygen species necessary for mitochondrial improvement[5]. The IOC consensus is explicit: any supplement intended to improve performance must be tested in training or in simulated competition before being used in real competition[6].
Best practice
Start a targeted course 4 to 8 weeks before the event, based on a blood test (ferritin, 25-OH vitamin D, erythrocyte magnesium), and test each product over at least two intense training sessions before the competition.
Common mistake
Trying out a new energy gel, sports drink or supplement on the morning of competition exposes you to an unpredictable digestive risk. The Swiss Sports Nutrition Society’s official rule is clear: no new product on the day[8].
Why does a course replace neither diet nor training?
A course complements an athlete’s diet; it never replaces the nutritional strategy of the 3 days preceding the event, from breakfast to the last meal. Within this window, the priority goes to a high carbohydrate intake adapted to the duration of the event — rich in starches and complex cereals — to a reduction of fibre to limit digestive disorders, and to sustained hydration with water and a suitable sports drink[4]. The triathlon review published in Nutrients recalls that optimising muscle glycogen and planning intakes before and during the event remain the most powerful performance levers for preparing the body for a long-duration effort[7]. A poorly calibrated micronutrient course has no compensating power over these fundamentals; training habits, sleep and mental preparation remain the pillars onto which the course is grafted.
Frequently asked questions about a course before a sports effort
How long before a competition should you start a course?
Start the course 4 to 8 weeks before the event, depending on the targeted micronutrient. This timeframe gives the body’s reserves time to rebuild, particularly for iron which requires at least 8 weeks at 100 mg per day to improve the performance of deficient female athletes[1]. Probiotics require a minimum of 28 days of continuous intake to alter the gut microbiota[3]. A course started the week before competition will not have time to act and exposes you in addition to a risk of digestive intolerance on the day of the event.
Is a multivitamin course useful during the week of a race?
No, unless it has already been under way for several weeks. A multivitamin complex introduced the day before or in the week of a competition does not have time to produce a measurable biological effect. The IOC consensus statement on supplements recalls that a solid nutritional programme matters more than any acute supplementation[6]. No product that has never been tested in training should be introduced in the immediate run-up to the event, a principle shared by the official guide of the Swiss Sports Nutrition Society[8], at the risk of disturbing digestion or sleep.
Should you have a blood test before starting a pre-competition course?
Yes for any course targeted at iron, vitamin D or magnesium. A biological work-up (serum ferritin, 25-OH vitamin D, erythrocyte magnesium) confirms that a deficiency really exists before supplementing. The iron course only shows a measurable benefit in deficient athletes; in an athlete without deficit, it provides no documented gain in performance[1]. For female endurance athletes, an annual ferritin measurement is a standard best practice, regardless of competition planning.
Is a course of high-dose antioxidants useful for better recovery?
Not necessarily, and it may even harm training adaptations. Antioxidants taken at high doses over a prolonged period can neutralise reactive oxygen species, signals needed to improve mitochondrial functions[5]. Acute supplementation just before a goal is still being explored, but the scientific evidence remains limited[5]. It is better to rely on a diet rich in fruit and vegetables — the whole food matrix provides a balanced mix of polyphenols without the overdose risk of isolated supplements.
What are the risks of contamination or doping with supplements in Switzerland?
The risk of contamination by banned substances exists, even on legally sold products. The IOC consensus statement on supplements warns about this point: taking supplements exposes you to an unintentional violation of the anti-doping code[6]. In Switzerland, the Swiss Sports Nutrition Society maintains, with the support of Swiss Sport Integrity, a Nutritional Supplements Guide which classifies molecules by level of scientific evidence[8]. Athletes in official competition favour products certified “Informed Sport” or equivalent, and keep an intake log over 12 months.
Sources and references
8 sources- Pengelly M., et al. (2024). Iron deficiency, supplementation, and sports performance in female athletes: A systematic review.
- Tarsitano M.G., et al. (2024). Effects of magnesium supplementation on muscle soreness in different type of physical activities.
- Kearns R.P., et al. (2024). Do probiotics mitigate GI-induced inflammation and perceived fatigue in athletes? A systematic review.
- Mlinaric J. and Mohorko N. (2025). Nutritional strategies for minimizing gastrointestinal symptoms during endurance exercise.
- Larsen S. (2024). Acute antioxidant supplementation and performance — Should this be considered.
- Maughan R.J., et al. (2018). IOC Consensus Statement: Dietary Supplements and the High-Performance Athlete.
- Miguel-Ortega Á., et al. (2025). Triathlon: Ergo Nutrition for Training, Competing, and Recovering.
- Swiss Sports Nutrition Society (SSNS). Nutritional Supplements Guide.