Keto, fasting and low-carb: what the Spanish consensus says

Every August it happens again. Somewhere between the parasol and the second round of calamari, someone in your training group announces they are starting keto in September. Or intermittent fasting. Or that they are cutting carbs because carbs "bloat you". The Spanish Nutrition Society has just published a consensus document reviewing those diets one by one, and its answer will not fit on a gym poster: it depends.

Seven fashionable diets and no winner

The document, coordinated by Juan Mielgo-Ayuso and published in December 2025 in the journal Nutrients, reviews the evidence behind seven strategies you will certainly recognise: high-carb, low-carb, ketogenic, intermittent fasting, vegetarian, paleo and carbohydrate periodisation. Rather than crowning one of them, it does something far more useful: it gives each one its moment, its dose and its small print.

The underlying idea is that "which diet is best?" is the wrong question. The right one is "best for which sport, in which phase of the season, and for which person?" That sounds like a politician's answer, but it is the exact opposite: it is what happens when somebody bothers to look at the data instead of selling a method.

Carbohydrate is still the floor everything stands on

Here the consensus is blunt: for long, hard efforts, eating enough carbohydrate remains "the most consistent approach" to sustaining performance. And it comes with numbers, which is what you want. On easy days, 3 to 5 grams per kilo of body weight per day. With moderate one-to-two-hour sessions, 5 to 7. With long two-to-three-hour sessions, 7 to 10. And in a loading phase before competition, up to 12 grams per kilo across the previous 36 to 48 hours.

During the effort itself, 30 to 60 grams of carbohydrate per hour if it lasts between one hour and two and a half, and 90 to 120 grams per hour beyond that. With one very sensible warning: you have to train your gut for it. Taking on 100 grams an hour on marathon day without ever having practised is the quickest way to get acquainted with the portable toilets at kilometre 30.

For recovery, the document talks about 1 to 1.2 grams of carbohydrate per kilo per hour during the first four hours, alongside roughly 0.3 grams of protein per kilo. Translated into actual food, that is more or less what we already covered about what to eat after a run without obsessing over it: an ordinary plate, not a ritual.

Keto, low-carb and paleo: they have a time and a place

None of the three is banned, but all three are fenced in. A low-carb diet (below 20-25% of total calories) is placed "only during specific phases, such as pre-season", and the consensus asks you to avoid it in competition: it worsens running economy at precisely the intensities you compete at, and reduces anaerobic capacity in team sports. If you lift, it also insists on keeping protein at 1.6 grams per kilo per day or above.

The ketogenic diet carries the most warnings. Under 50 grams of carbohydrate a day, applied in short two-to-three-week blocks in pre-season, and under medical supervision, including monitoring of kidney function and lipid profile. It impairs high-intensity performance and is not recommended for speed or team sports. And there is one very August-shaped detail: it raises the risk of dehydration, so trying it in the middle of summer is starting the house from the roof. If you are training in this heat, you have more urgent homework in our anti-dehydration guide.

Paleo, at 22-40% carbohydrate, falls short for endurance sport and carries a risk of calcium and vitamin D deficiency. The consensus suggests a reasonable workaround: slot potato, sweet potato or nuts around your key sessions instead of applying the dogma to the letter.

Intermittent fasting is not magic, it is a tool

The tone here is surprisingly kind. Intermittent fasting appears as a "useful tool" if your aim is to reduce body fat, and of all the variants the 16:8 window comes off best, for adherence and for lower risk. The practical advice is to start with 12 hours of fasting and build up, rather than waking on a Monday with an 18-hour window and a fartlek in the diary.

The caveats: risk of losing lean mass, less tolerance for very intense loads, and a poor match with highly glycolytic sessions. Plus a clear recommendation to avoid it during competitive periods. In other words, it is for the quiet weeks, not for race week.

Vegetarian: yes, but with a shopping list

Vegetarian and vegan diets come out of the document as "compatible with performance if well planned", and all the small print sits in that planning. Protein at the top of the range, 1.6 to 2 grams per kilo. Vitamin B12 supplemented without argument for vegans. Keep an eye on vitamin D, calcium, iron and zinc, pair non-haem iron with vitamin C, and consider creatine if you lift. Nothing dramatic; it simply cannot be improvised.

The sentence that sums it up

The document closes with an idea that all of us who train for the love of it, reading headlines on our phones mid-stretch, should probably have tattooed somewhere.

There is no single dietary strategy universally recommendable for all athletes.

Consensus document of the Spanish Nutrition Society (SEÑ)

So the next time somebody in the group announces their definitive diet, there is no need to argue. Just ask them which phase of the season they are in. If they cannot answer, you have conversation for the entire walk home.