Swap some meat and dairy protein for beans and bread: in 790 Spanish teenagers it made no difference to how fit they were

Ask a coach what a growing athlete should eat and you will often hear about eggs, milk, chicken and fish. Animal proteins carry a fuller set of essential amino acids and are easier to digest, so the thinking goes that they build muscle and performance better than protein from beans, grains or nuts. Most of the evidence on that question comes from adults. A study published in Frontiers in Nutrition on 28 October, by researchers in Spain, Chile, Ecuador, Korea and Andorra, looked at teenagers instead.

The team used the Eating Healthy and Daily Life Activities (EHDLA) study, run in all three secondary schools of the Valle de Ricote, a rural valley in Murcia in south-east Spain. After exclusions, 790 adolescents aged 12 to 17 had both a food questionnaire and a full fitness test: a 20-metre shuttle run (the 'bleep test') for stamina, handgrip strength, a standing long jump, a 4 x 10 metre shuttle sprint for speed and agility, and a sit-and-reach for flexibility. Their 45-item food frequency questionnaire covered the past year, and each food was sorted by its main protein source.

These teenagers ate plenty of protein: a median of about 111 grams a day, or 1.9 grams per kilogram of body weight, roughly double the usual recommendation for their age. About three quarters of it came from animal foods. The researchers then used a statistical method called isocaloric substitution, which asks what fitness would look like if 5% of a teenager's daily calories shifted from animal protein to plant protein while total calories, fat and carbohydrate stayed the same. At this group's median intake, 5% of energy works out to roughly 30 grams of protein a day, a big shift on paper.

The answer was nothing measurable. After adjusting for age, sex, family affluence, sleep, physical activity, screen and sitting time, body mass index and total energy, none of the six fitness outcomes moved in either direction, and the confidence intervals sat comfortably across zero. The result held when the group was split by activity level and by whether protein intake was adequate.

The team also broke protein down by food. Swapping egg protein for plant protein went with a shorter jump and a slower sprint; swapping oily fish went with a slower sprint; swapping cereal protein for animal protein went with a faster sprint, and swapping root-vegetable protein (mostly potatoes) for animal protein went with lower overall fitness. Those findings sound like a menu, but the authors are clear they are not. They ran 78 food-level comparisons, and once the numbers were corrected for testing so many at once, none of them held up. For some foods a 5% energy swap is also bigger than anything a teenager would actually eat.

The limits matter. This was a single snapshot, so it shows associations, not effects, and nobody changed their diet. Protein was estimated from a questionnaire with standard portion sizes, not weighed, and the study had no data on training history or past sport. The sample came from one valley in Spain. What it does suggest is that for teenagers who already eat enough protein, where it comes from may matter less for fitness than the overall diet, which leaves room for families who want more beans and grains on the plate.