Most youth sport nutrition advice is written as though every young athlete is the same. They are not, and the differences are not small — the UK government's own reference intakes set different figures for girls and boys from around age 10 onwards.
GrowStrong Kids has published separate figures for girls and boys from the start, because the numbers genuinely differ. This page pulls the girls' side together in one place: what changes, when it changes, what to watch for, and where the real risk sits.
What actually differs
Three things, and they arrive at slightly different times.
Iron rises sharply, and it is the big one. For ages 6–9 the UK reference intake is 8.7mg a day for everyone. From around age 10 it splits: 11.3mg a day for boys, and 14.8mg a day for girls once periods begin. That is roughly a 30% higher requirement, and it arrives at exactly the age training volume tends to increase.
Calcium goes the other way. Boys aged 10–16 need 1,000mg a day, girls 800mg. Lower, but no less important — the teenage years are when the majority of lifetime bone density is laid down, and that window does not reopen.
Energy needs are lower than boys' at the same age, but higher than most people assume. The figures by age are on our nutrition by age guides, and every one of them lists girls and boys separately. Lower than a boy the same age does not mean small — a girl in regular training needs a genuinely substantial amount of food.
Why iron matters more than anything else here
Iron carries oxygen to working muscle. When it is low, the effect is not subtle, but it looks like something else entirely.
A girl who is short of iron fades in the second half. She stops recovering properly between sessions. Her times stop improving, or her decision-making drops off late in a match. She looks tired despite sleeping fine. Almost universally, this gets put down to fitness, effort, attitude or a plateau — and she gets given more training, which makes it worse.
Low iron is common in female athletes, and it is one of the few genuinely fixable things in this entire subject. The foods that cover it:
- Red meat two or three times a week — the most efficiently absorbed source there is
- Lentils, beans and chickpeas
- Fortified breakfast cereals — check the box, most are fortified
- Dark green vegetables
- Chicken, fish and eggs
One practical detail that makes a measurable difference: pair plant sources with vitamin C at the same meal. A glass of orange juice alongside a fortified cereal, or peppers and tomatoes in a lentil dish, substantially improves how much iron the body actually absorbs. Iron from plants is poorly absorbed on its own.
Signs worth a GP conversation
None of these are cause for alarm on their own. Together, or persisting, they are worth a blood test rather than guesswork:
- Unusual tiredness despite sleeping well
- Recovery between sessions getting slower
- Performance dipping without an obvious reason
- Looking pale
- Concentration dropping off
- Periods becoming irregular or stopping
That last one deserves its own paragraph. If periods become irregular or stop during a period of heavy training, see a GP. It is not a sign of fitness and it is not something to train through. It is frequently a signal that the body is not getting enough energy to cover both training and normal development, and the long-term cost lands on bone.
The real risk is eating too little, not too much
This is the part we will not soften.
There is no page on this website that will tell a girl under 16 to eat less, and there never will be. The risk in youth girls' sport is under-fuelling, not over-eating — and under-fuelling does not look dramatic from the outside. It looks like a girl who has stopped improving, gets injured more often than her teammates, recovers badly, and is quietly losing bone density at the exact age she should be building it.
It is more likely in some environments than others. Gymnastics, dance, athletics and the weight-category combat sports all carry more of it, partly because appearance or weight gets discussed openly in front of young girls. Any coach, programme or plan that tells a girl under 16 to cut food or fluid to make a weight or a look is wrong, and you are entitled to say so.
Weight categories are never a reason to cut food or fluid in a young athlete. If a category is a genuine struggle, the answer is a different category.
Strength training will not make her bulky
This myth keeps girls away from the single most protective thing available to them.
Girls carry a higher rate of knee injury than boys in pivoting sports — football, netball, skiing, basketball. Stronger legs and a stronger core improve landing mechanics and change-of-direction mechanics, which is precisely what reduces that risk. Age-appropriate strength work is injury prevention, and avoiding it costs something real.
Our safe strength training guide sets out what is appropriate age by age. The bands apply to girls exactly as written.
When fussy eating is something else
Fussy eating in younger children is usually about texture, familiarity and control, and it tends to ease with time. In girls approaching and going through puberty, a narrowing diet can occasionally be something different.
The tell is usually what is being removed. Fussiness rejects textures and unfamiliar things. Restriction removes whole categories — particularly anything seen as fattening — and is often accompanied by more interest in food generally, not less.
This is not a reason to panic about a child who will not eat mushrooms. It is a reason to notice if a previously broad eater starts cutting things out, and to raise it with a GP rather than manage it at the dinner table. Our fussy eaters guide covers the ordinary version.
Where to go next
Every sport page on this site now carries a girls-specific section — the sport changes which nutrient matters most, and it is worth reading the one that applies:
- Football — running volume plus iron, the most common combination
- Gymnastics — where under-fuelling risk is highest
- Athletics — where iron shows up most directly in performance
- Swimming — high volume, and non-weight-bearing, so calcium matters more
- Rugby — contact increases both iron losses and bone loading
- Judo, boxing, kickboxing — weight categories, handled properly
And for the full figures by age, with girls and boys listed separately throughout, start with Support Growth and the nutrition by age guides.
Common mistakes
Planning a 13-year-old girl's meals around a younger child's requirements. The iron jump at puberty is substantial and easy to miss entirely, because nothing announces it.
Treating fatigue as a fitness problem. More training is the standard response to a girl who has gone flat, and it is the wrong one often enough to be worth checking iron first.
Assuming lower energy needs than boys means small portions. It does not. A girl in regular training needs a lot of food.
Avoiding strength work. It is protective, particularly for the knee, and the bulking worry is unfounded.
Treating a stopped period as a sign of hard training well done. It is a signal to see a GP.
Frequently asked questions
Does my daughter need an iron supplement?
Not automatically, and not on guesswork — too much iron carries its own problems. If the signs above are present, ask a GP for a blood test and act on the result. Most girls can cover 14.8mg a day through food, and food should be the first move.
At what age does the iron requirement actually change?
The UK reference figures split from age 10, and the 14.8mg figure applies from the onset of menstruation. In practice, once periods begin, plan for the higher figure regardless of the exact birthday.
Should she eat differently during her period?
There is no separate period diet. The iron requirement is an everyday figure, not a monthly one, and covering it consistently across the month is what matters. If cramps or fatigue are affecting training specifically, that is worth raising with a GP rather than managing through food alone.
She has gone off food since starting secondary school and training more. Is that normal?
Appetite genuinely moves around in this window, but going off food while training more is worth paying attention to rather than waiting out — the combination is the one that leads to under-fuelling. Look at whether whole food groups are disappearing, and talk to a GP if they are.
Is it true girls shouldn't do weights until they've stopped growing?
No. Age-appropriate strength work — bodyweight and band-based at younger ages — is safe and protective. Our safe strength guide sets out what belongs at each age. What is not appropriate at any age under 16 is heavy loading or maximal lifting.
My daughter is the only girl in her squad. Does any of this change?
The nutrition does not, but the environment can. Team plans, coach advice and squad meal arrangements are often built around the boys' figures without anyone intending it — the iron figure in particular. Worth checking rather than assuming.
The figures on this page come from UK government dietary reference values. GrowStrong Kids provides general nutrition information for young athletes and their families, and is not a substitute for individual advice from a GP, registered dietitian or registered nutritionist.