If you are a parent, coach, athlete interested in youth athlete nutrition, or a woman who was a competitive athlete in her teens and is only now connecting the dots between those years and how your body feels today, this article is for you.
There is a pattern I have started to see, and once you see it, you can’t unsee it.
A fifteen-year-old girl is training six days a week. She’s fast, dedicated, the kind of athlete coaches dream of. Her parents are proud, but exhausted, ferrying her to early-morning sessions and weekend competitions and physiotherapy appointments for injuries that never quite resolve. Then one of three things happens:
• Her periods stop, or never properly arrive.
• She fractures a bone in her foot that should not fracture in a healthy fifteen-year-old.
• She simply stops improving, i.e. plateaus then slides backwards, and no one can work out why.
A sixteen-year-old boy is in the same picture. He is leaner than he was a year ago, despite eating what looks like lots of food. He catches every virus going through the school. His mood has flattened. His coach mentions, almost in passing, that he just doesn’t have the same spark in training.
These aren’t separate stories. They’re the same story, expressed through different bodies. And the answer is rarely what the medical system reaches for first.
What these young athletes have is, almost always, a fueling problem. Not a talent, discipline, or psychological problem. A biochemical problem that conventional sports medicine still routinely misses, and that the supplement industry actively makes worse.
I want to walk you through what is actually happening, because if you are raising, coaching, or treating a young athlete, this information can change the trajectory of their career, and more importantly, their long-term health.
Teenage Athletes Are Not Small Adults
This sounds obvious, but it is missed constantly in practice.
A teenage athlete is doing three biologically demanding things simultaneously: training at intensities approaching elite adult levels, navigating the most rapid period of growth and sexual maturation since infancy, and building the peak bone mass that will protect their skeleton for the rest of their lives. That bone-building window closes in the early twenties. Whatever is laid down now is, literally, the bank they draw from for the next sixty years.
Each of these processes — training, growth, bone accrual — carries its own energy and nutrient cost. Stack them together, and the metabolic demand on a fourteen-year-old elite swimmer or a sixteen-year-old cross-country runner is staggering.
In practical terms, this often means 3,000 to 4,000 calories a day for a female endurance athlete in her mid-teens, and considerably more for many young male athletes. Most are not eating anywhere near this. The reasons are layered:
• Sport culture rewards leanness.
• Social media has distorted how teenage girls relate to food and their bodies.
• Training schedules leave little time to eat.
• High training loads suppress appetite through changes in hunger hormones, a recognized biological effect that many athletes and coaches do not know exists.
• Well-meaning adults, raised in a diet culture that has confused thinness with health, often do not see the warning signs.
The result is what the International Olympic Committee now formally calls Relative Energy Deficiency in Sport, RED-S. It is not anorexia. It is not always disordered eating in any clinical sense. What it is, very often, is an unintentional and chronic gap between what a young body needs and what it is being given.
What Under Fueling Actually Does to a Young Body
When energy availability drops below what a body needs to train, grow, repair, and run basic metabolism, the body makes a choice. It downregulates the functions it considers non-essential to immediate survival. The hierarchy is consistent and biologically logical.
Reproductive function goes first
In female athletes, this means menstrual disruption. It starts with irregular cycles, then missed periods, then the complete absence of menstruation. The mechanism is suppression at the hypothalamus, which collapses the downstream hormone cascade that drives estrogen production. In male athletes, the equivalent is reduced testosterone It’s less obvious, but with real consequences for mood, muscle mass, bone, and recovery.
Many young female athletes, and the adults around them, interpret the absence of periods as a convenience, or as evidence that training is going well. It’s the opposite. The body is declaring that it can’t afford to reproduce because it can’t afford the energy cost. This is a serious clinical signal.
Bone remodeling slows
The young athlete who fractures a metatarsal during a normal training week is not unlucky. She is showing you that her skeleton is no longer keeping pace with the demands placed on it. Stress fractures in adolescents almost always indicate energy deficiency, often compounded by low estrogen or testosterone, and by inadequate vitamin D, calcium, and K2.
Roughly 90 percent of adult bone mass is laid down by age eighteen. A girl who under-fuels through her competitive teenage years walks into her thirties with a skeleton several years older than it should be, and into the perimenopausal years, when estrogen-driven bone loss accelerates, with significantly less in reserve than her peers.
This is where the story of the teenage athlete connects directly to the health of the menopausal woman. The body keeps a record. What is built, or not built, in youth tends to surface decades later.
Thyroid output falls
Under fueling reliably suppresses the active thyroid hormone, T3 through reduced conversion from T4, and through downregulation at the hypothalamic level. This is not thyroid disease in the conventional sense. It is a metabolic adaptation that expresses itself as fatigue, cold intolerance, brain fog, and a stubborn performance plateau that no amount of additional training will move. TSH and T4 often look normal on standard panels. Free T3 and reverse T3 tell the real story.
Immune function falters
The teenager who catches every cold, who is always slightly unwell, or who takes longer to recover from minor illness is not a victim of coincidence or weakness. Chronic under fueling suppresses immune defenses at multiple levels. Recurrent illness in a young athlete is a symptom, not a personality trait.
Mood and mental performance decline
Serotonin and dopamine, the neurotransmitters that regulate mood, motivation, and the capacity to engage with hard training, require specific amino acids, cofactors, and a functioning methylation pathway to be produced. A teenager who is chronically under fueled is not producing enough of them. The flatness, the irritability, the lost spark: these are biochemistry, not character.
The Nutrients That Make or Break a Young Athlete
The deficiencies I see are remarkably consistent across this population. Here is what I look for and why.
Iron
This is the single most common deficiency I see in young female athletes, and it is consistently under-investigated. Menstruating girls lose iron monthly. Endurance training adds further losses through sweat and the mechanical destruction of red blood cells with each foot strike. Growth demands more. The iron requirement of a fifteen-year-old female cross-country runner is dramatically higher than her peers.
The mistake most clinicians make is checking only hemoglobin. By the time hemoglobin drops, iron depletion has been developing for months. Ferritin, the storage form, drops first. A young athlete with ferritin below 30 ng/ml is already underperforming, regardless of whether her hemoglobin reads normal. Many elite athletes function best with ferritin above 50.
Vitamin D
Indoor-sport athletes, athletes training in northern latitudes, and athletes with darker skin are routinely deficient. Vitamin D affects bone, immune function, muscle force production, and mood. I look for serum levels in the 50–80 ng/ml range, well above the conventional threshold, which was set to prevent rickets, not to support an elite athlete’s physiology.
Calcium, Magnesium, and Vitamin K2
These work as a system. Calcium alone, without the cofactors that direct it into bone, is at best wasted and at worst deposited where it should not be. Magnesium is depleted by sweat and training stress, and it is required for vitamin D to activate. K2 directs calcium into bone rather than soft tissue. Young athletes need all three, first from food, and then precision supplemented where intake falls short.
Omega-3 EPA and DHA
Training generates inflammation. Adaptation depends on it. But chronic, unresolved inflammation impairs recovery and accelerates the systemic toll of high training loads. Adequate EPA and DHA are precursors to the molecules that actively resolve inflammation, not merely oppose it. They also support ongoing brain development and, critically for contact-sport athletes, appear to play a meaningful role in concussion recovery.
Most teenage athletes on a standard fish oil capsule are not getting what they think. Omega-3 index testing, measuring EPA and DHA as a percentage of red blood cell membrane fatty acids, is the only way to know with certainty.
The Genomic Layer
Two young athletes on identical training plans and identical diets will respond differently. Much of that difference is genomic.
Variants in the MTHFR gene affect how folate is metabolized and how well the body clears homocysteine, with downstream implications for neurotransmitter production, recovery, and the safety of standard B-vitamin supplementation.
Variants affecting vitamin D receptors influence how efficiently a given serum vitamin D level translates into actual cellular activity. Other variants shape antioxidant capacity, mitochondrial function, fast-twitch to slow-twitch fiber ratio, and caffeine metabolism.
This is not theoretical. Personalized nutrition informed by functional genomics is what differentiates the athletes who break down from the athletes who keep building.
What Actually Helps
The intervention is rarely glamorous, which is part of why it is overlooked.
• Fuel first.
Caloric intake matched to training load and growth. Carbohydrate treated as fuel, not as a category to be feared. Protein distributed across the day rather than concentrated in one meal. Real, varied fats included rather than restricted.
• Protect the menstrual cycle.
A regular cycle is one of the most reliable signals that a young female athlete’s physiology is well-resourced. Its absence is a medical issue, and not a convenience. It should never be masked with hormonal contraception without addressing the underlying cause.
• Test what actually matters.
Ferritin, transferrin saturation, 25(OH)D, RBC magnesium, zinc, B12, free T3, reverse T3, and an omega-3 index. Standard bloodwork misses most of what is relevant in this population.
• Prioritize sleep.
Recovery and growth hormone release peak during sleep. A young athlete sleeping six hours a night is not able to optimize training gains regardless of how many sessions she completes. Adolescents have a naturally delayed circadian rhythm, so early-morning training carries costs that need to be understood and managed.
• Personalize.
Genomic data, functional lab work, and clinical observation, integrated. Not a generic protocol applied to an individual body.
The Longer View
The decisions made, and the deficits accumulated, in the teenage athletic years cast a long shadow.
Many of the adult women I work with today were competitive athletes in their teens and twenties. The patterns I see are remarkably consistent: thyroid systems that never fully recovered from years of under fueling; declining bone density; hormonal systems entering perimenopause already running on debt. The body keeps a meticulous record.
This is not a reason for guilt for anyone reading this who has lived that history. It is a reason for clarity about what we owe the next generation of athletes coming through.
If we can fuel them properly now, support their physiology rather than override it, and approach their development with the respect that young bodies deserve, we change the trajectory of the rest of their lives. Not only their athletic careers.
The Question Worth Asking
If you are watching a young athlete struggle with recurrent injury, fading performance, missed periods, frequent illness, or the quiet loss of spark, the most useful question is not “what’s wrong with her?”
It’s: what is she not being given enough of?
That is the question orthomolecular medicine begins with. And in young athletes, the answer is almost always the same: energy, key nutrients, recovery, and the kind of informed, personalized approach that recognizes a developing body for what it is: a remarkable system that responds beautifully when it is properly resourced, and breaks down quietly when it is not.
The talent in our young athletes deserves better than the broken system currently surrounding them. So do the futures they are building in their bones.
Is this resonating with your story or with someone you love?
Whether you are a parent trying to protect your child’s health, a coach wanting to be better for your athletes, or a woman looking back at your own athletic years and recognizing these patterns in your body today, I am here to help.
My practice integrates functional genomics, orthomolecular medicine, and targeted nutrition to find and address root causes. I work remotely with clients across North America, Europe, Africa, Asia, and Australia.
Book a free call or learn more at www.brigittespurgeon.com
Brigitte Spurgeon is a Doctor of Orthomolecular Nutrigenomics and Functional Genomics Practitioner. She works with clients worldwide to prevent and reverse chronic disease through personalized, root-cause nutrition, which includes youth athlete nutrition.
This article is for educational purposes and does not constitute medical advice.
