Even if you work out regularly — running, cycling, lifting, taking classes — there's a good chance no one ever sat you down and explained how to eat for your activity level. You figure it out as you go.

But when fatigue creeps in, your mood shifts and your workouts start feeling harder than they should, the problem might not be your training. It might be your fueling.

"I truly wish RED-S was talked about more frequently," says Amanda Beaver, a registered dietitian at Houston Methodist. "Every avid exerciser should know about this — especially women."

Your likely first question: What is RED-S?

The term stands for "relative energy deficiency in sport" and may be behind a frustration you've experienced but never been able to explain. You're training hard, eating what seems like enough and still battling fatigue and performance that won't budge.

"It happens when your calorie intake is too low for your activity level," explains Beaver.

Why RED-S symptoms are easy to write off

"So many adults I meet are recreational athletes," Beaver says. "And most never had a coach or dietitian teach them how to eat for their activity level."

That lack of awareness is part of what makes RED-S so easy to miss. Many of its earliest signs can seem like normal consequences of training hard or juggling a hectic schedule.

Early signs of RED-S are subtle, marked by changes in mood and energy:

• Fatigue during workouts

• Low energy throughout the day

• Increased anxiety

• Depression-like symptoms

"You might notice that you don't have the same energy you once had during workouts. You might feel more sluggish during the day, or earlier in the day, than usual," Beaver explains.

These signs are fairly nonspecific and easy to blame on a busy life — especially if you've been (unknowingly) dealing with RED-S for a long time.

Another sign is a stall in your training progress. "You're putting in all the work, but you're not noticing the impact you expect," Beaver adds. This happens because your body doesn't have the nutrition needed to fully capitalize on the work you're putting it through.

RED-S can affect far more people than you may think

Formally defined in 2014, RED-S expands on an older sports medicine term: the female athlete triad, which focused on three symptoms — calorie shortfalls, bone loss and menstrual irregularity — and only in women.

The change to RED-S recognizes that the phenomenon affects not just more bodily systems but also men.

RED-S disrupts hormones in both women and men

When your calorie intake is too low for your activity level, your body is forced to ration its limited energy supply — dampening down functions it considers nonessential.

"An example of this is how RED-S affects the production of hormones regulating reproduction and development. For women, estrogen drops. For men, testosterone lowers. Lower levels of these sex hormones can lead to reduced libido.

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Amanda Beaver, registered dietitian

These hormonal shifts can also cause irregular or missed periods in women. "The treatment for irregular periods is often birth control,” explains Beaver. "But, if the root cause of period irregularity is RED-S, taking birth control doesn't solve the problem — it just masks it."

In both men and women, RED-S can also increase stress hormones such as cortisol. (Related: Should You Stress Over Cortisol?)

That's one reason Beaver recommends that active adults work with a primary care physician who has training in sports medicine, so an issue like RED-S doesn't get missed.

RED-S weakens bones, gut function and immunity

It's not just your hormones. RED-S can lead to a cascade of problems throughout the body if the energy deficit persists over time.

Bone health is among the most serious concerns, especially for women. "RED-S decreases bone rebuilding and increases resorption," Beaver explains. This increases the risk of stress fractures, osteopenia and early-onset osteoporosis.

RED-S can also slow the digestive tract, causing bloating, constipation and gas — mimicking IBS symptoms.

Over time, the energy deficit can also affect heart muscle, leading to an abnormally low heart rate, dizziness and lightheadedness. Of note, while a low resting heart rate is often seen as a signal of cardiovascular fitness, it can also mean your metabolism has slowed to conserve energy.

And the long-term effects don't stop there.

"When we're not meeting our calorie needs long-term, our immune system can become suppressed," Beaver adds. "We might get colds more frequently — or, when sick, the illness may be more severe or symptoms may linger longer."

(Related: 6 Ways to Boost Your Immune System)

RED-S can develop in anyone who exercises regularly

Historically linked to young female athletes, Beaver stresses that energy deficits related to exercise can happen to both women and men — at any life stage and competitive level.

Sports where RED-S occurs more frequently include:

• Endurance sports like long-distance running and cycling, since calorie demands are so high that shortfalls happen easily

• Sports with an aesthetic emphasis, such as dance, figure skating and weight-class sports like wrestling

• High-intensity programs like CrossFit and Hyrox

"The condition can be entirely unintentional. Sometimes we genuinely just don't know we're not meeting our needs," says Beaver. "But RED-S can also coexist with disordered eating and eating disorders."

People trying to lose weight while maintaining a heavy exercise routine may also be at risk, particularly because weight loss requires a calorie deficit. Those on GLP-1 medications while training heavily should be especially cautious. "If we're doing high volumes of exercise while taking a GLP-1, the risk of RED-S can increase significantly as does losing large amounts of skeletal muscle," Beaver warns.

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RED-S prevention starts with consistent fueling habits

What can you do to avoid developing the condition? Beaver's advice is fairly straightforward: "Stick to the core things: fuel before a workout, refuel after a workout and aim for balanced meals. These simple steps can take you so far."

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If you suspect RED-S, Beaver recommends working with a primary care provider who specializes in sports medicine, who will have more knowledge of fueling-related conditions. A registered dietitian specializing in sports nutrition can help you understand your actual energy needs and recommend adjustments to your nutrition plan.

"The good news is that RED-S is both preventable and treatable," says Beaver. "Once someone understands what their body needs and starts fueling appropriately, they often feel stronger, recover faster and enjoy exercise more. Having a plan for fueling exercise will help you perform at the top of your game.”