How Many Calories Do You Need?

Weight loss is science, right? Bodies burn calories to create energy. Each body is unique, and differentiating factors–amount and type of exercise, height, weight, age, and gender–factor into the simple balance of calories in versus calories out.

Exactly how many calories, then, does a specific body need?

Eating to Lose Weight

scale balancing calorie needs and weight lossI’ve been dealing with an unrelenting, unexplained fatigue. In 2015, I ran the Paris Marathon; the next month (May), I attempted an ultra in North Carolina (and wrote up that 50K DNF), completed a half marathon on the trail, and then headed to West Texas for fitness camp in August.

Then I crashed. Big time.

Despite my quest to determine what caused that crippling lack of energy, doctors hadn’t supplied any answers.

Having given up on why, I’d desperately begun to tackle my resulting weight gain. Surely cutting the right amount of calories and giving up “bad” food would help. After all, how much did I need to eat if I couldn’t exercise?

My New Mexico woo-woo moment, when the hot stone masseuse reminded me “your body is a perfect machine” put me back on a sensible track. Damn it, I thought, it’s time to repair my health.

Once home, I scheduled with my nutritionist. At the outset, she’d evaluated my eating habits; that crappy low energy had to originate somewhere other than poor nutrition. Still, the fatigue lingered and I’d gotten larger. If my new normal involved little exercise, I reasoned, reevaluating my caloric intake made sense.

The nutritionist asked me to keep a food log, which I sent in before our appointment. As we talked over why I’d returned–my desire for a healthier weight and (fingers crossed) some reclaimed energy–she tapped a pen thoughtfully against her lips.

“I don’t think you’re eating enough,” she said, flipping through my food log.

I laughed out loud. “Yeah, no. No matter what I don’t eat, I keep getting bigger,” I replied. “If I’m not eating enough, at some point, I’d lose weight. If I ate more, God knows how huge I’d be!”

My nutritionist talked about nutrients needed before cleverly applying logic: “Humor me. Let’s get your resting metabolic tested. That way, we’ll know exactly what calories you need before tweaking your diet more.”

What Is Resting Metabolic Rate?

scale balancing calorie needs and weight lossAccording to the Medical Dictionary for Health Professions and Nursing (2012), resting metabolic rate is “the minimum number of calories needed to support basic functions, including breathing and circulation.”

In other words, RMR is the energy your body needs to just be.

So how do you measure RMR? Fortunately, the Fitness Institute of Texas, part of The University of Texas at Austin’s Kinesiology and Health Education department, had the answer.

FIT and I go ‘way back. When I was coached at Rogue Training Systems, FIT tested my V02 max. (I’m not going to go down that information rabbit hole now but let’s just say V02 max provides fascinating training feedback for every dedicated marathoner.)

At Austin Fit Magazine, I worked closely with FIT’s executive director, Phil Stanforth. FIT provided testing equipment, student volunteers, and sponsorship for the then-new AFM Fittest event. Years later, I turned to FIT for a consultation (what they call “Exercise Rx”) to shake up my workout regimen.

With our history, I naturally sought out FIT for RMR measurement; their Calorie Fit assessment provided the information I needed, so I scheduled my test.

How to Determine Your RMR

Instructions were simple:

  • No food after 11 p.m. the night before; only water.
  • Skip workouts (including, even, the short, 15-minute walk to UT’s stadium, where FIT is located).
  • Wear comfortable clothing.

scale balancing calorie needs and weight lossAfter parking in front of Bellmont Hall and being escorted to the FIT offices (BTW, it’s fun to ride in the elevator with the jocks), we entered the small testing area. I answered questions about current exercise, height, and weight; my heart rate and pulse were measured; my temperature taken. Then, the nutrition expert explained the test.

FIT uses a handheld machine, a BodyGem Indirect Calorimeter, which analyzes exhaled chemicals. After securing a tight clamp across the bridge of my nose, I’d serenely breathe in and out through my mouth into the BodyGem mouthpiece (a la snorkeling). The 10-minute test would be administered three times, outlier result tossed (there’s alway one), and an average RMR determined.

I settled into the comfortable recliner. The nutrition expert covered me with a soft blanket, dimmed the lights, and left the room. Despite the plastic nose clip (honestly, the worst part of the entire experience) and mouthpiece, I easily zoned out. Ten minutes of zen flew by. When the machine beeped, the tester returned, reset the machine, and repeated the procedure.

What Did My RMR Reveal?

scale balancing calorie needs and weight lossAfterward, we reviewed the results. And I was shocked.

My smart nutritionist had been correct: I wasn’t eating enough.

Without exercise, my total daily expenditure is 1596–1824 calories. With the type of exercise I was currently doing, ingesting 1460 calories daily should result in a half-pound weekly weight loss.

For the last three years, I’ve been digging an unhealthy hole of caloric deficiency. When fatigue hit, I immediately cut back on food: no need for fuel if there’s no workout. Lack of necessary nutrients created a vicious cycle; however my mystery fatigue had originated, I couldn’t know if it resolved–there weren’t enough calories to stoke my engine.

With only about 1000 daily calories, no wonder my energy level was low.

I emailed the printed results to my nutritionist. Her plan upped my  protein, fat, and carbs proportionally within a greater number of daily calories. Even though science had verified these numbers, this was scary. Purposefully adding foods I’d been avoiding had me hyperventilating. Eating more was almost panic-inducing.

Surely my body would completely mushroom. Could I really risk gaining additional weight?

Deep breaths, Leah. Trust the science. 

Did More Calories Make a Difference?

scale balancing calorie needs and weight lossWithin a week, I had noticeably more energy. I began sleeping better. My GI tract bothered me less, especially after eating. Getting out of bed in the morning was easier. A busy day or early workout didn’t send me into an afternoon stupor.

The biggest difference? I’m running again.

In two months, I’ve gone from barely running a mile to regular weekly runs (plus walks and aqua jogging). On Sunday, I comfortably completed a 10-miler with friends. As my workout routine returns, I feel more like myself. And that makes me very, very happy!

As to weight loss–well, not yet. But then again, I haven’t gained any weight. My wonderful nutritionist is measuring my body composition; she wants to track all the positive changes occurring internally. Her latest recommendation? Weight lifting.

Slowly, surely, I trust the scale will reflect my new healthy balance.

Scale calories weight balancing energy


Find Out More

Fitness Institute of Texas

Video explaining V02 max testing

BodyGem Indirect Calorimeter

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Published by Leah Nyfeler

I'm a writer, editor, runner, and adventurer who is always looking for the next new story, exciting adventure, and good meal/book/movie. My focus is on helping people find their best, healthiest self through sharing what I know and how I've come to learn it. In addition to my blog "Enjoying the Journey: Observations on the Fit Life" at, my articles have appeared in a variety of print and online magazines. You can hear me as part of the 2015 Austin cast of Listen To Your Mother.

3 thoughts on “How Many Calories Do You Need?

  1. Leah,

    I’ve just joined your blog and so very impressed with your insight, knowledge and honesty. The issues you describe are so real, and touch so many of us in our everyday lives. I have recently dealt with dietary issues due to early stage kidney disease. It’s a bit of a nightmare trying to figure it all out. But I’m doing good and making progress. As an alternate therapy I received accupuncture 2x/week and my eGFR increased from 47 to 61 over 3 months. I believe the accupuncture increased the blood flow to my kidneys, although at the time I was also really tweaking my diet and particularly focusing on my gut health. At any rate, it’s good to realize that I have so much power to make choices that increase my well-being.

    Also, I loved your little comment about riding in the Bellmont Hall elevator! That’s my girl!

    Diane G.

    1. My friend–thank you for reading! You’ve always focused on education in health and been willing to try new things–whether yoga, diet, acupuncture. I admire your spirit! Take care.

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