Within-Day Energy Deficits Can Hurt Health And Performance, Especially For Female Athletes

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Eating enough to fuel an athletic life is essential for health and performance. And it goes beyond making sure you have enough energy availability on 24-hour cycles. Recent studies indicate that even controlling for overall daily caloric intake, excessive within-day deficits can have negative impacts on hormones and metabolism, likely impacting bone health and physical adaptation as well, particularly for female athletes.

Kylee Van Horn is an RDN who counsels many of the athletes I coach on these issues (you can contact her at Fly Nutrition here), and as we review some of the studies, her general advice is important to keep in mind. “There is often a focus on manipulation of large food groups or macronutrients or timing of meals, but for most athletes, the top priority should simply be consistently having enough energy availability to support training and life.”

A wonderful 2019 article in the International Journal of Sport Nutrition and Exercise Metabolism laid out the stakes. Low energy availability is when energy expenditure through athletics and lifestyle is mismatched with energy intake, leading to either short-term or long-term caloric deficits. Chronic low energy availability can cause major health issues. Check out this article from last year for more, and a 2020 review in the journal Nutrients.

Today, though, I want to zoom in. 

When thinking about food intake, it’s tempting to think of it as a day-to-day thing. If an athlete tells me, “I didn’t eat enough on Tuesday,” I know to make sure we back off a bit and don’t force a workout or longer run on Wednesday in a depleted state. But the research indicates that negative impacts don’t just depend on daily caloric intake.

Excessive within-day deficits, classified by the studies as the number of hours with a 300- to 400-plus-caloric deficit, may play a role too (depending on the athlete and the number of hours in deficit). And it has far-reaching implications for how athletes need to think about fueling activity and life.

Before getting to the studies, a quick reminder. 

These issues can be hard. They can be hard because the science isn’t always settled and because individuals are unique, for sure. But nutrition practices are also complicated because they wrap up complex issues of culture and body image and psychology in an overstuffed burrito where it’s nearly impossible to get each ingredient with each bite. So as we talk about specific studies, possibly having some ingredients spill out along the way, remember that whatever you are going through, you are not alone. Talk to a nutritionist like Kylee or mental-health professional if you have questions.

Nutrition practices are also complicated because they wrap up complex issues of culture and body image and psychology in an overstuffed burrito where it’s nearly impossible to get each ingredient with each bite.

Danielle Snyder, a mental health expert who you can contact through her business Inner Drive Athlete, emphasizes that “it’s all about learning to celebrate what our bodies are capable of.”

“The goal is to find our strong. And what ‘strong’ looks like and the journey to get there may look different for each athlete. That can be complicated, but it’s also beautiful.”

The first study was published in the Scandinavian Journal of Medicine and Science in Sports in 2017, looking at within-day deficits for female athletes.

It started from a couple fascinating observations. Many female athletes with long-term energy deficiency maintain steady body weight and body composition in the normal range. In addition, athletes with eumenorrhea (regular menstrual cycle) and those with menstrual dysfunction had similar energy balance and energy intake, metrics traditionally measured on 24-hour cycles. If we view nutrition through a “calories in, calories out” framework, neither of those sentences make much sense. There must be variables that are not measured. What are they?

Other research found that the offset had to do with the endocrine system and how hormones influence metabolism and other physiological functions on different time scales. For more on this subject, read the amazing book Roar, by Dr. Stacy Sims, and follow Dr. Sims on social media.

The big breakthrough of the study was looking at energy balance and intake on one-hour cycles. By zooming in, the study found evidence that biochemistry could start to short-circuit before the 24-hour bell sounded.

The study involved 25 female athletes on the national or competitive club level between 18 and 38 years old. Participants went through tests related to menstrual function and athletic performance while recording activity and food intake in detail. Within-day deficits were classified as any 1-hour window where energy deficit exceeded 300 kcal. There were three big conclusions:

One: athletes with menstrual dysfunction had higher amounts of within-day deficits, even though total daily energy intake was not statistically different

Two: higher within day deficits led to suppressed resting metabolic rate and lower estrogen levels

Three: higher within day deficits led to higher cortisol levels

Those conclusions tied back to the big questions that motivated the study. First, the reduced metabolic rate from energy deficiency may keep body composition stable, even as the endocrine system changes and health is impacted. In fact, a 2000 study in Medicine and Science in Sport and Exercise found less favorable body composition as within-day deficits increased.

Second, the authors theorize that within-day deficits can account for menstrual dysfunction due to hourly hormone pulses during different phases of the menstrual cycle. To boil it down, the brain often thinks in terms of calendars, while biochemistry is using a stopwatch.

Four important caveats.

One, it’s an observational study of top athletes that relies partially on self-reporting (a 2018 article in the International Journal of Sports Nutrition and Exercise Metabolism went over some of the pitfalls of calculating energy availability).

Two, the results may not apply to other cohorts of athletes or across a broader population. In particular, post-menopausal women may have different considerations.

Three, the athletes all spent time in an energy-deficient state, but the athletes with menstrual dysfunction had 24 percent more deficient time. So it’s not about avoiding all gaps, but avoiding too many, which likely has individual variance, and may be subject to confounding variables.

Four, there was no difference in the largest hourly gap between the groups. Thus, transient energy deficits may not disturb reproductive function.

What does it all mean? 

It likely varies heavily on the individual female athlete, with the general point that eating enough means avoiding restriction on short time scales too. That includes avoiding fasted training in most cases. But a counterintuitive study finding may be instructive to take it a step further. Athletes with higher within-day deficiencies paradoxically had higher eating frequency.

These higher-eating-frequency athletes were eating often, but food with lower energy density. As the authors say, “Other dietary characteristics, behavioral attitudes, and taboos toward certain foods, including carbohydrate-rich foods, fats, and energy-containing beverages needs to be evaluated when counseling athletes at risk for energy deficiency.” In other words, safe snacks may obscure underlying issues when there are larger deficits from training and life.

There is no ‘one-diet-fits-all,’ but there is a principle of ‘not-enough-calories-hurts-all,’” Keely says. “I have found a lot of success with my athletes and friends when we collectively celebrate days we fuel properly. Food should be a sense of joy in our lives, not a sense of stress.

While the study doesn’t make broader conclusions, reading the parallel findings together with the 2019 review study on low energy availability indicates that increased within-day deficits are likely associated with other issues like reduced adaptation and reduced bone health.

Keely Henninger is a coach and Nike athlete who helps so many athletes in this area, and she has a great summary. “There is no ‘one-diet-fits-all,’ but there is a principle of ‘not-enough-calories-hurts-all,’” Keely says. “I have found a lot of success with my athletes and friends when we collectively celebrate days we fuel properly. Food should be a sense of joy in our lives, not a sense of stress. It allows our bodies to continue to do the amazing things we all do.”

“So eat up. Your body will thank you later.”

A 2018 study in the International Journal of Sport Nutrition and Exercise Metabolism looked at the question for male athletes.

It started from the same basic premise—thermodynamics and endocrine response don’t work on 24-hour cycles, so why should our view of energy intake?

The study had 31 participants that were classified as trained or well-trained between 18 and 50 years old. Similar data was gathered as the study on female athletes, but without the reproductive variables. Within-day energy deficits were classified as exceeding a 400 kcal deficit.

The male athletes with suppressed resting metabolic rate spent more time with within-day energy deficits. Larger single-hour energy deficits were associated with higher cortisol and a lower testosterone:cortisol ratio.

The authors theorize the problem gets back to catabolic processes when the body has excessive within-day deficits. That can affect endocrine levels, “which may reduce the ability to recover and increase the risk of overreaching and overtraining, thereby compromising athletic performance.” The study is subject to similar methodological limitations as before. In particular, these findings don’t mean that athletes needed to avoid all deficits, just too many too often.

For transgender athletes, these considerations could be slightly different, since a transgender/nonbinary person taking hormone-replacement therapy receives hormones through a different mechanism than a cisgender person (though the responses to stimuli are likely similar).

There are tons of questions left to answer in future studies for all athletes.

Do athletes adapt to different dietary approaches over time? How much does type of food play a role? What do the findings look like for female athletes over 40? Female and male athletes over 50 and beyond? Transgender athletes? How about athletes that train less or differently? How about those that are advised by a doctor to lose weight for health? And how can I save 15 percent or more on car insurance?

But those questions are for another day. For now, I’ll leave you with this anecdotal observation based on talking to experts like Kylee and Danielle and Keely, and coaching serious athletes over many years. If I had to distill what I have learned on the subject from seeing athletes progress (or regress) long-term, my takeaway for athletes would be pretty simple.

Eat enough, always. Eat too much, sometimes. Eat too little, never.

David Roche partners with runners of all abilities through his coaching service, Some Work, All Play. His book, The Happy Runner, is about moving toward unconditional self-acceptance in a running life, and it’s available now on Amazon.

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