What is Within Day Energy Deficit?

Over the past decade or so that I’ve been educating and coaching women athletes, the scene has exploded. 

There’s been a real democratization of the information available to and methods used for coaching women to find and develop their athleticism.  

Unfortunately, a solid and coherent understanding of the need to eat enough to fuel such health and performance goals just hasn’t kept pace. 

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And in far too many instances, the popular dietary paradigms have worked directly against goals such as gaining strength and muscle. (Intermittent fasting is one example of that.)

Over this same period of time, there’s been an evolution of sorts around mainstream nutrition advice for active and athletic women. 

The focus was, and still is, ensuring that individuals are eating enough. 

Even today, step one of sports nutrition coaching still focuses on ensuring an adequate energy intake – just throw calories at people – before then getting into how those calories are partitioned at a macronutrient level. 

Nutrition for Female Athletes

We can be much more nuanced in both the types and amounts of calories we are targeting. Specifically, consider balancing protein energy needs against non-protein energy (carbohydrate and fat) needs.

In my experience, many athletic women just don’t have a good concept of what “enough” is, both in terms of calories and actual food eaten. 

All too often, active women who train a lot default to calorie intakes barely sufficient for children, let alone very active adults. 

We can also take a more nuanced approach to the specific timing of this energy across the day. 

What is Within-Day Energy Balance?

It is becoming increasingly apparent that how you distribute your energy over a day is as important as making sure you’re eating enough in the first place. 

Put another way, you can be eating enough, as assessed over a 24-hour period.

However, if your within-day energy balance is off – you spend too many hours within a day in energy deficit – there can be negative impacts on your health and performance.

I regularly see and coach women who are in a state of low energy availability (LEA). As a result, they’re at risk for relative energy deficiency in sport (RED-S). 

A low energy availability occurs when energy intake, after accounting for the energy used during intentional physical activity, is not enough to fully meet the remaining needs of the body.

These basic needs consist of gut function, bone health, maintenance of muscle mass, and reproduction among others. 

Function, maintenance, and regeneration of these systems downgrade.

A Useful Analogy for Within-Day Energy Deficit

Let’s create an analogy to help you understand how within day energy deficits work.

Think having insufficient charge in your phone’s battery.

It automatically goes into power saving mode, reducing function and performance so that there is sufficient battery charge (energy) to support basic functions for slightly longer. 

Starting the day with your phone fully charged and ending the day with your phone fully depleted doesn’t tell you much about what happened in between.  

Its charge might drop from 100% to 20% in a very short space of time. Then, it might spend the rest of the day in power saving mode – with reduced function and performance – until you charge it again.

This is a different scenario than a fully charged phone that has its charge slowly decrease over the day. It maintains full function and performance throughout the day, requiring a relatively minor top up at night. 

If you simply looked at the ‘energy in, energy out’ picture after 24 hours, you’d miss the important ‘within day’ details.

Image Credit (1)

What Daily Energy Totals Miss

Typically, when assessing energy intake and energy output, either for research purposes or for your own self-tracking, you’ll add up your 24-hour energy intake and energy expenditure totals. 

But this 24-hour assessment doesn’t allow you to see what is happening within the day. 

In my background reading for the writing of this article, I saw a great quote which I’ll paraphrase here: You measure your day with a calendar, while your body is measuring its responses to that same day with a stopwatch.  

Your body is constantly making real-time metabolic adjustments to keep everything on an even keel. 

The typical 24-hour, end-of-day type assessments made tend not to take into consideration such real-time responses that can occur with large energy surpluses or large energy deficits.  

Image Credit (1) (simplified for clarity)

For example, an individual who eats a large dinner meal but very little over the day leading up to it – a common pattern in typical 16/8 intermittent fasting – may spend most of the day in a significant energy deficit, despite appearing to be in 24-hour energy balance.

In other words, even though there might be energy balance by the end of the day, very large chunks of the day might be spent in negative energy balance.

“Such an eating pattern would produce low blood sugar during the day, which would result in an elevated serum cortisol that is associated with low bone mineral density, catabolism of lean mass, and reduction in estrogen production. 

To compound these problems, the low blood sugar is likely to result in a hyperinsulinemia response at the next eating opportunity, which is associated with increased fat storage.

In addition, the appetite stimulating hormone, ghrelin, is turned off with a normoinsulinemia, but remains elevated with hyperinsulinemia, resulting in greater food consumption than normal.

None of these possible outcomes could be adequately predicted through a 24-hour energy balance assessment, and none are desirable outcomes for the athlete.” (2)

The above quote is an excellent summary of what I hear almost every day in athletic women, including female athletes over 40. 

Note: I often see women athletes fail to meet an end of day energy balance with 16/8 intermittent fasting. Often, you struggle to eat enough in the 8 hour fueling window.

And if you’re doing intense training and not refueling until hours afterward, you’ll stay in a catabolic state for even longer. This is especially unfavorable for women in peri-menopause or post-menopause because low estrogen means you’re already more catabolic.

The reason some people experience changes in weight while intermittent fasting comes down to creating a caloric deficit.

Female athletes need to be very cautious about introducing large caloric deficits due to the implications of LEA and RED-S.

How Within Day Energy Deficit and Intermittent Fasting are Related

It’s important to understand the implications of within day energy deficit. For female athletes, extended fasting – particularly in the first half of the day – is a mismatch.  

The day starts either not eating enough (or not eating at all if fasting). 

The energy deficit this creates requires higher cortisol levels to prop up the energy of the system. Cortisol stimulates energy production, including from the breakdown of the protein in your muscles.  

When you do eat, especially if it’s an energy dense meal, insulin levels are likely to rise higher than normal in response to that meal. Some of the energy consumed may be partitioned to fat storage.

This is the very thing many women are trying to avoid by fasting. Many athletic women believe that fasting is the ticket to lose weight / body fat or prevent weight gain.

Appetite hormones are stimulated, resulting in eating more food than you might otherwise, potentially further adding to fat storage.

This can also take a psychological toll because you might feel you have no willpower. Or you might feel guilty or shameful at your perceived “lack of control” around food.

And because the numbers “add up” to some preconceived 24-hour energy balance total, it’s common to be upset at your lack of progress.

This further perpetuates the trap of trying to eat less calories per day, fast longer, and / or exercising more to drive weight loss. 

What those 24-hour numbers do, however, is mask the real problem, which is the within-day energy deficiency.

What Does Research Say About Within-Day Energy Deficit?

“Traditional multiple day energy balance and energy availability assessments can mask the energy deficiency by the compensation effect, while within-day energy balance can detect the hidden energy deficiency with more physiological relevance…” (3)

A 2017 study examined this masking effect of within-day energy deficits in women athletes, specifically looking at their menstrual function and athletic performance. 

In women with higher within day energy deficits, estrogen levels were lower and cortisol levels higher. The women also showed signs of a suppressed metabolic rate.

Perhaps most surprising is that when the women with menstrual dysfunction were compared to those without, there were no differences in the total energy consumed over a 24-hour period. 

But those women with menstrual dysfunction did have larger within-day energy deficits. (4)

Compounding the within-day energy balance issue is what I’ve termed an absence of carryover effect. 

This is where you ran an energy deficit in the previous 24-hour period. But because you’re now into a new 24-hour period (the next day), you wipe the slate clean from the previous day.  

What Role Might Over-Eating Play?

There’s also a tendency to use the next day to compensate for any sensation of overeating in the hours before bed. 

Due to the recency effect, you might see that last meal(s) in the hours before sleep as representative of the whole day. 

There still might be an overall energy deficit in your day. But because your last sensation of the day is being overfull, you think you need to cut back on what you eat the next day to compensate.

In reality, this creates another round of energy deficits.

Despite feeling – based on a single most recent meal – that you’ve overeaten, your energy deficit from the previous day still hasn’t been corrected. 

For example, let’s say you needed to eat 2300kcal in a day, but came up 600kcal short.

You start the next day aiming, once again, to get to 2300kcal.

However, in reality, you might now need to get to 2600kcal over a couple of days – an additional 300kcal per day – to clear that 600kcal energy deficit.

Also, a protein energy deficit will have a different effect to a carbohydrate energy deficit. Both are likely to leave you feeling flat and sluggish.  

If this pattern occurs long term, pushing resets daily without making up for the energy shortfall, the deficit can soon stack to up to missing an entire day of eating.

The Drained Battery Analogy

So what does this within-day energy deficit look like in practice on a daily basis? 

Let’s go back to our analogy of a phone. 

At midnight your phone is at 100% charge and sitting idle running on its battery. 

Running on standby overnight drains only a small amount per hour. Let’s say 5% drain per hour for 6 hours.

At 1am, the phone has 95% of its energy available. 

Then 2am, the battery is at 90%, and so on. 

By 6am, the phone is sitting at 70% “energy availability.” Now you’re awake and actively using it. 

Between the alarm function, clearing messages, checking email, doing your social media scroll, and playing music while you make breakfast, the charge drops 20% in an hour.

Realizing you’re about to step out the door with only 50% charge, you quickly plug the phone in. This adds 5% to the battery before you have to leave. 

You listen to the Fuel Your Strength podcast on your morning commute and quickly call your mom. This reduces the available energy even more. 

Across the day, you use your phone a lot.

By mid-afternoon, the battery drops below a critical threshold – let’s say 20% energy availability. At this point, power saving mode kicks in.

This slows the performance of the phone and limits its capacity. You’ve hit low power mode.

You manage to nurse the charge until you’re home and can plug it in. This adds, in one big hit, 80% more energy.

You’re finally back at full charge.

What Does Within-Day Energy Deficit Look Like?

This analogy, though imperfect, is similar to what happens when you underfuel across the day. 

Again, you might be taking in a large enough number of calories to get you to energy balance for the day while spending large portions of the day in an energy deficit.

As you sleep, you’re naturally fasting, and you drift into an energy deficit. It’s small at first, but grows as the night rolls on. 

You wake up and start your day, and your energy expenditure increases. And if you don’t eat enough, this energy deficit gets even larger.

If you have a very small breakfast – or worse, no breakfast and just coffee – the energy deficit grows and can reach a critical threshold. This critical threshold is thought to be around 300kcal in women (4) and slightly higher in males.

At this point, your body starts making metabolic adjustments as it attempts to prop up the system and prevent energy depletion. 

A small, low energy density lunch and a snack barely lift your average energy levels up above the minus 300kcal critical threshold for long.

Yes, your body can handle dipping below the critical threshold from time to time. But when chronic low energy availability or within day energy deficits occur, it’s time to take notice.

Whether you train early in the morning (fasted) or later in the day, the effect is the same: it dramatically pushes the energy deficit into negative territory. This is a hole you may struggle to dig yourself out of across the day.

By the evening, you’re tired, hungry, and now have full access to the fridge, pantry, and food delivery apps. 

If you eat a massive meal at the end of the day because you’re ravenously hungry, the total amount of energy eaten in a relatively short period of time overwhelms a system that has already made several hormonal and metabolic adjustments. 

Chronic deficit across the day has turned into an acute positive energy balance at the end of the night. Remember, we’re looking at what’s happened hour-by-hour across the day, not just the end of day / 24-hour total.

This pattern can drive a state of metabolic suppression. 

This is where several hormonal changes can occur, including reductions in estrogen and testosterone, and higher cortisol levels, reducing resting energy expenditure (thus reducing the background energy drain), reducing muscle and bone mass, and creating menstrual cycle disturbances. (5)

But What About Energy Deficits for Fat Loss?

“But isn’t an energy deficit needed if I want to lose body fat?”  

Though I don’t coach intentional fat loss, I’d be naive to think that athletic women over 40 aren’t pursuing it. However, it may not even be necessary or a wise pursuit when performance is the primary goal.

And often times, women athletes go about it completely the wrong way, getting frustrated when those attempts backfire.

As I’ve previously written, you need to make a distinction between different types of energy. 

One thing you certainly want to avoid are large and persistent deficits in your protein energy. 

Yet this is exactly what can happen when you skip meals, have extended fasts, or simply keep high-quality protein serving sizes small (or avoid such foods altogether).  

The positive benefits of eating an optimal level of protein are best obtained by a steady distribution across the day. (6)

You can’t obtain these same benefits by eating only 1-2 large protein meals each day.  

Likewise, in most cases, you’d be better served by ‘front-loading’ more of your total energy earlier into the day. 

This would mean a more even distribution of the food / energy you consume across the day. Eating relatively more earlier in the day and relatively less later in the day is one approach.  

Mild energy deficits are more sustainable and achieve better longer term results when they accumulate gently across a day. This is because it won’t trigger the compensatory hormonal and metabolic adjustments mentioned previously in this article.

Natural deficits which can occur during physical activity, improving satiety with higher protein intake, and biasing toward fiber-rich carbohydrates are much more wiser ways to create a slight energy deficit for sustainable fat loss. See my article on metabolism and energy flux to dive deeper into this topic.

Can You Have Within-Day Energy Deficit Even If You Eat Enough?

“But I do eat enough!”

This is something I hear often, especially from those women whom I suspect are underfueling themselves. There are two traps here.  

1) The already discussed 24-hour total masking the large energy deficits occurring within a day.  

2) Confusing eating frequency with energy density.  

While eating more frequently might be a practical strategy for overcoming within-day energy deficits, a paradox exists within the research examining this issue. 

In some populations studied, a higher meal frequency was associated with greater within-day energy deficits. (4)

The authors noted that this may be due to athletes consuming low energy density foods or “safe snacks.” This can look like:

• Filling up on a lot of fibrous, raw vegetables
• Adding a lot of fiber to smoothies to bulk them out and keep them low calorie
• Snacking on low carb bars
• Eating too many low calories foods

(Note: The concept of “safe foods” also occurs in some forms of disordered eating and eating disorders. If you need resources for this, please see NEDA.)

It can feel like you’re eating a lot because:

a) your numbers at the end of the day “add up”,

b) you are eating frequently across the day, and

c) your last meal of each day (the one most recently in your mind when you ask yourself the question “did I eat enough today?”) is likely your largest.  

But even with all of these factors taken into account, you could still be running a substantial energy deficit across the day.

Nutrition is Complicated, Even for Athletic Women

I get that this information can be challenging for many. It was for me once, too. 

As an athletic woman over 40, you’ve have been fed a diet (pun not intended) of statements around what and how you should eat.

These range from the popularity of waif-like supermodels of the 1990s to oversimplified advice like “eat less, move more” to fads, outright lies and misinformation.  

When it comes to nutrition, body weight and body composition, the issues are complex.

It’s not even something you can “simply” boil down to nutritional biochemistry.

You’re also dealing with diet culture and the common taboos around certain foods. Beliefs, fears and anxieties around food, weight / body mass, self-esteem, etc are intertwined.

My mission is to help athletic women over 40 like you fuel, train, and recover smarter. I want you to build fitness, strength, and muscle.

Because feeling stronger and more powerful translates into a more expansive life outside the gym. Strength is a catalyst for more.

Apply to work with me on your strength nutrition

Summarizing Within-Day Energy Deficits

In this article, you explored the concept of within-day energy deficit.

• Looking at energy balance, surplus or deficit at the end of the day is only part of the picture. It doesn’t take into account long periods during the day you might be in an energy deficit. This is within-day energy deficit.
• The threshold for within day energy deficit in females is about 300kcal. When your energy baseline drops below -300kcal, this is territory for within day energy deficit. Small infrequent deficits at or slightly below 300kcal are fine. But when they become frequent and extended, as they often are for many women, negative consequences arise.
• When in a state of within-day energy deficit, especially the type seen during 16/8 intermittent fasting, metabolic and hormonal changes may occur to manage energy. These can include elevated cortisol, body fat storage, and overeating.
• Over-eating during the last meal of the day and grazing on low energy snacks can both provide a false sense of having eaten enough while still running a sizable within day energy deficit.
• If athletic women truly need fat loss, avoid fasting. Don’t skip meals for many hours during the day and pay attention to protein intake. Avoid eating the largest meal of the day later in the evening after fasting or eating very little.
• For best results, eat regular meals throughout the day with adequate protein-energy (and non-protein) energy. This includes quality protein, fiber-rich carbs, and fats. Pre-workout and post-workout meals and snacks can also help reduce the amount of time spent in within-day energy deficit.

Other Articles on Energy Intake

• What is Low Energy Availability (and How to Avoid It)
• Low Daily Energy: How Under-eating Affects Motivation & Training
• Calories In, Calories Out – What’s the Deal?
• Metabolism – How Does it Work?
• Metabolism Over 40 – What Works for Athletic Women

References

1. Benardot, D. (2013). Energy Thermodynamics Revisited: Energy intake strategies for optimizing athlete body composition and performance. Pensar en Movimiento: Revista de Ciencias del Ejercicio y la Salud, 11, 1-14.
2. Arroyo F, Benardot D, Hernandez E (2018) Within-Day Energy Balance in Mexican Female Soccer (Football) Players – An Exploratory Investigation. Int J Sports Exerc Med 4:107. doi.org/10.23937/2469-5718/1510107
3. Lee S, Moto K, Han S, Oh T, Taguchi M. Within-Day Energy Balance and Metabolic Suppression in Male Collegiate Soccer Players. Nutrients. 2021 Jul 30;13(8):2644. doi: 10.3390/nu13082644. PMID: 34444803; PMCID: PMC8398536.
4. Fahrenholtz IL, Sjödin A, Benardot D, Tornberg ÅB, Skouby S, Faber J, Sundgot-Borgen JK, Melin AK. Within-day energy deficiency and reproductive function in female endurance athletes. Scand J Med Sci Sports. 2018 Mar;28(3):1139-1146. doi: 10.1111/sms.13030. Epub 2018 Feb 5. PMID: 29205517.
5. Strock NCA, Koltun KJ, Southmayd EA, Williams NI, De Souza MJ. Indices of Resting Metabolic Rate Accurately Reflect Energy Deficiency in Exercising Women. Int J Sport Nutr Exerc Metab. 2020 Jan 1;30(1):14-24. doi: 10.1123/ijsnem.2019-0199. PMID: 31887723.
6. Jäger R, Kerksick CM, Campbell BI, Cribb PJ, Wells SD, Skwiat TM, Purpura M, Ziegenfuss TN, Ferrando AA, Arent SM, Smith-Ryan AE, Stout JR, Arciero PJ, Ormsbee MJ, Taylor LW, Wilborn CD, Kalman DS, Kreider RB, Willoughby DS, Hoffman JR, Krzykowski JL, Antonio J. International Society of Sports Nutrition Position Stand: protein and exercise. J Int Soc Sports Nutr. 2017 Jun 20;14:20. doi: 10.1186/s12970-017-0177-8. PMID: 28642676; PMCID: PMC5477153.