Can Undereating Causing Weight Gain?

This is a question many dieters sincerely ask themselves. Despite their best efforts to severely restrict calories and lose body fat, the scale doesn’t budge or even goes up. They feel certain they fall into the category of people whose bodies hold onto weight when calories are too low.

The notion that undereating paradoxically leads to weight gain is a frustrating and confusing belief. It leaves dieters feeling powerless in their weight loss journey, unsure of what actions to take. Many become discouraged and abandon their efforts, leading to disappointment.

Yet the idea that undereating causes weight gain, as intuitive as it may seem, stands in contradiction to the basic science of energy balance and metabolism. It is a myth that sabotages many dieters’ progress towards their health goals.

In this article, we will explore why undereating alone does not lead to fat gain. We’ll look at the common pitfalls that can stall weight loss, as well as more effective, sustainable strategies for shedding pounds.

With a deeper understanding of the dynamics of weight management, you can troubleshoot frustrating plateaus and rediscover a path toward success. Losing those stubborn pounds relies on changing behaviors and mindsets, not just slashing calories.

The Confusing Comment

If you’re a coach, you’ve probably experienced this.

A client or potential client comes in extremely frustrated at their lack of progress. And then they put forth a radical hypothesis and say:

“I think I’m putting on weight because I’m undereating.”

They then go on to explain that believe that repeated bouts of dieting have left their metabolism “damaged” or “broken.” They’re confused because eating fewer calories is supposed to correlate to losing weight. But now if they eat fewer calories, they gain weight? They’re worried they’ve entered some type of starvation mode.

And if the frustration and confusion weren’t enough, this idea has left them disheartened and disappointed, as though their previous attempts to get healthier have actually left them in a worse-off position.

You are compassionately skeptical of this claim. After all, undereating would mean a caloric deficit which should lead to weight loss, not gain. No one can violate the laws of thermodynamics, right?

You gently probe further. They show you their food tracking app, where you see they’ve consumed around 1000 calories per day for the past four days straight. Yet when they weighed themselves, the scale was up from their measurement from last week.

What is going on here? Is eating too little going to cause someone to gain weight?

Let’s explore this further and see what we can learn and how you can address this delicate issue.

What About Those Pesky Calories?

It’s old hat, but the role of calories in weight management has to be addressed.

At its core, the first law of thermodynamics tells us that weight gain or loss is determined by the number of calories consumed versus the number of calories burned.

While this calorie math may seem simplistic on the surface, its validity has been proven time and again through rigorous scientific research.

Likewise, studies of overfeeding demonstrate that a caloric surplus leads to fat gain in a dose-dependent manner. Differences in macronutrient proportions impact health markers, but calories are the ultimate driver of energy balance and weight change.

No matter what someone’s diet composition or exercise regimen is, they cannot violate the laws of thermodynamics. The human body is a complex adaptive system, but still bound by these physical principles.

In short, no one can synthesize new adipose tissue if they expend more energy than they consume. A sustained caloric deficit inevitably manifests as losing weight.

While factors like metabolic adaptation, hormone levels, genetics, and lifestyle behaviors absolutely influence rate of weight loss and overall health, they cannot fully prevent weight reduction that is congruent with the energy deficit. The calorie balance equation is the fundamental determinant of fat loss or gain.

When inquiring why weight loss has stalled despite diligent efforts, an objective look at caloric intake versus expenditure provides important clues.

What About the Diligent Food Tracking?

Despite the sincerest efforts, it’s impossible to be 100% precise with measurements. Small inaccuracies and margins of error creep in, even if clients are tracking diligently.

Calorie counts on labels can legally be up to 20% off their stated amounts. The calories in a given food item can vary based on size, density, cooking methods, and more. Portion sizes are notoriously hard to estimate by sight alone.

So, even if the client is actually weighing and measuring everything they eat, carefully preparing their own food, never dining out, and legitimately tracking every morsel they put in their mouths (which would amount to be the rarest of all clients), there is still a margin of error.

If not losing weight as projected, carefully reevaluate the counting methods.

What About the Scale?

The scale is the ubiquitous tool used by coaches and clients alike. Although it’s a bit of a blunt instrument, it can show a trend toward weight loss or weight gain over time. But it can be misleading in shorter time frames and here’s why:

Daily Weight Fluctuations.

Before assuming that undereating is leading to weight gain, it’s important to understand normal daily fluctuations in weight. Body weight is a dynamic measurement that can vary up to 2% (or more) day-to-day based on factors like hydration status.

If someone weighs 150 pounds, their weight may naturally fluctuate by 3 pounds in either direction. These changes represent changes in water balance, not actual fat tissue.

So let’s boil that down to a real-world situation. If as their coach, you weigh them on a day that they happen to be 153 lbs, through no fault of your own, you look like a bad coach. If, however, you catch them on a 147 lbs day, you look like a great coach even though you’re not responsible for it.

And if that weren’t enough, all too often clients let these transitory weight fluctuations dictate their mood, 147 lbs equals happy while 153 lbs equals frustration even though nothing has really happened.

Short-term weight fluctuation is often just shedding water.

Okay, so what would cause those short-term weight loss or weight gain readings on the dreaded scale?

There are several reasons. Eating carbohydrate-rich or salty foods causes the body to retain more water. Hot weather and exercise cause increased sweat output and water loss. A lower number on the scale may just mean you are dehydrated, not that fat was lost.

Likewise, higher scale numbers may simply reflect greater hydration, not actual fat gain. Women also retain water at different points in their menstrual cycle. Digestive waste and undigested food also contribute to normal weight fluctuations.

Therefore, it’s important not to overreact to daily weigh-ins and instead have multiple data points to track over time so both you and your client can see the overall trend.

Before making drastic changes to diet due to a perceived weight gain, get several measurements over a week or two. Look at the range and trends over time. This gives a more accurate picture and helps rule out hydration shifts as the cause of weight change.

What About Metabolic Damage?

Some argue that repeated dieting or severe calorie restriction damages your metabolism, causing it to burn far fewer calories than it should at a given weight. Experienced coaches have heard many clients ask about a “broken” metabolism. Proponents of this idea claim that once this “metabolic damage” occurs, it becomes nearly impossible to lose weight through calorie reduction.

Metabolic Adaptation

However, metabolism is dynamic and adapts both up and down to changes in energy balance, activity levels, age, and other factors. As sports scientist Dr. Mike Israetel explains, “Metabolic adaptation is a real phenomenon, but metabolic ‘damage’ or harm is likely not.”

The metabolic slowing that results from an energy deficit and weight loss can persist for a while after returning to higher calorie intake. However, metabolism will eventually normalize again if weight regain occurs.

As Israetel summarizes, “There are no studies that find a permanent, deleterious change in biomarkers of metabolism that do not return to normal levels after substantial weight regain.” In other words, metabolic changes are adaptive effects, not permanent damage.

So while metabolic adaptation is real, it does not fully prevent further weight loss, nor is it permanent. With continued caloric deficit, metabolism will readjust over time to continue burning fat, although at a slower rate. Metabolism is malleable, not damaged.

Evidence from Metabolic Wards

For those non-believers out there, there is more evidence.

Skeptics of “calories in, calories out” often argue that controlled lab studies don’t reflect the complexities of real-world weight loss. However, metabolic ward studies provide irrefutable evidence of this principle.

In these studies, participants are admitted to a research facility where every morsel of food is precisely measured and calorie intake completely controlled. Under continuous medical supervision, participants are placed on fixed calorie diets or fed different amounts.

Over a century of metabolic ward research shows that when calorie intake is reduced below expenditure, participants always lose weight. No instances of caloric restriction leading to weight gain have ever been demonstrated in these tightly controlled settings.

While metabolic adaptation causes somewhat less weight loss than predicted mathematically, a caloric deficit invariably leads to fat loss. This proves that being in an energy deficit is the driver of weight reduction, regardless of metabolism. Undereating does not cause weight gain when calories are precisely tracked.

So what does that uncover?

Undereating alone does not cause weight gain – but it can lead to overeating which does.

So What Is Actually Going On?

Let’s revisit our client and their claim that undereating is causing their weight gain.

They’ve shared the data from their food tracking app and it looks like they’re in a caloric deficit.

Armed with the previous information in this article, you press on.

They insist something must be wrong with their metabolism or hormones. But with an empathetic curiosity, you ask them to elaborate on what their weekends look like.

The client then admits on Fridays through Sundays they give themselves more “freedom” to “live a little!” This means frequent restaurant meals, drinks, appetizers on game day, and indulgent snacks.

Now the full picture comes into focus. While they have summoned immense willpower to get through deprivation during the workweek, their weekend indulgences likely erase that hard work.

Think Weekly Average, Not Daily Deficit

When trying to lose weight, it’s easy to become hyper-focused on daily calorie intake. Yet fixing attention solely on being in a deficit each isolated day can backfire. Our metabolism and hormones don’t completely reset every 24 hours.

Our bodies operate on a longer-term rolling average when it comes to food intake and energy expenditure. Focusing on weekly averages can be far more insightful than rigid daily targets.

For our example, the client may indeed be eating 1000 calories Monday-Thurday (a daily deficit) but then 3000 calories Friday, Saturday and Sunday (overeating).

While they may feel deprived during the week, their weekly average intake is around 1850, which in this case exceeds maintenance levels and spills into caloric surplus, which is why the client is gaining weight.

A Better Approach

A more sustainable approach is to determine your target weekly calorie average needed to lose weight. Then aim to spread these calories relatively evenly throughout the days of the week.

Periodic low or high days are fine, but the broader average and distribution matters more than isolated days of restriction. This builds consistency without deprivation and reduces urges to binge.

As their coach, you can delicately educate them on why undereating is not causing their weight gain. You explain that it comes down to total weekly calories, not just the extremes of restriction or indulgence on given days. Building this awareness empowers them to find solutions.

You can compliment their dedication through the weekdays. But you suggest focusing on more balance day-to-day versus rigidity and deprivation followed by weekend excesses.

You suggest creating strategies that align with their schedule, resources, and environment so they can sustain them through the week and long term. With support and new strategies tailored to their lifestyle, they can get off the yo-yo dieting rollercoaster and build lasting healthy habits.

The Power of Behavioral Science

While an understanding of nutrition and exercise physiology is crucial, knowledge alone rarely leads to sustainable change. Successful weight loss requires behavioral strategies that make healthy habits easy to stick to long-term.

This is where leveraging insights from psychology and behavioral science becomes invaluable.

As a coach, identifying each client’s unique barriers, motivations, preferences, and environmental influences allows you to tailor solutions that work for them. This type of education can allow you to guide your clients to sustainable change instead of the pendulum swings between deprivation and excess.

Guiding clients towards self-awareness of why they engage in certain eating patterns enables you to collaboratively develop replacements aligned to their personality and context. This creates healthy habits that are intrinsically rewarding and can withstand internal and external obstacles.

Sustainable change requires meeting people where they are and customizing approaches that align with their personality, resources, preferences, and environment. Behavioral science offers a roadmap to make healthy lifestyles satisfying.

The Value of Behavior Science-Powered Coaching

For personal trainers and coaches looking to maximize their clients’ success, the Behavior Science-Powered 360 Wellness Coaching Certification delivers the skillset you need.

This powerful certification equips you to guide clients toward sustainable habit change by integrating behavioral science with health coaching.

You will learn how to:

  • Uncover the root causes driving each client’s behaviors using assessments and exploratory skills
  • Design personalized wellness plans tailored to their unique personality, preferences and environment
  • Apply evidence-based strategies for motivation, habit formation, and overcoming setbacks
  • Leverage psychology to make healthy lifestyles intrinsically rewarding

This transformative certification delivers the techniques to help your clients overcome obstacles, embrace new behaviors, and achieve lasting results. Meet them where they are and empower real change through compassion and psychology. Learn more about the Behavior Science-Powered 360 Wellness Certification here.


Frustration and confusion over stalled weight loss efforts often arises from myths about metabolism and undereating.

Sustainable weight management relies on understanding energy balance along with tailoring personalized behavioral changes.

While calories alone do not provide the full picture of health, their role in weight gain and loss cannot be overlooked.

Of course, rigid calorie restriction is just one part of successful weight management. Monitoring totals should be balanced with diet satisfaction, healthful nutrition, and lifestyle factors to ensure sustainability.

Equipping clients with self-awareness, habit strategies, and coaching aligned to their individual needs empowers true lifestyle transformation.

Be guided by the science of nutrition, metabolism, and human behavior to help your clients succeed on their wellness journeys.