Calories In, Part I: What Happens When We Gain or Lose Weight

This month is all about the metabolism, and while last week dealt with “Calories Out,” today we’re talking about “Calories In” – how our metabolism (“calories out”) interacts with the food that we eat (“calories in”).

This can be a complicated topic, so I’m going to be splitting this post into two parts this week. Also, when ideas get complex, I like to use analogies to make them simpler. My favorite analogy, when it comes to calories, is spending and budgeting.

In general, our bodies do not like to lose weight, just like we don’t like to see our savings dwindle. Instead, we like to maintain weight by maintaining an “energy balance,” and our bodies also tend to take advantage of opportunities to store weight for future famine to ensure survival.

Now, let’s translate that into spending to help some concepts make more sense. If your living expenses add up to $2,000 a month, it’s important for financial health that you make at least $2,000.  If you make exactly $2,000 a month, you can pay your bills, but you can’t add to your savings, either. You would maintain your savings that you already have. You would not have to dip into your saved money to pay bills.

If your living expenses were $2,000 but you made $3,000, you would have the ability to save up resources for the future if you didn’t spend the money. You would be in a surplus.

If your living expenses were $2,000 but you made $1,800, you would have to dip into existing savings to cover your costs. You would be in a deficit.

Similarly, if you burn (on average) 2000 calories per day, if you eat less than that per day consistently, your body will dip into stored resources (fat) to cover your energy needs. If you eat more than 2000 calories per day consistently, your body will add onto its energy storage to cover future shortages.*

*Keep in mind that this “2000” number is a general number that I’m using as an example, not a rule – as I’ll discuss later, our needs are individual. 

The problem is that saving money is a good thing, but saving lots and lots of calories for future use is not. Why? Because when it comes to money, it’s quite likely that we will need the money. But when it comes to calories, it is unlikely that we will need the calories. This hardwired proclivity for storing up for future famine is unnecessary for most of us. For most of my readers, life is physically easy and food is not scarce.

In essence, we are not living in the environment for which we were hardwired.

But because this process isn’t 100% straightforward, I want to get into more detail about surpluses and deficits. Let’s start with what happens when you gain weight. 

Make More, Spend More

Think about when you are in college and have no money. You live on ramen and have a roommate. You make $2,000 a month at your job. But your living expenses are only $1,800, so you can save $200 per month if you don’t spend it. You’re not making or saving a lot of money, but you’re not spending a lot of money, either.

Now, let’s say you’re 30, you make $4,000 a month. Does this mean that you are saving $2,200 a month for an emergency? Probably not.

Why not?

When we make more, we tend to spend more, in addition to building our savings. Instead of a $500 apartment, we have a $1,500 apartment. Instead of eating ramen, we eat $10 poke bowls and go to brunch on Sundays. Our living expenses gradually increase to match our income.

The process of gaining weight is similarly dynamic. If you eat at a surplus and gain weight, something counterintuitive happens – your metabolism will increase. Even though you’re eating more, you’re also spending more, which means that your weight tends to increase gradually or reach a stabilization point, rather than ballooning out of control. This is the opposite of what most people think. We intuitively think that someone heavier, with more body fat, has a slower metabolism, when in reality, they are probably burning more calories per day than a lean person.

Remember, if you want to calculate your resting metabolic rate, your height and weight play a role. In general, the bigger your body is, the more calories you will burn on a daily basis to maintain your body weight. Your body takes work to maintain, and the more mass you carry, the harder you have to work. Climbing stairs is harder. Detoxifying your blood is harder. Everything is harder.

On a complete side note, this is why I find it so reprehensible that – culturally – we accept and even idealize the lean woman being a foodie and chowing down on burgers and nachos, while we quietly ignore or openly shame overweight women for making “unhealthy” food choices. It’s not only rude and presumptuous, but also illogical. 

But what about weight loss?

Make Less, Spend Less

In perhaps the cruelest twist of metabolic logic, the inverse is also true.

When you lose weight, your metabolic rate decreases as your weight decreases. If you made $4,000 a month and had a $1,500 apartment, and then lost your job, you would downgrade back to a cheaper apartment until you could get your income back up again.

This is what your body does, outside of your control, when you lose weight. This is called “metabolic adaptation.” If you burned 2200 calories a day at 180 calories and then lost 20 pounds, you automatically will lose a few hundred calories of burn per day. In other words, when your income decreases, you also cut back your energetic budget – what you “spend.” Your body takes care of this without your conscious knowledge.

If you went from 180 pounds to 160 pounds, your metabolic rate would probably drop to 2000 instead of 2200, which means that you permanently need to regulate your eating and exercise to adjust to this new metabolic rate and maintain your new weight. 

Furthermore, there’s another layer to metabolic adaptation, but because I’ll be discussing it more in later weeks, I’m only going to touch on it now. On top of the few hundred calories that you lose due to weight loss, your body will down-regulate your metabolism even further than the weight loss would seem to warrant, to prevent further weight loss. Remember, your body does not like to lose weight – it’s not optimal for survival. If caloric restriction is too sharp or consistent and/or if weight loss is too rapid, your body may think something is wrong, and sound the alarm, dampening thyroid function and increasing cortisol.

In other words, if you burned 2200 calories at 180 pounds, you should burn about 2000 calories at 160 pounds. But in reality, you may burn more like 1800-1900 after the 20-pound weight loss, because your body is guarding against further fat loss. Your body is unhelpfully slowing down to protect itself from starvation. This effect can last for years, and can be counterproductive to maintaining weight loss if you don’t approach this phenomenon with awareness and intention.

This is also why people with a higher percentage of body fat can – at first – lose weight very rapidly. When calories are reduced, their bodies can tap into the stored fat for energy very efficiently, without a problem – just like a person with lots of savings can lose their job and survive temporary unemployment without disaster. But as weight loss continues, the savings dwindle, and you reduce your budget to anticipate leaner times.

Many people sense this phenomenon as they lose weight, but can’t label it. It’s frustrating, because we intuitively think that leaner people should have faster metabolisms, when the inverse is actually true. It can be a rude awakening when you lose weight, and is partly what makes weight loss maintenance challenging.

This is why there are two aspects of fat loss that matter more than what diet you use or what workout gets the best results, and those two factors are individualization and habit formation.


Calories matter, because different people have different needs. Let’s create two different women who both want to lose 20 pounds.

Let’s go back to our example of a woman who is 180 pounds, and let’s add in details that she is 5’7″, 30 years old, is active (exercises vigorously five times a week), and has a healthy metabolism from consistent weight maintenance for several years. To lose weight at a steady rate, she probably would not have to dip her intake lower than 1800-2000 to effectively and noticeably get the scale moving.

But for a woman who is 5’2″ and 130 pounds, even if she’s equally active and has an equally (relatively) robust metabolism at 30 years old, getting the scale to move would require a much more significant dip in calories – a daily intake of probably around 1400-1500 on average.

Even though these women have the “same goal,” in reality, these two women are extremely different and should use different strategies to achieve their goals.

This is a problem, however, because most commercial women’s magazines suggest meal plans that sometimes don’t exceed 1300 calories per day. If you are 5’2″ and 130 pounds, this meal plan would be challenging to implement, but would induce weight loss at a moderate pace, perhaps without terribly affecting energy or cravings. But if you’re 5’7″ and 180 pounds, this would be beyond challenging, and would induce rapid and unsustainable fat loss.

A woman who needs to be eating 1800 calories per day to just feel “okay” will truly suffer on a 1200-calorie plan. She will be sluggish. Her performance at the gym and at work (and in life) will suffer. She may feel moody and emotional. The metabolic adaptation that happens as she loses weight will be tremendous. It is likely that her body will fight back against the weight loss by triggering binges and periods of overeating. She won’t be able to sustain it, and therefore won’t have a change to form new habits. 

This is why individualization is so incredibly important, and why vague goals of “eating properly,” “eating right,” or “eating clean” can be so damaging. Someone who idealizes “eating right” and ends up under-eating (for her needs) will live in a state of constant failure, unable to live up to her own standards.

We are all apples and oranges. Calorie reduction induces weight loss for everyone, but what number works for you is highly individual, and how you can strategize the achievement of that goal is also highly dependent on your unique lifestyle and energy needs. There’s no one “right” way to do it, and specific knowledge is empowering.

This is where the second factor becomes extremely important: permanence. 

Habit Formation

The easiest way to lose weight is to do a crash diet.

However, that’s not the easiest way to maintain weight loss, which in reality, is what people actually want when they say they want to lose weight. They don’t want to lose 20 pounds for a month – they want to lose 20 pounds and keep it off permanently.

To that end, it’s extremely important that your calorie goals are, ultimately, sustainable. The secret to sustainability is habits.

While temporary stretches of caloric restriction (2-4 days, for example) can help jumpstart weight loss, I think that it is much more important to become accustomed to your maintenance lifestyle, and preemptively build and practice routines supporting that goal. For example, if we use the first woman from my example above, let’s say she is 180 pounds at 5’7″, is 30 years old, exercises five times per week, and wants to lose 20 pounds.

If she types her numbers into MyFitnessPal, it is quite likely that she will pick a goal that is too aggressive, and will receive a calorie assignment of somewhere around 1500. Losing weight faster is better, right?

However, her maintenance number for 160 pounds (her goal weight) will be roughly 1800-2000 calories, depending on how much her metabolism slows in response to weight loss. 

This means that her strategy is – unknowingly – to completely change her eating from something like 2200 calories per day, to limit herself to 1500 calories per day until she reaches 160 pounds… and then completely switch it up again, increasing calories to 1800-2000 calories a day.

This is difficult for the average person to negotiate. It’s a lot of change – not just at first, but also once the goal weight is reached (if it’s reached). This is partly why people often bounce back up to their original weight, once they achieve their goal – they never made the habits that would maintain their goal weight, and so they never make the transition from loss to maintenance.

Therefore, it would be more intelligent for someone who is 180 pounds to learn to consistently eat “at maintenance  for 160 pounds, by eating 1800-2000 calories per day, than it would be to implement radical caloric restriction techniques to quickly lose 20 pounds, and then adopt an entirely new lifestyle all over again. 

If you restrict calories too tightly, not only do you start bumping up against your basic needs, but you also are not equipping yourself with the tools of maintenance. You’re not making habits – you’re just firing off temporary strategies that won’t fit your lifestyle once you actually achieve your goal. It is infinitely wiser to build the new habits while you lose weight, with the knowledge that you are going to keep using this repertoire of habits every single day, indefinitely.

Therefore, in “Part II” of this post, I’ll explore the purpose of calorie counting, how to leverage its benefits, how to sidestep its potential weaknesses, and how to keep all weight loss strategies in the context of long-term maintenance.

Follow-Up Questions from Last Week

If you want to jump into this free healthy living project (with weekly strategies to try out), sign up on this page to get these questions delivered directly to your inbox each week!

Last week in “Calories Out: How Does Your Metabolism Burn Calories?” we explored how your body uses calories for energy.

Answer the following questions about your experiences with last week’s post:

  • Question 1: Have you ever found yourself affected by fear-based journalism? Click-bait headlines? Scary health stories on the morning news?
  • Question 2: How could you get more steps in throughout the day, to increase your level of NEAT?
  • Question 3: How could you incorporate more exercise more consistently into your lifestyle?

E-mail these answers to me! 

Rachel Trotta

I am a Certified Personal Trainer, Fitness Nutrition Specialist, Physique and Bodybuilding Specialist, and Women's Fitness Specialist. I live in New Jersey in the NYC metro area, and I coach clients online all over the world. As a trainer and health writer, my mission is to make healthy living sustainable for the average person. I’m also a wife, mom, nature lover, runner, avid cook, weightlifting aficionado, history nerd, travel addict, and obsessive podcast listener. Get in touch!

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