Calories In vs. Calories Out: Is Weight Loss/Gain That Simple? Part 1

Yes! Well, kinda. With the exception of this thing, and that thing, and well, it isn’t that simple. It is and isn’t at the same time (such is life, right?). But, luckily, with the new position stand by the ISSN on diets and body composition, which can be found here, we are able to get a clearer picture of weight loss and all of its complexities. I dove into the paper and this article is my conclusions about weight loss/gain based on the research that was presented and the position stand.

Weight change (loss or gain) is a process that can have many factors that influence the amount of weight that is added or lost including, but not limited to, diet composition, health status, and occupation. This combination of factors makes for a cocktail of confusion but, luckily, there is a general rule of thumb when we are talking about weight change-calories. Yes, I said it. The ol’ calories in vs. calories out adage does hold true for a large majority of people. We’ll go into more detail about this along with the changes your body makes to fight the imbalance of weight loss/gain.

Generally, if you want to lose weight, put yourself in a caloric deficit, consume less than you use to fuel daily activity, and vice versa for weight gain. Daily activity means every little thing you do from walking and bending down to pick up something to actual exercise. Here’s a term that will make sense of what I mean. Non-Exercise Activity Thermogensis (NEAT) refers to all the energy used for moving and activity you are doing that is not voluntary exercise such as walking, fidgeting, your job, or typing blog posts at 10:56pm on a Saturday. Keep this concept in mind as it will pop up again in a bit!

Here’s the tricky part, we’re humans. The ISSN puts it well by stating that “involuntary adaptive shifts separate humans from machines”¹. What this means is that we are not simply input/output machines. Unfortunately, we can’t just drop our calories below what we use for total daily energy expenditure and expect weight loss. Now, this may work for a period of time, but one cannot expect weight loss forever after dropping calories once.

There’s a term known as homeostasis that refers to the body’s desire to maintain a balance and stay where it’s at as much as it can. Your body will fight you throughout the process to remain in homeostasis because that’s what it’s used to and likes. Think about it from an evolutionary standpoint; our ancestors did not want to lose weight because food was so scarce. So, in that case, the slowest metabolism was actually desired because they could survive for longer with less food! Their bodies were so accustomed to holding onto energy and using it sparingly whereas now we want ultra-fast metabolisms to burn off all the calories and never gain weight. Our goals have flip-flopped with our great great ultra-great grandparents from survival to looking good, causing unwanted physiological adaptations that now work against us instead of with us². 

Over time, these were the people that survived and passed down their genes, which is why we have internal functions that fight us when we try to lose weight. Some of which include slowing down your metabolic rate (adaptive thermogenesis) so you burn less and, therefore, require less calories to be in homeostasis. Another method is to produce more of the hormones that trigger hunger, which may lead to consuming more than you want to satisfy your body’s instinctual need for calories and energy.

What about weight gain? Well, there has not been any evidence to show that adaptive thermogenesis exists when in a caloric surplus. This means that when in a surplus, your body does not appear to make any changes that may impede weight gain³ beyond the physiologic increases in energy expenditure that come with increasing intake and weight. Referring to that last sentence, your body does increase its metabolic rate when you consume more food. This is known as the Thermic Effect of Food (TEF). Ironically, it costs energy to get energy from food. Metabolic processes that occur to break down food in forms that it can extract energy and nutrients cost something. That is TEF.

One barrier to gaining weight if that’s your desired goal is NEAT among other things. If you’re an individual who has a labor-intensive job or are just moving a lot for work or other reasons, you may burn a lot of calories from NEAT, so you may need to increase calories more than expected initially if this is the case. For example, there was one study that put subjects in a 1000 calorie surplus. Some of the subjects burned upwards of 692 calories per day that was attributed to NEAT while others burned less than half of that4. NEAT is a factor that is highly variable between people so keep this in mind when planning to gain weight.

There’s so much to this equation of weight loss right? Did I just leave you more confused? Shit. Sorry. BUT! There’s some good news. For the average person trying to lose weight or gain weight, it may just be a matter of playing around with your intake and seeing what helps you towards your goals. So, in a very general way, yes. It is as simple as calories in calories out for weight loss/gain.

How do we ensure that the weight we loss/gain is the actual type of weight we want to lose, meaning, in most cases, fat? Just because you lose/gain weight doesn’t mean it’s all fat or muscle. How can we establish a caloric surplus/deficit in a healthy way without harming ourselves physically or mentally? This will be in the part 2 discussion of this topic. We’re not done yet! Thanks for reading!



²Oʼrourke R. Metabolic thrift and the genetic basis of human obesity. Ann Surg. 2014;259(4):642–8.

³Joosen A, Westerterp K. Energy expenditure during overfeeding. Nutr Metab (Lond).

4Levine J. Role of nonexercise activity thermogenesis in resistance to fat gain in humans. Science. 1999;283(5399):212–4.

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