skinny and chubby

Why Some People Get Fat and Others Don’t

skinny and chubby
image courtesy of James D Waller

For most people getting fat is a permanent condition that can’t be reversed.

Is it physiologically impossible to lose weight once you gain it?  No, of course not.

On some level our bodies are just a collection of different biochemical feedback loops which interact with one another that result in net weight gain or weight loss.   If you change the amount of energy your body receives and hormones it uses to utilize it then the scale will likely move in one direction or another.

But for many reasons losing weight is an extremely difficult task.  A survey recently done by Penn State University found that 86% of overweight people couldn’t lose weight and keep it off.

In addition to being difficult to reverse getting fat is becoming more prevalent.  Here’s a revealing chart from the CDC on changes in BMI since the year 2000:

obesity rates usa
Over 3/4 of adults are overweight or obese

More than 3/4 of adults over the age of 18 are either overweight or obese.  (Caveat:  The percentages for all the groups adds up to over 100, which is impossible for this sort of thing, so these numbers can’t be 100% accurate).

But what’s mysterious is why some people, despite living a similar lifestyle and eating similar foods, can have radically different weights.

Fat Behaves in Mysterious Ways

I used to chew on how people can have similar lifestyles but radically different weights when I was in college and went to parties.  (Oh, those were the days!  They seem like a distant memory at my ripe age of 29).  I’d run into a group of girls from my dorm off campus and they’d all be drinking scores of alcohol.  Most of them were really skinny but a few of them would be quite obese.  Logic would suggest that the difference in weight wasn’t because one girl was literally eating twice as much as the others.  I knew for a fact that they often ate together, and probably drank together too.  My subjective observation is that the heavy girls were more self-conscious about what they ate than the skinnier ones to boot.

So why the extreme variations in weight?

The best way to answer this question is to backtrack to some important observations about how the fat on our body works:

  • The amount of fat cells you have stays fairly constant.  Throughout your adult life you don’t gain or lose a significant amount of fat cells.  When you’re growing a spare tire what’s happening is that the fat cells you do have begin filling up, like when you blow air into a balloon.
  • The fat you have is regulated by hormones, not caloric intake.  Admittedly this point is up for debate, so I won’t be so strong as to say it’s 100% unequivocal fact.  It’s also not an either/or proposition…it has to do with both.  But after reading and contemplating the issue for about six years I’ve decided that it’s hormones (primarily insulin) and not the raw amount of calories you eat that primarily determines how tight your tummy is.  (Keep in mind that losing fat is not the same thing as losing weight, even though the two often go hand in hand).  This might seem like an opinion that’d only be held by a silly blogger trying to sound clever, but the idea is more intuitive than you think.

Anyone who’s been overweight can relate to the difficult feelings that come from watching someone who’s eating the exact same thing as you……….but they weigh 30 pounds less!!

What are they doing that’s so different than me?

The answer is that their body is regulating their “flow” of fat differently than yours, and have a lower propensity to store it.  Our third point illustrates why.

  • You lose fat when the rate of fat going out of your fat cells is greater than the rate at which they enter.  It’s useful to think of your fat cells as a bucket with a hole on the bottom.  Or a bathtub.   Your body has a set of hormones that activate the flow of fat into the cells and they simultaneously act to block enzymes that enable fat to leave it.  When there’s more fat going into your cells than going out your body stores fat.  If your body is shuttling fat out of its fat cells faster than it’s coming in your body loses fat.

What causes fat to go into and out of your cells? There are three different ways.

De Novo Lipogenesis.  This is a fancy way of saying that your body can make fat internally all by itself.  For most reasonably healthy people de novo lipogensis doesn’t move the needle very much.  It’ll comprise about 5% of the fat going into your cells. However, it might be more of a problem for people who are already obese and are overfeeding on carbohydrates.

Re-esterification. This is the fat you get from your diet.

Lipolysis.  Translated literally, this word means “fat breaking” in latin.  It’s an enzymatic process that’s turned off by insulin, and activated by enzymes like hormone sensitive lipase (HPL) and glucagon.  Lipolysis happens when your short term glucose stores have been used up and your body hasn’t received any signals that more is on the way through your mouth.

The Fat Burning Equation

This’ll be the only equation used in this post, but we can summarize the process of fat flow like this:

DNL + R = L.

That is, the amount of fat created internally by the body (DNL) plus the amount of fat digested through the diet (R) equals the amount of fat that’s released through the cells (L).  When your body does this the “flow” of fat into and out of your cells is the same and you’re not gaining or losing any fat.

(Note:  Anybody can be in fat balance.  This doesn’t say anything about how what kind of shape you’re in.  The above equation can just as easily apply to a bodybuilder as a couch potato).

For you to lose fat the rate of lipolysis (fat breakdown) has to be greater than the rate of fat going into the cells. We know that DNL (de novo lipogenesis) doesn’t move very much, except for extreme circumstances.  It’s also easy to see how the amount of fat you get from your diet (R in the above equation) fluctuates according to what you eat.

What’s interesting is how your rate of fat burning (the “L” in the above equation) changes.

You probably know someone (perhaps yourself?) that’s been very frustrated with the lack of results in their diet even though they’re eating very little and exercising a lot.

The reason is that despite a reduction in calories your body isn’t mobilizing its own fat for energy, which is an enzymatic process regulated by hormones. The processes your body uses to activate its fat utilization pathways is long and complicated, but it’s broadly accurate to say that it’s primarily regulated by the peptide insulin.  When insulin is secreted from your pancreas after a meal a cascade of switches is turned on that allows your body to sweep triglycerides into your fat cells and silences the enzymes and pathways needed to release them into your blood.

How People Typically Lose Weight

When people successfully lose weight they’re usually using one of two methods:

  • Ornish/McDougall/Vegan/Nutritarian method – These are the more traditional diets popularized in the 1970’s that emphasize lots of carbs that are high in fiber and very little fat.  These people have very little digested fat and probably lower rates of fat burning since high carbohydrate diets give you more readily available blood glucose from carb burning.
  • Atkins/Paleo/High Protein method – These diets switch the ratio of macronutrients around.  You eat lots of fat with little carbs.  These people lots of lard, fat, meat, etc.  Suffice to say these people probably have very large amounts of digested fat but a very high rate of fat utilization, presumably because their diet doesn’t induce any sort of insulin response.  In many cases high enough to overcome their hypercaloric diets.

Is one better than the other? For the purposes of weight loss I’m sympathetic to the low carb route for most people, but of course the answer depends on a few things.

Why the slight nudge for low-carb?

For one, some very high quality studies (double blind and randomized) have compared groups of people on different diet methods over the course of a year to see how they lose weight over long periods of time (over one year), and so far it seems like low-carb diets do just a little bit better than the alternatives.

Two useful papers are here and here, but my favorite was published in JAMA in 2007.  It followed 311 obese, premenopausal women over one year on four popular diets (atkins, ornish, zone, and LEARN) to see which one was most effective in accelerating fat loss and other risk factors.  In my opinion it’s one of the most conclusive papers written on the subject of fat loss.

From the abstract:

Weight loss was greater for women in the Atkins diet group compared with the other diet groups at 12 months, and mean 12-month weight loss was significantly different between the Atkins and Zone diets (P<.05)…..Weight loss was not statistically different among the Zone, LEARN, and Ornish groups. At 12 months, secondary outcomes for the Atkins group were comparable with or more favorable than the other diet groups

So is this the equivalent of rapture for people who eschew grains?


I have no doubt others could find studies which say the opposite. And maybe what’s true for obese pre-menopausal women isn’t universally true for the population at large.  But the studies I linked to here tick off all the check  boxes for quality you’d want to see better than any others that I’ve read. However,  let’s keep the following points in mind to make sure we’re interpreting this information in the best way possible:

  • Despite the clinical success of low carb diets, about 20-25% of people don’t respond well to them.  This fact is grabbed from the above paper.  The reasons could be numerous and are probably multi-factorial.  Some people are genetically pre-disposed to digest starch better than others.  Lots of high carb diets restrict lots of other foods besides fat.  And it’s entirely possible that eating a high fat diet could lead you to poorer food quality if you’re not careful.
  • Carbohydrate intake vs glycemic load.  It’s one thing to have a diet that’s 60% carbohydrate (or 80%, 90%, etc), but it’s something else entirely to suggest that same eating pattern would cause a commensurate increase in blood sugar.  As I’ve eluded to before, not all carbs are created equal.

This is a great graph to illustrate my point:

starch insulin response
the insulin response of different types of starch

It examines the insulin response for the same ingredient (wheat starch), but processed in different ways.  As you can see, the difference between a wheat shaft and a wheat thin is huge.

What About Successful High-Carb Dieters?

There are lots of examples of groups of people who have long lifespans and a diet that consists primarily of carbohydrate.

These populations all had a few things in common:

1).  They were physically active.

2).  They consume low amounts of sugar.  The carbs they did eat were usually from vegetables and slowly digested, which means throughout their life their bodies likely had a very mild endocrine response to the food they ate.

3).  Their diets have a low glycemic load.  This is closely related to point #2.  Glycemic load is a measure of how much a food increases your blood sugar.  These people were eating the carbs, but due to food quality and lifestyle reasons weren’t seeing an insulin response because of it.

So what does this mean?

More than anything, I think it suggests that we ought to consider carbs an elective macronutrient.

For physically active people they can make a lot of sense.  They can also work if you’re going to eschew other foods that could interfere with their digestion (processed fats, large amounts of sugar, etc). But for people who are just gurgling along trying to do their best with the least amount of effort despite being in a socioeconomic environment that presents bad food choices from a fire hose, moderating their intake might be the easiest way to get the results you want.

(Addendum:  I realize that after proofreading this post you might think that I think the girl on the far right in the featured image of this page is fat.  (I’m talking about the one with the three girls in tank tops).  I don’t.  In fact, I think she’s beautiful.  If I knew nothing else about those women and was choosing one strictly for their looks I’d choose her, hands down.  It’s just that I thought an image with different sized women would be easier to look at one with someone morbidly skinny or morbidly obese).

4 thoughts on “Why Some People Get Fat and Others Don’t”

  1. I am 53, started low glycemic diet about 4 years ago after finding out I was POST menopausal. I progressed to a no processed foods, healthy fats only and gluten free grains. My weight 4 years ago was 156, today it is 124. I have been at this weight for 2 years. So, I think I agree with your point on hormones and insulin processing. 🙂


    1. Thank you Vicki 🙂

      It’s amazing to think about how bad diet advice has been the last 30 years, although I am optimistic that popular opinion is beginning to converge on the truth.


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