Obesity and the Physics of Weight Gain
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Obesity is a dangerous epidemic on an upward trajectory, but management is shrouded in misunderstanding, misinformation, and taboo.
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Obesity rates in the United States have now reached greater than 40% of the population, increased from 15% in the 1970s. By sheer numbers, this means there are 100 million more obese people in America than there was 50 years ago. It’s starting at younger ages and reaching greater extremes, both of which compound the damaging effects.
Obesity is fundamentally a case of energy imbalance. If a person takes in more energy than they use, they will store it as fat for later use. This results in increased body mass. If a person uses more energy than they consume, they will break down previously stored energy for immediate use, resulting in decreased body mass.
The first law of thermodynamics is a universal physical principle which states that energy cannot be created or destroyed, but can only change form. This applies without exception to obesity and weight gain. The equation for weight gain or loss can be as simple as calories in vs calories out, but in reality, a more complex understanding can be helpful.
There are certain genetic factors that play a role, such as basal metabolic rate, which is how many calories the body burns to carry out basic processes necessary to sustain life. This rate is not predetermined, though, and can be affected by body composition, or the muscle to fat ratio a person maintains. The state the body is in when calories are consumed also plays a significant role. Carbohydrates ingested during active exercise will be used for immediate energy needs rather than stored as fat. Foods vary in their nutrient content, which can influence how aggressively the body stores the energy and how satiated, or full, you feel afterward, and how quickly you get hungry again.
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Obesity and the Physics of Weight Gain
Obesity is often referred to as an epidemic, and with good reason. Its prevalence has skyrocketed over the past half century, going from approximately 15% in the 1970s to well over 40% today. Extreme obesity (BMI >40) was essentially non-existent 50 years ago, but now more than 1 in 10 people fit this description. Childhood obesity has grown at an even faster rate, from about 5% in the 70s to near 20% today. This is even more concerning, because the negative health effects of being in an obese state tend to compound over time, so the earlier obesity sets in, the more severe negative effects become. Unlike an epidemic of an acute infectious disease which often causes extensive panic, obesity is an epidemic of chronic disease. Since realization of its symptoms take years to manifest, it doesn’t inspire near the same level of alarm.
America doesn’t have the highest rates of obesity worldwide, but it does stand out significantly among other G7 countries (countries with highly developed economies). Unsurprisingly, our neighbors to the north vie for second place among these countries, with a rate of around 27%, but still significantly lower than our >40%. England, with whom we also have many cultural connections, has rates that hover in the upper 20s as well. Germany’s current rate hangs out just above 20%, France just below, and Italy and Japan are both near 10%. All G7 countries have seen some upward trend in obesity since the 1970s.
Being overweight is defined as a BMI between 25-30, obesity >30, and extreme obesity >40. BMI is a measure based on a person’s height and weight. It’s not perfect, but it does correlate very well with increased risks of metabolic syndrome (the combo of high blood pressure, high blood sugar, high cholesterol, and excess abdominal fat) and death from any cause. It’s important to note that the risk of serious disease doesn’t increase linearly with increasing BMI, but exponentially. Once a person starts to move into and through the obese range, the rate at which they suffer from severe disease increases even faster with each step. The longer time spent at any given abnormal BMI also increases the likelihood of developing a chronic disease.
BMI is oft disparaged, and not just by the body positivity crowd. Again, it’s not perfect, but it’s actually far more likely to tell an unhealthy person that they’re healthy than a healthy person they’re unhealthy. It’s very common for someone who is unhealthy to have a normal BMI (as an extreme example, think of a drug-addicted person who is rail thin but that you’d never classify as healthy). The straw man argument about NFL running backs being obese based on BMI is always brought up, but this is countered with a simple heuristic: “If you can’t see your abs, trust BMI.” Based on body weight, many NFL running backs who are in great shape would be classified as obese, but they are the rare exception due to above average muscle mass. While being at higher weight, even when it’s muscle, still does increase the risk of high blood pressure and other abnormalities, it’s far less for someone with low body fat. So unless you have a 6-pack, you would benefit from losing weight if you fall in the overweight or obese category.
Obesity not only increases the risk of developing a number of diseases, it worsens the outcomes for other issues not necessarily incited by increased caloric intake. Obesity is more than just excess fat cells, but also persistent mild inflammation of these cells. The inflammatory signals released by adipose tissue amplify the effects of the disease beyond just adding excess weight. For example, it became abundantly clear that obesity is a major risk factor for negative outcomes from COVID. In fact, measures of IL-6 (one of the inflammatory signals released specifically by fat cells) in hospitalized COVID patients was one of, if not the best predictor of eventual transfer to the ICU.
Obesity also makes pregnancy much more dangerous. A common statistic paraded around is that maternal death rates are higher in America than other developed countries, and this is then blamed on America’s lack of free health care. First, the mortality rates between the US and other developed countries differ by less than one tenth of one percent in most cases. Second, these numbers don’t account for obesity rates and their negative effects on outcomes. Not only does obesity increase the risk of gestational diabetes, pre-eclampsia, and C-sections, it increases overall maternal mortality rate (chances of the mother dying during or near childbirth) by 2-4x compared to a non-obese person. This increased risk due to obesity and the increased rates of obesity in America more than compensate for the disparities in maternal health outcomes in other developed countries.
At any age, an obese person’s chance of dying from any cause is doubled compared to a person of normal weight. Someone who develops obesity in their 20s or 30s is likely cutting nearly 15 years off their life expectancy, and childhood obesity is new enough that we don’t even have great numbers for what early obesity does to their lifespan. Again, these negative outcomes don’t increase at the same rate as BMI increases, but the rate at which they increase increases with higher BMIs.
Obesity is fundamentally a case of energy imbalance. If a person takes in more energy than they use, they will store it as fat for later use. This results in increased body mass. If a person uses more energy than they consume, they will break down previously stored energy for immediate use, resulting in decreased body mass. Energy intake occurs by eating or drinking. The calories in our food are a measure of heat, or energy. When we talk about calories in food, technically we’re talking about kilocalories. Our body breaks down molecules of food and captures the energy released by the breaking of chemical bonds. This energy is then used as the cells in our body perform the necessary processes to maintain life. (For more about investigating energy from first principles, read our essay on mitochondria.)
The first law of thermodynamics can be simplified to say that energy cannot be created or destroyed, but can only change forms. This is a fundamental physics principle and holds true across the known universe at micro and macro scale. Not too many principles are that broadly applicable, but this law is one of them. Our bodies store excess energy as fat because it’s the most efficient way to hold on to energy. One gram of fat can hold nine calories, while one gram of carbohydrates or protein only hold four. We are predisposed to desperately hang on to every available calorie because humanity has spent most of its time in environments where food isn’t readily available, and it was often necessary to go extended periods of time without access to a good energy source.
To put it simply, fat is stored energy. We store it when we have excess energy (or more precisely, excess substrates that can be converted into energy) at one time and want to save it for later. If we are burning more energy than we’re consuming, we are not storing energy as fat and we have to burn the fat we have to get energy. You cannot gain fat when in an energy deficit, or when you’re burning more calories than you eat. People commonly complain that they’re eating miniscule amounts of healthy food, but still gaining weight. In these cases, the people still must absolutely be eating more calories than they’re burning, because otherwise it is physically impossible to increase or maintain fat stores. Nothing can be said with more certainty in medicine, because it is based on a fundamental principle consistent throughout the known universe: if you are gaining fat, you are eating more calories than you’re burning. Since this is an absolute statement with no wiggle room, it is important to point out a caveat that body weight can fluctuate from things other than fat, such as water weight. Weight may vary day to day or during certain times of the month based on how much water the body is retaining, and this is different from weight due to increased fat. Unless your body is increasingly retaining more and more water, though, body weight will decrease over time if you are in an energy deficit. The body can also break down muscle to gain energy, but it preferentially uses fat, and even if the energy use was coming from muscle instead, it would still lead to weight loss.
This energy balance is very precise. A positive energy balance means you’re taking in more energy than you’re burning, which paradoxically is usually a negative thing. If you’re trying to put on weight or grow taller, as in the case of growing children, a positive energy balance is a good thing, but that’s about it. We want to keep our energy intake and expenditure as close to equal as possible (or a negative energy balance if we’re trying to lose weight). Even something as little as a persistent 1% positive energy balance leads to about a 20-pound weight gain over the course of a decade. This is more than enough to take a healthy weight person into the overweight category, and another 20 pounds could easily move them into obesity. If your total energy expenditure in a day is 2000 calories, this is the difference of only an extra 20 calories a day. That’s not a lot. (20 calories is the equivalent to ¼ of a banana, 1 oz of chicken, or looking too long at an Oreo.)
This energy balance rule holds true no matter what. If you expend more energy than you take in, you will lose body fat, even if it’s low-quality food. The best example is the “Twinkie Diet”, performed in 2010 by a nutrition professor from Kent State. He limited his calorie intake to 1800 calories a day, with at least 2/3 of the calories coming straight from junk food, like Twinkies or Nutty Bars. Over 10 weeks he lost 27 pounds. He was probably hungry for most of the 10 weeks and a diet like this would be very difficult to maintain over a longer period, but it shows that when calories in are less than calories out, even when those calories are trash, your body has to derive its energy from previously stored fat reserves, resulting unequivocally in weight loss. Also, in only 10 weeks, the negative effects of chronically high blood sugars and insulin resistance would not yet manifest.
The body has regulating mechanisms in place to help maintain the energy intake and expenditure balance. It’s impossible to continually track calories burned from exercise compared to calories consumed through diet, so the body has a number of ways to influence the balance in whichever direction is necessary. Unfortunately, since all these internal processes are geared towards survival, and survival depends on access to energy, it is always easier to override the signals in place to stop eating than ignore those that tell us to start eating. In a resource poor environment, during the rare times when food is available, it’s best to eat as much as possible and store it as efficiently as possible. Lack of food will kill you a lot faster than excess food. It’s easier to ignore the feeling of fullness compared to the feeling of hunger. This impulse to eat more than necessary when food is almost always easily available pushes towards an excess energy balance. External forces, such as quality or ingredients of certain foods, can also make it easier to bypass limiting mechanisms. The problem with much of the processed food that is so readily available is that it is calorically dense, but nutritionally poor. It provides a lot of energy, but not much else. This is why they’re called “empty” calories.
In terms of pure energy content, a calorie is a calorie. One dietary calorie contains the same amount of energy regardless of its source. However, in terms of nutrition and metabolism, not all calories are equal due to the differing effects of macronutrients in the body.
For example, proteins have a higher TEF (thermic effect of food), which means your body burns more energy processing proteins than it does carbohydrates or fat. This means it’s a less efficient energy source, which is why your body preferentially uses the other two macronutrients if available.
Carbohydrates, especially highly processed ones, induce the release of insulin. Insulin is an anabolic hormone that triggers the body to hold on to energy even more aggressively than it normally would (aka store more as fat). After a rapid increase in blood sugar there is often a rapid decrease, which triggers hunger. Since this is brought on due to a drop in blood sugar, you often crave something sugary that will bring sugar levels back up quickly, developing a feedback cycle detrimental to your health. Also, chronically high insulin levels due to repeated ingestion of processed carbohydrates is one of the first steps of insulin resistance which then leads to metabolic dysfunction and obesity.
Along with the type of nutrients, the state which your body is in when you eat affects how the food is processed. A body at rest is going to store energy for later, while a body in motion is going to put energy to immediate use. It takes energy both to store energy and then to break it back down to a usable form, so your body will use the most readily available energy source. Working out in a fasted state forces the body to derive its energy from stored resources because there is less easily available energy. This doesn’t necessarily mean that you’ll lose more weight working out before a meal, because most people tend to just consume more calories later in the day in a resting state, and the body quickly replaces the fat burned during exercise with new stores.
Eating carbohydrates right before or during exercise means the energy will be put to good use rather than stored. In this same vein, there is a common practice in Indian culture with ancient Ayurvedic roots to “take 100 steps after every meal.” Even a short walk helps regulate blood sugar as muscles will naturally take sugar out of the blood and ease the insulin-production burden on the pancreas.
It Is often very difficult to rely on exercise to lose weight. You can Is out-eat a good exercise routine. For example, walking for 30 minutes will burn 100-200 calories. Jogging, cycling, or swimming for that same amount of time will burn somewhere between 200-400 calories, depending on intensity. To match calorie expenditure to intake, if you average 300 calories burned in a workout, on the healthy side you get 300 calories from 2 apples, 60 baby carrots, 4oz of salmon, or 1/3 cup of almonds. From unhealthy foods, 300 calories is 10-15 potato chips, ½ of a cheeseburger, 1/3 of a large pizza slice, or our favorite benchmark, 6 Oreos.
This is where nutrient density vs calorie density comes in to play. Your body will get more of what it needs in smaller portions from nutrient-dense foods. Because of this, these foods are generally more satiating. You will fill much fuller after eating 60 baby carrots (probably uncomfortably full and maybe a little orange) compared to 15 potato chips. The chips will also spike your blood sugar and create a feedback loop of cravings and snacking. Sometimes foods can be both nutrient- and calorie- dense. If you’re trying to lose weight, you may try and shift to foods that are higher in nutrients while still being lower in calories ( i.e. baby carrots instead of salmon, or celery instead of steak). If you’re trying to maintain a healthy weight, or just get the most bang-for-your-buck from your food, you’ll probably be ok eating calorie dense foods, as long as they’re also very nutritious (salmon, chicken, steak, eggs, avocados, nuts). Processed foods tend to be more calorically dense with fewer nutrients – a double whammy of negativity. Many of them are specifically manufactured to create a rush of taste that subsides quickly, which induces over-indulgence. (This is the same principle that makes certain drugs more addictive than others: there’s a fast and strong onset followed by a rapid diminishment of the effect.) Aside from vegetables and some fruits, foods with low calorie density but good nutrient density could be something like soup, or other foods with high water content. The water helps fill you up faster and deters over-eating. Nutrient density is generally more important than calorie density. If you’re getting the nutrients you need, your body’s regulating mechanisms can function as designed. When you’re not getting the nutrients, your body will drive you to eat more in an attempt to obtain those nutrients, which, if you’re eating high calorie low nutrient food, will only exacerbate the problem.
Even with regular exercise, it still only accounts for around 10-20% of the daily calorie expenditure for most people. Most of the calories your body uses will be from carrying out behind-the-scenes, everyday functions of living, like breathing, cell growth and division, and maintaining body temperature. This calorie expenditure that is necessary to maintain appropriate bodily functions is called the basal metabolic rate (BMR). Since this accounts for the overwhelming majority of energy usage, it’s important to understand influencing factors, which we will briefly discuss here.
First, the average BMR for women is 1,400-1,500 calories per day, while for men it’s around 1,600-1,800. (This is driven largely by more muscle mass and less fat in men. Muscle is more metabolically active than fat.) The standard deviation of BMR has been reported at 5-8%, meaning that 95% of women will have a BMR between 1,350 and 1,650. The ranges for men would be slightly higher. There’s always outliers, though, and one study in Scotland that measured BMR of 150 different people found a range from 1,027 calories a day at the low end to 2,499 on the high end.
Body composition and size play a large role in setting BMR. Someone who is larger has more cells requiring energy to function, and thus a higher BMR. If the largeness is composed primarily of muscle, then even more energy is required. Fat is still metabolically active, even if it is at a lower rate than muscle. One pound of muscle will burn 6-10 calories a day at rest, while one pound of fat burns 2-3 calories. Even with increasing fat, BMR still increases, so to continue gaining weight in an obese state requires more and more caloric intake.
Age is also another large determinant. BMR tends to decrease with age. Much of this is attributable to change in body composition, (again, more fat and less muscle) but age is also an independent variable. It is reasonable to assume that a person’s BMR could drop about 170 calories a day from age 30 to age 60, assuming weight stayed roughly the same.
There is also genetic variation in BMR that can be difficult to ascribe to specific causes. Genes can regulate hormone production, which is another major driver. Thyroid hormone is integral in BMR, and individuals with low thyroid levels will have lower BMRs. There are a number of other hormones that will have genetic variation and affect BMR, including insulin, cortisol, adrenaline, growth hormone, leptin, glucagon, and sex hormones (testosterone and estrogen). These can also be influenced strongly by environmental factors, so it can be difficult to separate genetics from environment. Either way, 95% of people will be within 10% or so of the mean BMR for their gender.
Out of all the factors above, some can be modified, and some can’t. Body composition can be altered by exercise and appropriate diet. You may not have the genetics to be the most ripped person at the gym, but everyone has the capacity to build muscle. Age is a little tougher to control, but maintaining appropriate body composition as you age will help mitigate the effects. Hormones can also be altered/optimized through various lifestyle changes or medication, which is beyond the scope of this essay.
After all is said and done and you’ve optimized contributing factors to your greatest extent possible, what if you’re still not losing weight? It is an undeniable fact of physics that you must still be taking in more energy than you’re using. Multiple studies on calorie restriction have shown that almost undoubtedly, people in the study not losing weight were underreporting calorie intake and overreporting exercise. The studies also found that this was more common in overweight and obese people. Whether it is due to cognitive dissonance, innocent rationalization, or intentional misrepresentation, one study found that, on average, overweight and obese patients underreported calorie intake by 51% and overreported exercise by 47%. With correct reporting, essentially all patients began losing weight as predicted.
Without precise studies, it’s impossible to know your BMR exactly, and even then it will fluctuate throughout your life. It’s also very difficult to correctly measure exercise calorie expenditure, although wearable devices are making it easier to estimate, although they’re not completely accurate. And it’s unreasonable to track calorie intake every day for the rest of your life. Undoubtedly, some experimentation is in order. If the goal is to lose weight, you can start by cutting calorie intake to 1,500 or 1,200 calories a day. Track your input for a week or two, until you have a good idea of what your input is, and then stick to the same or similar meals. Your weight is a rolling average of this caloric balance, so it may take days to weeks to notice a different in the average, reflected by a decrease in weight on the scale. But if results are still lacking, it means you’re either underestimating your intake or overestimating your output. This is an undeniable law of physics that holds true across the known universe. To make things easier, focus on nutrient dense whole foods and maintain some regular level of activity so your body’s innate mechanisms can function as intended and maintain a healthy weight. This is far from easy and there are numerous organizations across a range of industries with incentives directly opposite of yours. It’s a fight you must fight yourself, but having associates fight alongside you can also be helpful. While you can’t control the outside environment (you may drive past 20 fast food restaurants on your way to work), you can control what you bring inside your home. As Confucius said, “He who conquers himself is the mightiest warrior.” Above all, remember it’s a journey and not a destination. Implementing healthy habits will benefit your life, regardless of numbers on a scale.
I like how you just state facts instead of sugar coating everything. 😄 This is very interesting. It's stuff we have all learned, but with better explanation and study. I'm trying to drop processed foods for more natural foods, but it's a tough battle. This gives me some fresh motivation!