the math & science of obesity

The rate of obesity in the U.S. has skyrocketed. Between 1970 and 2024, the percentage of people with obesity has increased almost tenfold, from 4.5% to over 43%.

Compared to other health problems, obesity is, relatively speaking, a new kid on the block. It was quite rare before the 20th century.  For example, in the late 19th century, only 1 in ~ 20 middle-aged white men were obese, but today, that number is 1 in 2. This means that 50% of middle-aged white men in the U.S. are now classified as obese, defined as having a body mass index (BMI) of 30 or higher.

The above data don’t include those who are overweight, which is defined as a BMI between 25-29.9. Overweight accounts for more than 25% of the population. This means that –depending on which state you live in–only about 20-25% of US adults have a normal BMI. It’s almost as if having a normal body weight is becoming an endangered condition; and this is not just in the U.S. but in many countries around the world.

The above graph shows the trend in obesity prevalence among U.S. adults from 1970 to 2024. Obesity rate has increased almost tenfold in the last 5 decades (Source: data from CDC, NIDDK, USAFacts, and Harvard School of Public Health). 

Why is this the case?

Obesity can be explored through the lens of mathematics. To understand obesity you need to understand the energy balance equation.  At its core, obesity is an issue of energy imbalance in this formula:

Energy Intake − Energy Expenditure = Δ Body Fat

Our modern lifestyle has significantly disrupted the energy balance equation.  First, there is an abundance of calorie-dense foods and drinks that are high in sugar and unhealthy fats, making it easy to consume more calories than our bodies need. 

Second, modern conveniences such as cars, elevators, dishwashers, and remote controls have led to a much less active lifestyle, reducing the amount of physical activity we get each day.  Many jobs today require sitting at  a desk for long hours, which can lead to boredom, mental fatigue, and muscle loss. As a result, people tend to be even less physically active outside of work, choosing to spend their leisure time ‘decompressing’ in front of screens and snacking.

Caloric consumption among US adults from 1970 to 2020. Based on data from USDA and NHANES, there has been a steady increase in caloric intake over the decades.

Let's explore the components of the energy balance equation (Energy Intake − Energy Expenditure = Δ Body Fat) and how they contribute to obesity.

ENERGY INTAKE

Calories In: Every food and drink you consume provides energy measured in calories. The primary sources of calories are 3 macronutrients, and alcohol:

• Carbohydrates and Proteins: Each gram provides 4 calories.

• Fats: Each gram provides 9 calories.

• Alcohol: Each gram provides 7 calories.

The total daily caloric intake should be aligned with your body's energy needs to maintain weight. If intake exceeds expenditure, weight gain occurs.

ENERGY EXPENDITURE

Calories Out: Your body expends energy through several processes:

1 . Basal Metabolic Rate (BMR): 

This refers to the number of calories your body needs to function at rest. It's like the idle fuel consumption of a car. Your BMR does not include the calories you burn from normal daily physical activities or exercise. 

You can use a BMR calculator online, which factors in your age, sex, weight, and height.  BMR accounts for about 60% of your daily energy expenditure. Factors such as age, gender, weight, and muscle mass influence your BMR. People with more muscle mass tend to have a higher BMR, which means they burn more calories even when they're not active.

There are many equations out there to estimate BMR. The Harris-Benedict and Mifflin-St. Jeor equations are two of the commonly used ones. At 148 pounds and 5'7", my BMI is 23.2 (Yay!). According to the above BMR calculator, my Basal Metabolic Rate is 1,504 calories per day. This means that if I do absolutely nothing all day, my body needs about 1,500 calories just to maintain my weight and basic functions.  

2. Thermic Effect of Food (TEF):  

This refers to the energy your body uses to digest, absorb, and process nutrients from the food you eat. It accounts for about 10% of your daily energy expenditure. 

Foods high in protein, fiber, and healthy fats have a higher TEF, meaning your body burns more calories digesting them. On the other hand, foods that are mostly refined carbohydrates with little fiber, protein, or fats have a lower TEF, resulting in fewer calories burned during digestion. 

Eating a balanced diet that is rich in fiber (like your green veggies and fiber-rich fruits) can help increase your TEF and support weight management.

3. Physical Activity 

Exercise vs. NEAT: The Two Key Types of Physical Activity

When we think of physical activity, our minds often jump to structured exercise like running, cycling, or lifting weights. However, there’s another equally important type of physical activity that plays a crucial role in our health and metabolism: NEAT.

What is NEAT?

NEAT stands for Non-Exercise Activity Thermogenesis. It encompasses all the physical activities you do outside of formal exercise, such as walking the dog, gardening, cleaning the house, or even fidgeting. These everyday movements might seem minor, but they significantly contribute to our daily energy expenditure.

Research shows that NEAT can account for a large portion of our total daily caloric burn. For instance, a study published in the journal Mayo Clinic Proceedings found that NEAT can vary by up to 2,000 calories per day among individuals depending on their lifestyle and occupation. This means that someone with a more active lifestyle can burn significantly more calories without setting foot in a gym. To learn more about NEAT, I highly recommend this excellent book, Get Up!, by Dr. James Levine. 

The Role of Exercise:

On the other hand, structured exercise is intentional physical activity aimed at improving fitness and health. Activities like swimming, jogging, and weight training enhance cardiovascular health, strengthen muscles, and boost mental well-being. The Centers for Disease Control and Prevention (CDC) recommends at least 150 minutes of moderate-intensity aerobic activity per week, coupled with muscle-strengthening activities on two or more days per week. I personally think that exercise should be a daily routine. Do it every day around the same time of the day and combine it with something fun (e.g music).

Exercise has been shown to provide numerous health benefits, including reducing the risk of chronic diseases, improving mood, and enhancing cognitive function. Additionally, regular exercise boosts your metabolism.  A very comprehensive scientific review published in 2015 highlighted that regular exercise can improve insulin sensitivity, reduce inflammation, and support healthy body weight. 

My bike and gear on the GAP trail, Pennsylvania.

Balancing Both for Optimal Health:

To achieve optimal health, it’s important to incorporate both exercise and NEAT into your daily routine. While exercise can provide targeted health benefits (e.g., increased aerobic fitness, muscle mass and strength, and improved mood and concentration), increasing your NEAT can help you stay active throughout the day and boost your metabolism. Simple changes, such as going for a walk, taking the stairs instead of the elevator, or opting for a standing desk, can increase your NEAT and enhance your overall well-being.

Energy Expenditure—The Case of Tony vs. The Olympic Skier:

Tony, who leads a sedentary lifestyle working a desk job, spends the majority of his daily calories – about 70-75% – on his BMR, which covers breathing, circulation, and staying awake. Only around 10% of his calories go to the Thermic Effect of Food (TEF), while a modest 15-25% is used for physical activities (walking to the garage, standing and walking a few times at home and work, and preparing breakfast and dinner.

In contrast, an Olympic skier has a dramatically different energy profile. Their BMR makes up a 40-50% of their daily energy use, but the intense physical activity involved in their training and competition skyrockets their calorie burn from exercise to 40-55% or more. The TEF for the skier remains relatively constant at about 10-15%. This highlights the tremendous difference in the modes of energy expenditure between Tony’s relaxed lifestyle and the demanding regimen of an Olympic skier.

You don’t need to become an Olympic skier, but aiming to burn a higher percentage of your daily calories through physical activity can make a big difference. Additionally, swapping simple carbs for complex carbs and protein can help you burn more calories. This change boosts your TEF, meaning your body uses more energy to digest these healthier options.

The difference between Tony and the Olympic Skier in the ways they burn calories per day. Tony, a 45-year-old with a weight of 230 pounds and a height of 5'8". This gives him a BMI of 35, classifying him as having class 2 obesity. Tony’s BMR is 1,900 calories, the Thermic Effect of Food (TEF) adds 250 calories, and he only burns 400-500 calories through daily activities totaling ~ 2,550 calories burned per day. Now, consider our Olympic skier, aged 30, weighing 165 pounds, and standing at 5'9" tall, with a BMI of 23.6. The Olympian’s BMR is 1,700 calories. With a TEF of 350 calories and an incredible 2,000 to 2,500 calories burned through religious daily workouts and skiing (a non-negotiable for feeling good), the skier burns between 4,000 to 4,500 calories each day!

The 500-Calorie Rule:

The "rule of 500 calories" is a simple guideline that helps explain how daily calorie intake affects weight gain or loss.  The idea is that eating 500 more calories than your body needs each day can lead to gaining about one pound per week.  On the flip side, cutting out 500 calories a day, either by eating less or moving more, can help you lose a pound a week.

Why 500 Calories?

• Fat Facts: One pound of body fat contains roughly 3,500 calories. That means if you accumulate a 3,500-calorie surplus, you’ll gain a pound. Spread out over a week, that’s 500 extra calories per day (3,500 ÷ 7 = 500).

Applying the Rule:

• Weight Gain: If you indulge in an extra slice of cake every day without increasing your activity, expect to see the scale tick up by about one pound a week.

• Weight Loss: Skip that dessert or take a brisk walk that burns 500 calories daily, and you’ll likely see a pound drop each week.
  
  

Keep in Mind:

• Metabolism Matters: Everyone’s metabolism is different. Some people burn calories like a roaring furnace, while others are more like a slow cooker.

• Muscle vs. Fat: Muscle tissue burns more calories at rest than fat. So, the more muscle you have, the more calories you burn. Keep lifting weights!

• Body Adaptation: Over time, your body might adjust to a lower calorie intake by slowing down your metabolism, which can affect weight loss. This leads us to the concept of the Set Point Theory. 

The Set Point Theory

This theory suggests that your body has a natural weight range it tries to maintain, much like a thermostat. When weight decreases, your body may slow metabolism and increase hunger to regain the lost weight, making it challenging to maintain weight loss.  

Think of this as your body's thermostat for weight. Just like a thermostat keeps your home at a set temperature, your body strives to keep your weight within a certain range. But the key difference between the set-point theory of body weight and a thermostat is that while it's relatively easy to increase your body's set point, lowering it can be much more challenging.

How Set-Point Theory Works:

• Hormonal Control: Your body uses hormones like leptin and ghrelin to regulate hunger and metabolism. Leptin, produced by fat cells, signals your brain to reduce appetite when you've eaten enough, while ghrelin stimulates hunger when your body needs energy.

• Metabolic Adjustment: If you lose weight, your metabolism may slow down to conserve energy, making it harder to keep the pounds off. Conversely, if you gain weight, your metabolism might speed up to burn more calories.

Doc, I’ve Always Been Heavy. I’m Not Too Bothered By It. What Do You Say?

You might think carrying a few extra pounds is no big deal, but let me show you why it’s more than just a cosmetic issue.

The Heart: Picture your heart as the powerful engine of your body. It loves efficiency and hates dragging around unnecessary weight. When you're carrying extra pounds, your heart has to pump harder to move blood through more miles of blood vessels. This constant overdrive is a recipe for burnout.

High blood pressure is a common issue, with around 75% of hypertension cases linked to obesity. More weight equals more pressure on your heart, leading to premature heart failure and arrhythmias. Add to that the problem of clogged arteries. Obesity messes up your cholesterol balance, increasing bad fats (LDL) and decreasing good fats (HDL), which leads to heart attacks and strokes.

Diabetes: Imagine your cells as efficient little factories needing insulin to let in glucose for energy. When you're obese, those cells resist insulin's knock, resulting in high blood sugar levels and type 2 diabetes. More than 90% of people with type 2 diabetes are overweight or obese. It's a vicious cycle that's tough to break.

Cancer: Obesity doesn’t just harm your heart and pancreas; it’s a known risk factor for several cancers. Extra pounds can actually fuel cancer cell growth. Breast cancer, especially in postmenopausal women, colorectal cancer, driven by higher levels of insulin and growth factors, and endometrial cancer, due to elevated estrogen levels in obese women, all become more likely.

Breathing: Extra weight doesn't just slow you down; it can literally take your breath away. Obesity is a major cause of sleep apnea and other respiratory issues. Fat deposits around your neck can obstruct your airway, leading to poor sleep and chronic fatigue.

Joints: Think of your joints as the suspension system on a high-performance vehicle. Extra weight is like driving a loaded truck on fragile springs, leading to pain and breakdown. Every extra pound puts additional pressure on your joints, making osteoarthritis more likely. Losing just 10 pounds can reduce your risk of knee osteoarthritis by 50%.

The Mind: Carrying extra weight can heavily burden your mind, too. Obesity is linked to higher rates of depression and anxiety. Plus, the social stigma can lead to a vicious cycle of low self-esteem and mental health issues.

Life: Finally, obesity can shave years off your life. Studies show it can reduce life expectancy by up to 10 years – that’s a decade of adventure, joy, and time with loved ones you don’t want to lose.

I hope that I convinced you that obesity isn’t just a superficial concern; it’s a full-scale health crisis. But here’s the good news: you have the power to change. By making smarter food choices, getting active, and ditching the couch for a hike or bike ride, you can reclaim your health.

Doc, You Convinced Me. But Why Can’t I Just Rely on Ozempic to Lose Weight?

Great question!  Ozempic and similar weight loss medications (collectively, let’s call them Ozempic for short) have gained attention for their weight loss effects. I personally think it’s a wonder drug and when covered by health insurance I’m willing to prescribe it to my patients as part of a comprehensive medical weight loss plan.  However, relying solely on Ozempic for weight loss may not be the best approach for several reasons.

• Ozempic is Not a Magic Bullet

While Ozempic can aid in weight loss, it is not a cure-all solution. Studies have shown that combining medication with lifestyle changes like diet and exercise is more effective than medication alone. For instance, a study published in the New England Journal of Medicine found that participants who combined semaglutide (the active ingredient in Ozempic) with lifestyle intervention lost significantly more weight than those who relied on medication alone. This highlights the importance of a comprehensive approach to weight management.

• Potential Side Effects

Ozempic can have side effects, including nausea, vomiting, diarrhea, and abdominal pain. More serious side effects, such as pancreatitis and gallbladder problems have also been reported. It is essential to consider these potential risks and discuss them with your healthcare team before relying on the medication for weight loss.

• Sustainability is Key

Long-term weight maintenance requires sustainable lifestyle changes. Relying solely on Ozempic may not teach you the healthy habits necessary to maintain weight loss once you stop taking the medication. 

• Cost and Accessibility

Ozempic can be expensive, and not all insurance plans cover it. Additionally, relying on a medication that may not always be accessible could leave you without a reliable weight management strategy. Investing in lifestyle changes can provide a more cost-effective and sustainable solution.

• Addressing Underlying Issues

Weight management often involves addressing underlying issues such as emotional eating, stress, and sleep. Focusing on these factors, along with medication, can lead to more successful weight loss and improved overall well-being. 

In summary, while Ozempic can be a helpful tool for weight loss, it is most effective when used in conjunction with healthy lifestyle changes. 

Final Thoughts

If you remember nothing else from what I wrote above, please remember these points: 

1. While obesity is a chronic disease influenced by genetics and environment, understanding the energy balance equation can empower you to make informed decisions about diet and exercise. 

2. To maintain a healthy weight or lose weight, focus on a balanced diet rich in whole foods. Prioritize fruits, low starch veggies, lean proteins, and whole grains while limiting processed foods high in added sugars and unhealthy fats. Practice portion control to avoid overeating, and stay mindful of calorie intake. Fill your day with other activities (preferably social ones) so that you don’t think about food all the time. 

3. Remember NEAT.  If you don't move much during the day, losing weight can be very difficult, even if you eat less. This is because weight loss relies on both consuming fewer calories AND burning more calories through physical activity. When you're very sedentary, your body doesn't burn many calories, making it harder to create the necessary calorie deficit to lose weight. To effectively lose weight, it's important to combine a healthy, calorie-controlled diet with regular physical activity to increase energy expenditure.

4. Remember that exercise isn't a punishment; it's an investment in your future self. Think of it as compound interest for your body. The more you move, the more you burn, and the healthier you become. Find activities you enjoy: Whether it's hiking, biking, swimming, or dancing, find something that makes you want to get up and move.

5. Stick to healthy habits that you genuinely enjoy–and preferably, that involves social interactions. If you don't like what you're doing, it won't stick. That's why most prescription diets and strict exercise programs fail—they just aren't sustainable.

   Finally, Determination and willpower are key. As a determined Chia fan armed with knowledge, math, and the willpower to change, obesity doesn’t stand a chance.  Remember, this journey isn’t just about losing weight; it’s about gaining health, confidence, and a life well-lived. So put on your game face, grab your calculator, and let’s get to work. I’m here to help.