If Losing Weight Were Just About Willpower, We’d All Be the Same Size

Your body has settings you were born with, and they shape more of your physiology than you may realize.

These settings are influenced by things like your genes, hormones, metabolism, how your brain responds to hunger and fullness, and even how your body uses and stores energy.

Your lifestyle and daily decisions do matter. But if weight were only about those choices, then two people who eat the same foods and move the same amount would look the same. That rarely happens. One person might store or gain weight much faster than the other, even under the exact same circumstances.

Because of these settings, weight is not something the body controls only through effort. Biology plays a major role, so when we talk about things like overweight or obesity, we’re talking about how the body works, not just the choices someone makes.

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Body Mass Index

One of the most common tools doctors use to talk about weight is something called Body Mass Index (BMI). You can check yours here.

BMI is a number based on weight and height. It’s meant to help compare body mass across people of different sizes. Since taller people naturally weigh more, dividing weight by height helps even that out.

BMI Categories:

• Underweight: < 18.5
• Healthy weight: 18.5 – 24.9
• Overweight: 25 – 29.9
• Obese:
  • Class I (Mild): 30.0–34.9
  • Class II (Moderate): 35.0–39.9
  • Class III (Severe): 40.0 or higher

Doctors use these ranges to predict possible health risks like heart disease or type 2 diabetes. But BMI has limits. It only looks at height and weight. It doesn’t show whether weight comes from muscle, bone, or fat. Two people can have the same BMI but look completely different depending on what they have more of (muscle or fat).

Waist measurements, body fat percentage, or more advanced tools like DEXA scans (Dual-Energy X-ray Absorptiometry) give a clearer picture. DEXA isn’t always easy to access and taking measurements takes time, but both are becoming more common in clinics and will likely play a bigger role in the future¹. So for now, BMI is still helpful. It just makes more sense when you also look at how a person lives, too. For example:

• A person who exercises regularly, eats balanced meals, and moves often likely carries more muscle.
• Someone with low muscle tone, fatigue, or limited movement may have more fat mass relative to muscle.

So, BMI gives a quick picture of body size, but lifestyle and movement show what that number really means.

How Genetics Shape Weight and Metabolism

Genes account for about 40 to 70 percent of differences in body weight. This means that a large part of what affects a person’s weight comes from the genes they were born with, not only from their lifestyle, choices, or habits.

Certain genes affect how fat cells behave; how easily they expand, how many new fat cells the body creates, and how quickly they release stored energy.

Genetics influence how strongly the body defends its current weight, which affects how quickly metabolism shifts when someone eats more or less than usual.

Other genes influence the brain’s reward pathways, which can change how strongly a person responds to food cues or cravings, even when physical hunger is low.

Some people carry more fat around the abdomen, called visceral fat. This type of fat is more active and can release chemicals that affect blood sugar, inflammation, and heart health.

Others store more around the hips and thighs, called subcutaneous fat (under the skin)². This type of fat is less reactive and less likely to lead to these problems.

Proof That Bodies Respond Differently to the Same Calories

A well-known study published in The New England Journal of Medicine followed 12 pairs of identical twins who were asked to eat about 1,000 extra calories a day for three months³.

It was designed to test how bodies respond when they consistently take in more energy (calories) than they use. The twins’ food intake and activity levels were supervised to make sure they actually ate the extra calories and maintained the same amount of movement.

Here’s what happened:

• Everyone gained weight, but the amount and where they gained the weight were different.
  • Within each twin pair, weight gain and fat distribution were almost identical.
  • Between separate twin pairs, the results were completely different; some people gained nearly three times more than others.

So, even with the same diet and activity, genetics strongly influences:

• how much weight a person gains,
• how much of the weight gained becomes fat versus lean tissue (muscle, bones, organs, water),
• where the body stores fat (belly, hips, thighs), and,
• how metabolism responds to extra calories (how efficiently the body uses or stores energy).

Quick side note: The study only involved men. That’s a common limitation in research (women-specific data is slowly catching up), but the findings still show how big a role genetics plays in weight.

Genetic Types of Obesity

Certain genes, like FTO⁴, MC4R⁵, and those that affect leptin, help regulate appetite, hunger, and how the body processes energy (calories) from food. When these genes don’t work the way they should, it can be harder to feel full, easier to store fat, and more difficult for the body to use extra calories for things like daily movement or basic cell functions before they get stored.

The main genetic types of obesity⁶ include:

Polygenic obesity (about 80-95% of cases)

• This is the most common form of obesity.
• It involves many genes working together, each playing a small part in influencing appetite, fat storage, and how much energy the body burns.
• For many, this can mean their body handles the same calories differently than someone without the risk.
• For example, if two people drink the same 12-ounce can of soda, one person’s body might naturally use most of the energy through normal movement. The other person’s body, because of their genes, may store more of that energy instead.
• Some research estimates this difference can be in the range of about 200 calories a day.

Monogenic obesity (less than 5% of cases)

• A less common form of obesity.
• Caused by a change, or mutation, in a single gene.
• The most common mutation involves the MC4R gene, which can make the brain think the body needs more food even when it does not.
• For example, imagine two people eating the same exact meal. One person’s hunger signal turns off when they’ve had enough. The person with the MC4R change may not get that “I’m full” signal as clearly, so their body keeps asking for more food even when their stomach is already physically full.

Syndromic obesity (fewer than 1% of cases)

• A very rare form of obesity.
• It shows up as part of a larger genetic condition that affects several body systems and goes beyond appetite.
• For example, a person with a condition like Prader–Willi syndrome⁷ might feel hungry almost all the time and also have lower muscle tone. This means their body uses fewer calories during daily movement. So the same amount of food affects their body differently than someone who doesn’t have the condition.

Finding What Works for You

Treating obesity needs the right approach depending on the type and cause of it, how long someone has lived with it, and what resources a person has available.

Polygenic obesity, which is the most common, responds best to steady lifestyle changes and supportive care. Monogenic or syndromic obesity may require more medical management, but even then, healthy habits still make a difference in how the body responds, functions, and feels.

The starting point to address any type of obesity is usually lifestyle intervention: balanced nutrition, appropriate movement, and consistent sleep. These are the foundation for everyone, even when other treatments are added later. But lifestyle change alone can still be difficult. Stress, poor sleep, or even having healthy food available but not affordable or appealing can all make it harder to follow through. That’s why treatment needs to be both scientific and practical.

Support can come from several places. You don’t need to tackle everything at once. You can focus on one at a time or layer them as you go:

• Nutritional counseling: This includes learning how to shop and cook in a way that fuels your body and matches your energy needs. Small changes, like eating enough protein and fiber, can help you feel full longer and cut down on cravings for sweets. Balanced eating also means allowing for treats sometimes, but making nutritious choices most of the time. It can help to make snacking a little less convenient, like going out for ice cream or a bakery treat instead of keeping it at home. A nutrition professional can help plan meals that fit your culture, budget, and routine.

• Behavioral strategies: Tools that make healthy habits easier to repeat. Placing reminders where you see them (sneakers by the door, a water bottle by the fridge) and changing small cues (turning off screens while eating) help the brain form new patterns. Over time, these simple cues make healthy choices automatic instead of forced.

• Medication (GLP-1s): Medicines that lower appetite and improve how the body handles blood sugar. They can make it easier to manage portions and hunger while you form new habits and make lifestyle changes. Medication is often most effective when paired with consistent movement and balanced eating.

• Therapy or support groups: Emotional and social support can help uncover the root causes of eating or movement patterns, such as stress, trauma, or habits that formed early in life and carried into adulthood. Working with a therapist or group can reduce stress eating, rebuild self-trust, and improve motivation and confidence.

• Medical or surgical care: Options like metabolic or bariatric surgery may be recommended for severe obesity or when other treatments haven’t worked well. These require medical screening but can lower health risks and improve mobility and quality of life.

• Personal trainers or coaches: An exercise professional (especially one who specializes in corrective exercise) can create safe, progressive programs that build strength, protect joints, and improve posture and movement patterns. Movement should feel supportive, not punishing. You can try a few different activities to find something that feels fun and sustainable. Many programs, including virtual or at-home options, can offer guidance without requiring hours in a gym. For many, this is an emotionally safer place to start.

Not everyone can afford multiple specialists at once, and that’s okay. Progress can start with small, affordable steps like getting better sleep, learning how to cook a few nutritious meals, walking more (even at home), or working with one trusted professional who has experience helping people living with obesity.

The Takeaway

Obesity is complex and strongly linked to biology. It doesn’t change overnight, and it can’t be fixed with quick plans. For many, it’s a lifelong condition that needs steady care.

Just like with anything new, the beginning can feel like the hardest part. When energy is low and food or TV is one of the few comforts, taking that step toward change can feel like losing something that helps you cope. Sadness, some depression, and frustration are completely normal in this phase. It doesn’t mean you’re failing. It’s actually the opposite. It means your body and brain are healing and adjusting, and that takes time.

Small, steady steps like eating balanced meals at regular times or finding a type of movement that feels doable, even if it’s only a few minutes from a chair at first, make the shift. With consistency, sleep improves, energy starts to come back, and life begins to feel good again.

Then eventually, those early tough moments start to fade. You notice you’re moving more easily, breathing and sleeping better, or not needing as many breaks during everyday things. Normal daily tasks feel lighter. These changes come from patience, not perfection.

And if you fall off track, that’s part of the process. You’re learning what works for you. Real change is about showing up again.

It all takes time, but the time will pass either way. If you start now, a year from today, you’ll already see and feel the difference. And you’ll know it was you who made that shift possible.

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Resource

1. Sibal, R., Balamurugan, G., Graham, Y., & Mahawar, K. (2025). Moving away from BMI: a new era of diagnostic criteria in obesity. Expert Review of Endocrinology & Metabolism, 20(5), 403–413. https://doi.org/10.1080/17446651.2025.2537160 https://www.tandfonline.com/doi/10.1080/17446651.2025.2537160?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub%20%200pubmed ↩︎
2. https://link.springer.com/article/10.1007/s00125-014-3214-z ↩︎
3. https://www.nejm.org/doi/10.1056/NEJM199005243222101?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub%20%200www.ncbi.nlm.nih.gov ↩︎
4. Asuquo EA, Nwodo OFC, Assumpta AC, Orizu UN, Oziamara ON, Solomon OA. FTO gene expression in diet-induced obesity is downregulated by Solanum fruit supplementation. Open Life Sci. 2022 Jun 15;17(1):641-658. doi: 10.1515/biol-2022-0067. PMID: 35800074; PMCID: PMC9202533. https://pmc.ncbi.nlm.nih.gov/articles/PMC9202533/ ↩︎
5. https://www.ncbi.nlm.nih.gov/gene/4160#:~:text=4160%20%2D%20Gene%20ResultMC4R%20melanocortin%204,with%20the%20Monocarboxylate%20Transporter%208. ↩︎
6. https://www.sciencedirect.com/science/article/pii/S2667368124000123?via%3Dihub ↩︎
7. https://www.mayoclinic.org/diseases-conditions/prader-willi-syndrome/symptoms-causes/syc-20355997 ↩︎

Photo Credits

Weight scale by Kana Design Image from Getty Images

Abstract Representation of DNA Molecule with Genetic Structure E by berkay08