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8 Key Causes of Weight Gain & Obesity

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Weight gain is one of the biggest health issues of modern day life. Obesity rates continue to rapidly rise with over 60% of the U.S. population now classified as overweight or obese (Flegal et al., 2008).

These statistics continue to get more shocking with most recent data suggesting that over a 1/3 of children are now classified as overweight or obese by the mere age of ten! These worrying statistics for our children future really highlights the overwhelming obesity epidemic we face today, which is continually getting worse each year.

The main problem is obesity is an extremely complex disease resulting from many contributing factors.

Despite what some self professed “guru’s” may argue, there is no single reason (i.e. carbs) why people become overweight. Obesity is a multifactorial disease and this article will discuss some of the key research supported reasons why people gain body fat in the first place.

1. Energy Imbalance Causes Weight Gain

An energy imbalance is ultimately the key underlying biological cause of obesity.

On many occasions I have reiterated the fundamental importance of calories in managing weight status. Put simply, if you eat more calories than you use everyday for days, weeks, months and years you’ll gain weight and over the long term.

That is the fundamental basics but of course there are many factors that influence your energy intake and expenditure and thus your energy balance.

Sadly, the modern day lifestyle is only fueling the obesity epidemic with daily energy intake rapidly increasing due to the over consumption of readily available, high calorie dense (and tasty) foods, coupled with the significant reduction of daily activity levels.

This energy imbalance is the key biological cause of weight gain. Many of the other topics discussed in this article are simply contributing factors to the energy balance equation.

As shown below, this is basically the sea-saw like scale of weight gain. If energy out is more, you will lose fat. If energy in is higher then you will gain weight.


2. Decreased Physical Activity Causes Weight Gain

As mentioned, decreased physical activity is one of the most important contributing factors to a decrease in total energy expenditure.

Energy expenditure (also referred to as energy output), i.e. how many calories you burn per day, plays a crucial role in determining overall health and weight gain.

Over the years, daily energy expenditure has progressively decreased. This is in part due to an increased reliance on cars and other advances in technology allowing us to do most things from the comfort of our own home (Strasser, 2013;Dwyer-Lindgren et al, 2013).

For example, a sample of over 5,000 people were analyzed by their self-reported time spent doing moderate to vigorous physical activity (MVPA), watching TV (TV), and sedentary time. Not surprisingly, researchers found that those who spent less time watching TV, being sedentary, and who exercised more were much less likely end up being obese (Maher et al., 2013).

weight gain graph

Physical activity encompasses two different forms: structured workouts such as gym sessions or classes and Non Exercise Activity Thermogenesis (NEAT). NEAT is the term used to describe all the non structured physical activity that you do on a daily basis such as walking the dog, taking the stairs, cleaning the house, gardening etc.

In the fitness and research world, many experts strongly believe that the decrease in energy expenditure is the key factor in the obesity crisis. More specifically, the reduction in daily NEAT (Levine, 2006).

It has been observed that obese people spend much more time in sedentary positions. Even something as simple as standing and walking can influence the odds of being lean versus obese (Levine, 2006).


weight gain graph


If you’re struggling to lose weight or finding yourself slowly gaining weight then adding in structured workouts 3 – 6 times a week is certainly recommended. However, in addition to this, one recommendation which is often forgotten is to increase our NEAT.

Simply increasing you daily activity levels by walking to work, taking the stairs instead of the elevator, taking a walk on your lunch break is a great method to improve health and reduce unwanted fat gain.

3. Increased Reliance On Processed Foods Causes Weight Gain

On the other side of the energy balance equation is energy intake. Over time, our daily intake has risen, largely as a result of the over consumption of highly calorie dense, tasty processed foods (Prentice & Jebb, 2003).

Looking back 50 or so years, these refined foods did not make up such a large percentage of daily intake as they do today. These days, highly processed foods have been made more available than natural whole foods and are relatively inexpensive, hugely convenient and very palatable, hence most individuals over reliance and over consumption.

While I don’t advocate a completely restrictive diet all the time, these types of foods when eaten regular and / or in large quantities can quickly result in weight gain and increase risk of other serious health conditions, such as Type 2 Diabetes.

One of the main issues with processed foods such as pizza, chips, fries, candy and ice cream, is that they are extremely calorie dense. This means that per portion size/ serving they contain a lot more calories than low calorie dense whole foods, such as lean meat, vegetables and fruit.

Try eating 1500 calories of vegetables, it’s basically impossible. Try eating 1500 calories of pizza and ice cream… no problems, that is about 3-4 slices and a small bowl of ice cream!

Reserach suggests that consuming a diet full of energy dense foods can increase your total energy intake, and subsequently increase the likelihood of becoming obese, both in childhood and in the adult years (Blatt et al., 2012; Pérez-Escamilla et al., 2012).

For example 100g of milk chocolate contains around 550 calories, compared with 100g of blueberries which only contains around 50 calories. The portion size is the same for both but the amount of energy (calories) provided is very different.

High energy dense foods tend not be very satiating (filling) either, the 100g of blueberries will definitely keep you fuller for longer (as it has fiber and water) than the 100g of chocolate. As a result, this means that high energy dense foods are commonly over-consumed, therefore providing a surplus of calories.

As you can see, not only do high energy dense foods provide loads more calorie per portion/ serving but they are also often eaten in too large a quantities. This is an instant recipe for weight gain as your body is consuming more calories than is expended through the body and physical activity.

Small amounts of processed food in moderation can have a place in calorie control diets such as flexible dieting. I recommend you take an 90-10 approach. 90% of the time you consume single, whole food ingredients, leaving 10% for the odd treat. This is a good way to help reduce daily caloric intake and therefore aid weigh loss and benefit health. A little balance is key to long term success.

4. Rewarding Yourself With Food Causes Weight Gain

Another common reason you may be gaining weight, even if exercising, is if you food as a reward.

This comes in many forms, with people using food as a psychological reward for a hard day’s work, successful achievement, a celebration or simply a reward for hitting the gym and working out.

Food can certainly be a great part of an enjoyable social life, however it shouldn’t be the only pleasure or daily reward that you seek.

A common reason people can gain weight particularly when starting a new fitness program or fat loss regime is that they reward themselves with the overconsumption of unhealthy foods as they believe they have earned it or now deserve it.

Sadly, this approach is extremely flawed. You simply cannot out exercise a poor diet with one small candy bar providing more calories than a whole 45-60 minute workout!

Using food as a psychological reward is a bad habit to fall into, as when you’re having a bad day or are feeling stressed then you will often revert to binge eating and making poor food choices as a psychological relief.

Instead, try and find a more healthy and sociable thing to look forward to, such playing a team sport, socializing with friends or family, taking up a new hobby etc.


5. Following The Wrong Diet Plan Causes Weight Gain

With the vast amount of information readily available online these days it’s no surprise that many people embark on the wrong diet plan.

It seems that every week a new fad diet emerges, promising you all these amazing benefits which sadly aren’t supported by any research nor have any mechanistic backing for their use.

Far too often I hear from people who have fallen into this trap.

Even though some of these fad diets may cause short term weight loss (due to the ridiculous calorie or food restriction) they are rarely sustainable nor do they educate you on the importance of it becoming a long-term lifestyle change.

In addition, due to a lack of emphasis on key nutrients such as protein or the right type of training,  a lot of the weight loss comes  from muscle and carbohydrate stores, fooling the individual into thinking the diet is successful. In reality, they’ve actually lost very little body fat and once they return to the old diet, it soon piles back on.

Remember there’s no quick fix to losing fat which has probably accumulated over a number of years. Instead of following a fad diet you stumbled across, stick with the basic principles that make up a healthy lifestyle, such as a whole food based diet, high protein intake, strategic refeeds, exercise, sleep and hydration.

6. Not Tracking or Counting Calories Correctly Causes Weight Gain

Even if you are trying to monitor your calorie intake there are large discrepancies in food tracking and errors in calculations which can lead to weight gain.

During the last decade working with clients, I have had many puzzled fans contact me unsure why they are gaining weight even though when they are tracking their daily intake down to a single calorie and gram using fitness apps such as my FitnessPal.

I’ve discussed in detail HERE all of the errors that can arise in food calculation and reporting. In summary, your daily calculations can be wrong by up to 30%. This large discrepancy can easily explain the difference between not losing fat and even gaining fat.

This is very common, whenever I look at food journals from experienced clients (including personal trainers, pro bodybuilders and elite athletes), it’s clear to see that the calculations and figures that they are quoting do not mirror the nutritional values of the foods they are eating.

This may also be the case for you, if you’ve been tracking your intake yet failed to lose fat or worse still gained it then I recommend you read the article linked above.

7. Being A Weekend Binger Causes Weight Gain

Another extremely common cause of weight gain is being a weekend binger.

This is where someone will follow a restrictive diet they don’t enjoy during the week and then as a reward or for social reasons, they’ll completely blow out and binge eat on Saturday and Sunday.

More often than not, this large binge / weekend blowout can easily negate any weight loss benefits achieved in the week. Over the long-term, this approach can even cause significant weight gain, especially if the individual isn’t strictly dieting in the week, but still binging at the weekends.

When embarking on a new diet or healthy lifestyle, you should understand that if you’ve got a lot of fat to lose you do not necessarily need or deserve a large refeed or cheat meal every week to begin with. Instead you need to follow an enjoyable healthy diet which you can happily adhere to all week and weekend.

This is just one of the reasons why I strategically place refeeds into the 90 Day Bikini Program and also teach you how to eat the foods you love without excessive binging or restriction.

8. Drinking Your Calories Causing Weight Gain

Another common error that people make is to consume a fairly high proportion of their daily calorie intake from calorie dense drinks.

Calorie dense drinks include pretty much every drink apart from unsweetened coffee, tea, water, sugar free soda, or calorie free flavored waters. While of course the occasional calorie laden beverage is okay, most people consume at least one everyday, if not more. Without realizing these drinks can easily provide 20% + of your daily calorie requirements, or, over 500 empty calories.

The problem with these sugar laden beverages is that they don’t signal normal satiety pathways in the body. In other words, your body consumes them and does not realize that it’s taken in 300, 400, 500, or 600 calories and therefore, will continue to eat or remain hungry on top of this.

While this can be great for people that need to gain weight fast (in medical cases or athletes) this doesn’t apply for the majority of the population who want to drop fat or maintain a healthy bodyweight.

Some researchers have also suggested that these sugar laden beverages can actually increase hunger and appetite, causing you to overeat. In fact, in a review of over 20 studies on the topic, it was determined that “the majority of the prospective studies found positive associations between intake of calorically sweetened beverages and obesity” (Olsen & Heitmann, 2009).

Other research has found a strong association between soft drink consumption and obesity in adults (Chaloupka et al., 2011). For instance, as the percentage of adults who drank one or more soda per day rose, so did the percentage of adults who were overweight or obese (Babey et al., 2009).

weight gain graph

If you want to lose weight or just generally improve your health one of the most important and first recommendations I would make is to eliminate all calorie based beverages. This can include alcohol, sugary sodas, juices, smoothies, added sugar in hot drinks and so on.

The only exceptions to this are lower calorie milk without added sugar and protein based drinks such as a whey protein shake.

One of the most common errors people make when it comes to calorie based beverages is consuming smoothies and particularly juices because they are falsely advertised as health drinks or beneficial to your health.

In actual fact, juices and most smoothies are one of the worst drinks you can consume for your diet. They are crammed full of sugar, calories and provide little nutritional benefit compared to the whole fruit.

If you’re into juices and smoothies eliminating them and opting for low calorie, highly nutritious and fiber dense fruits such as berries is an extremely simple yet important nutritional strategy you should adopt. The only exception to this is home-made smoothies, which skips the juice and uses low sugar fruits, combined with plenty of protein from natural yogurt / whey.

Worse of the lost are possibly milkshakes (as much as I love them!!), especially those with an ice cream base. Many store / restaurant bought milkshakes have in excess of 1000 calories 🙁 – equivalent to a whole personal pizza – and many people chose them as an addition to a highly 1500 calorie meal.

If you want to gain large amounts of body fat, then this is the perfect combo!

Avoid These Fat Traps

You should now be able to see some of the most common ways that can lead to unwanted weight gain, some of which may not have been completely obvious to you before.

By avoiding all the points discussed in this article you can quickly improve your diet, health and promote weight loss without the need for a restrictive diet/ fad diet.

Once you’ve eliminated these errors, take your physique one step further. There’s a ton of useful information on the site, to get you started, check out my Metabolic Resistance Training workouts and read some more information on dieting / nutrition!


Flegal, K. M., Carroll, M. D., Ogden, C. L., & Curtin, L. R. (2010). Prevalence and trends in obesity among US adults, 1999-2008. Jama, 303(3), 235-241.

Strasser, B. (2013). Physical activity in obesity and metabolic syndrome.Annals of the New York Academy of Sciences, 1281(1), 141-159.

Dwyer-Lindgren, L., Freedman, G., Engell, R. E., Fleming, T. D., Lim, S. S., Murray, C. J., & Mokdad, A. H. (2013). Prevalence of physical activity and obesity in US counties, 2001–2011: a road map for action. Population Health Metrics, 11(1), 1.

Maher, C. A., Mire, E., Harrington, D. M., Staiano, A. E., & Katzmarzyk, P. T. (2013). The independent and combined associations of physical activity and sedentary behavior with obesity in adults: NHANES 2003‐06. Obesity,21(12), E730-E737.

Levine, J. A., Vander Weg, M. W., Hill, J. O., & Klesges, R. C. (2006). Non-exercise activity thermogenesis the crouching tiger hidden dragon of societal weight gain. Arteriosclerosis, thrombosis, and vascular biology, 26(4), 729-736.

Prentice, A. M., & Jebb, S. A. (2003). Fast foods, energy density and obesity: a possible mechanistic link. Obesity reviews, 4(4), 187-194.

Blatt, A. D., Williams, R. A., Roe, L. S., & Rolls, B. J. (2012). Effects of Energy Content and Energy Density of Pre‐Portioned Entrées on Energy Intake. Obesity, 20(10), 2010-2018.

Pérez-Escamilla, R., Obbagy, J. E., Altman, J. M., Essery, E. V., McGrane, M. M., Wong, Y. P., … & Williams, C. L. (2012). Dietary energy density and body weight in adults and children: a systematic review. Journal of the Academy of Nutrition and Dietetics, 112(5), 671-684.

Olsen, N. J., & Heitmann, B. L. (2009). Intake of calorically sweetened beverages and obesity. Obesity Reviews, 10(1), 68-75.

Chaloupka, F. J., Powell, L. M., & Chriqui, J. F. (2011). Sugar‐sweetened beverages and obesity: The potential impact of public policies. Journal of Policy Analysis and Management, 30(3), 645-655.

Babey, S. H., Jones, M., Yu, H., & Goldstein, H. (2009). Bubbling over: soda consumption and its link to obesity in California. UCLA Center for Health Policy Research.

About the author

Rudy Mawer, MSc, CISSN

Rudy has a 1st class BSc in Exercise, Nutrition & Health and a Masters in Exercise & Nutrition Science from the University of Tampa. Rudy currently works as a Human Performance Researcher, Sports Nutritionist and Physique Coach. Over 7 years he has helped over 500 people around the world achieve long last physique transformations.

He now works closely with a variety of professional athletes and teams, including the NBA, USA Athletics, World Triathlon Gold Medalists, Hollywood Celebrities and IFBB Pro Bodybuilders. If you would like to get in contact or work with Rudy please contact him on social media.

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