Eight Causes of Unintentional Weight Gain

weight gain
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Gaining weight unintentionally is so common in Western societies that it is almost the rule rather than the exception. And while in most cases the cause is what you think it is — storing extra fat because we consume more calories than we burn — there are other potential causes for unintentional weight gain.

If you are gaining weight and don’t know why, here are eight causes of weight gain you should be aware of.

1 - Diet and Eating Habits

This, obviously, is the big one. And while it’s true that the number of calories we store as fat can be reduced to a simple equation (i.e., calories consumed minus calories burned equals calories stored or lost), human physiology makes it much more complicated than that. To our bodies, food does not merely represent calories and nutrition; food is also data. The kind of food we eat tells our bodies something about our environment, and in response to that information our bodies change the way our food is handled and stored, adjust whether our appetites are stimulated or suppressed, and slow or speed up our metabolism. So what we eat has a major impact on our weight, in addition to how many calories we eat.

For instance, eating carbs with a high glycemic index boosts our insulin levels. We can think of insulin as fat’s jailer — it locks up fat in our fat cells, and refuses to release it so we can burn it off.

By avoiding carbs (especially the high glycemic kind) we can reduce our insulin levels and help mobilize our “incarcerated” fat.

Eating patterns can also affect weight gain. Eating five smaller meals a day, instead of two or three larger meals, has been associated with a reduced risk of becoming overweight.

Eating breakfast regularly is also thought to reduce the risk of weight gain. On the other hand, eating at night before bedtime has been associated with obesity (and sleep disturbances).

Keep in mind that as we age, the way our bodies handle calories changes, and we tend to gain weight far more easily than we did in our carefree youth. This weight gain often occurs without any discernible change in our diets or activity levels — so it is “unexplained.”

2 - Insufficient Physical Activity

Living a sedentary lifestyle is strongly associated with weight gain. And watching lots of media is the sedentary behavior that most increases our risk of diabetes and obesity.

3 -Sleep Deprivation

Getting less than 7 hours of sleep per night is associated with weight gain. The reason for this, at least partially, is that restricting sleep increases the appetite suppressing hormone leptin, and increases the appetite stimulating hormone ghrelin.

So, when we are sleep deprived we are hungrier and we eat more.

4 -Smoking Cessation

Smoking cessation, while a major boon to health, often leads to weight gain. In fact, experts suggest that smoking cessation programs always include dietary and exercise components to prevent this problem.

5 - Drugs

Several prescription medications have been associated with weight gain, including:

  • Several drugs used to treat psychiatric disorders such as schizophrenia, depression, and bipolar disorder
  • Steroids
  • Birth control pills
  • Some diabetes drugs
  • Some drugs used for seizure disorders

If you are being treated for any of these problems with prescription medication, talk to your doctor about whether your medicine might be contributing to weight gain.

6 - Endocrine Disorders

Several disorders of the endocrine system often lead to weight gain. These include:

You can check with your doctor to see whether you should be evaluated for any of these problems.

7 - Cardiac Disease

Heart failure is the result of several kinds of cardiac disease, and it is associated with weight gain because of fluid retention. While classically people with heart failure develop edema in the legs, as many as 10 - 12 pounds of fluid can be retained in the body without producing visible edema.

8 - Other Medical Problems

Several other medical problems can produce weight gain by causing fluid retention. For instance, kidney disorders, especially nephrotic syndrome, can lead to significant weight gain from edema. Similarly, liver problems such as cirrhosis can produce fluid retention, especially in the abdomen (a condition called ascites). Lymphedema, fluid retention caused by blockage in the lymphatic system, can also cause weight gain. 

The Bottom Line

For people who have unexplained weight gain the most common cause, by far, is related to poor dietary habits and suboptimal physical activity.

But if you are gaining weight for no apparent reason and you believe you are eating reasonably and getting a fair amount of physical activity, you should consult with your doctor about whether you might have one of the other causes of weight gain.


Allison KC, Grilo CM, Masheb RM, et al. Binge Eating Disorder and Night Eating Syndrome: a Comparative Study of Disordered Eating. J Consult Clin Psychol 2005; 73:1107.

Bhutani S, Varady KA. Nibbling Versus Feasting: Which Meal Pattern is Better for Heart Disease Prevention? Nutr Rev 2009; 67:591. 

Filozof C, Fernández Pinilla MC, Fernández-Cruz A. Smoking Cessation and Weight Fain. Obes Rev 2004; 5:95.

Greer SM, Goldstein AN, Walker MP. The Impact of Sleep Deprivation on Food Desire in the Human Brain. Nat Commun 2013; 4:2259.

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