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Sneaky reasons you’re struggling with unexplained weight gain

Published by Cheryl Mussatto, MS, RD, LD on Sep 24, 2023

If your clothes are getting tight or the number on the scale is going up, one of these issues could be the reason why

Here’s a common scenario when attempting weight loss:  You think you’re doing everything right – you’ve cut calories and increased exercise – but the pounds keep piling on, making it a puzzling and frustrating issue. While overeating and lack of exercise are commonly known causes, plenty of other factors are likely contributing to the problem. It’s essential to be aware of and identify these factors to address unexplained weight gain effectively.

Let’s look at sneaky reasons for unexplained weight gain that doesn’t make sense.

You’re getting older

Aging causes many physiological changes impacting weight. One of the most significant changes is the loss of muscle mass, which typically begins in middle age and decreases by around 1% annually.

Muscle loss can impact our strength and metabolism, affecting our ability to burn calories efficiently. When we have smaller muscles, our bodies require fewer calories, so if our diet remains the same, we may consume more calories than our bodies need, resulting in excess fat storage.

Chronic stress

As we age, managing stress becomes more difficult. If you experience frequent or chronic stress, your body will likely release more of the stress hormone called cortisol.

Cortisol’s role is to assist the body in replenishing energy stores. However, this hormone can indirectly promote weight gain in some individuals by increasing appetite (as the body believes it needs more energy) and storing unused energy as fat.

Stress can also lead to compulsive behaviors, such as indulging in ‘comfort’ foods that are often high in sugar, unhealthy fat, extra calories, and salt. This could be a significant factor contributing to difficulty losing weight, resulting in obesity.

Insomnia or disrupted sleep

As we age, our sleep quality may be affected. If you consistently get under six hours of sleep each night, it could impact the hormones regulating hunger. In particular, insufficient sleep can lead to increased levels of hunger hormones, decreased levels of satiety hormones, and higher cortisol levels, all of which influence weight gain.

Changes in sex hormone

As people age, they may experience a decrease in certain sex hormones. This decrease can lead to sleep problems and higher body fat for women with low estrogen levels. Men with reduced testosterone levels may experience a reduction in muscle mass.

Underlying health conditions

Experiencing unexplained weight gain, particularly if it’s a recent development, could be a sign of various health issues. One such example is that an individual with heart failure may experience weight gain due to fluid retention, which could manifest as swelling in the feet, ankles, legs, or abdomen, in addition to fatigue or difficulty breathing.

Other underlying conditions associated with weight gain include

  • diabetes
  • certain kidney diseases
  • sleep apnea (pauses in breathing during sleep)
  • thyroid problems

Medication side effects

Numerous medications can cause weight gain. For example, prednisone can cause fluid retention, leading to an increase in weight. Additionally, certain drugs can affect the brain’s chemical balance, increasing appetite and, thus, weight gain.

Examples include

  • antidepressants such as paroxetine (Paxil) or phenelzine (Nardil)
  • antihistamines that contain the ingredient diphenhydramine (the active ingredient in Benadryl)
  • antipsychotics such as clozapine (Clozaril) or olanzapine (Zyprexa)
  • beta-blockers such as the drug atenolol (Tenormin) or metoprolol (Lopressor)
  • sleep aids containing diphenhydramine, such as Sominex, Unisom SleepGels, or ZzzQuil.

Late-night eating

Waking up during the night to eat (late-night eating) is another contributor to potential weight gain. Depending on what and how much a person is eating during this time of night, it can lead to increased body fat, slowed metabolism, and daytime hunger.

Composition of your gut microbiome

There is a belief that weight gain may be caused by specific microbes in your gut, referred to as your microbiome. Research indicates that these microbes have the potential to affect your appetite, metabolism, blood sugar levels, and the way your body stores fat.

Research has also discovered that the gut bacteria of individuals who are obese differ from those who are lean in that these individuals may be genetically programmed to gain weight because of having a particular microbiome. Or it could be that people with obesity are eating differently than lean people, which may change the microbiome. These potential causes of hidden weight gain still need further research.

What you can do 

If losing weight is becoming more challenging or your weight gain has recently occurred rapidly or more excessively, see your doctor. Your doctor can check for underlying health conditions and evaluate if your medications are influencing your weight.

The best way to control weight as we age is to follow healthy living habits consistently:

  • Eat a healthy diet of fruits, vegetables, legumes, limited amounts of whole grains and starchy vegetables, and ample lean protein at every meal to help build and maintain muscle mass.
  • Avoid late-night eating.
  • Get seven to nine hours of sleep per night.
  • Exercise briskly every day for at least 30 minutes.
  • Strength train at least twice a week.
  • Practice stress reduction techniques daily

Another helpful habit is to step on a weight scale at least once a week. It’s best to weigh yourself daily, first thing in the morning, before eating, with minimal to no clothing on. This habit allows you to keep track of your weight and prevent unexplained weight gain from getting out of hand. It’s natural to experience weight fluctuations of 1-2 pounds during the week. However, if you experience weight gains of 3 or more pounds per week, it’s essential to consult with your doctor for further follow-up.



Cheryl Mussatto, MS, RD, LD

Cheryl Mussatto MS, RD, LD is a registered dietitian with a master’s degree in Dietetics and Nutrition from the University of Kansas and a bachelor’s degree in Dietetics and Institutional Management from Kansas State University. She is a clinical dietitian for Cotton O’Neil Clinics in Topeka and Osage City; an adjunct professor for Allen Community College, Burlingame, KS where she teaches Basic Nutrition; and is a freelance writer and blog contributor for Dr. David Samadi, Urologic Oncologist Expert and World Renowned Robotic Surgeon in New York City. Cheryl is also the author of The Nourished Brain, The Latest Science on Food’s Power for Protecting the Brain from Alzheimers and Dementia and The Prediabetes Action Plan and Cookbook, both available on Amazon in Kindle and paperback editions.

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