The Top 5 Not-So-Obvious Causes of Weight Gain

Causes Of Weight Gain

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by Rod Navajas

Let’s be clear, adipose tissue (or fat) cannot be created out of thin air. It does not magically appear in the body just before you decide to try on an old suit or dress. Nonetheless, in some cases the cause of fat gain is not as explicit as we would like it to be. 

You will find conflicting claims about weigh control. Some say that the only thing that matters is the number of calories you consume. Others will blame your hormones or perhaps your age and genetics. Quite confusing indeed! Who is right and who’s wrong? Here is the answer: They are both right and they are both wrong. Read on.

Although we always try to simplify things as much as possible, weight regulation is a bit more complicated than simple mathematical equations. There are several systems and metabolic pathways in play, completely intertwined and responding to all sorts of different stimuli coming from the various sensory mechanisms we have in the body. To make matters more complicated, during weight loss or gain the body will create an adaptive response making the maintenance of the lost or gained weight even harder. I know, it’s not fair! 

In the previous article entitled Top 5 Reasons Why You Are Not Losing Body Fat, I discussed some of the reasons related to the calories in x calories out model. In this article, I will explore some of the less obvious causes that could be stalling your weight loss journey by disrupting any of the mechanisms used to control body weight. 

1. Medications

Several medications can influence weight regulation. They will make you hungrier or more fatigued, slow down the rate that you spend energy or cause water retention for example. 

Medications used for mental health disorders such as bipolar and schizophrenia are well known to cause weight gain. About 70% of patients during treatment with those drugs will experience weight gain (1).

Antidepressants, much more commonly prescribed than antipsychotics, can also cause weight gain. Elavil, Phenelzine, Remerom and Celexa are different classes of antidepressants that have been shown to cause weight gain (1). 

Insulin used for diabetes, beta blockers used for hypertension and corticosteroids used to treat inflammatory and autoimmune conditions are some of the other types of medication that can cause weight gain (1)

2. Aging

Aging is perhaps the most frustrating of all causes listed in this article, but is not a fat gain sentence. 

Aging is an inevitable process that causes several physiological alterations in the body. Females are very familiar with the term menopause for example. During and just prior to menopause, the hormonal changes that cause various symptoms it will also make you more likely to store fat around your midsection (2). Reduced energy expenditure and an accelerated loss of muscle mass, which contributes to weight gain, have also been associated with menopause (3). 

The loss of muscle mass as well as the reduction in the mass of individual organs associated with aging irrespective of gender cause a reduction in metabolic rate favoring fat gain (4). 

The good news is that training and adequate nutrition can counterweight these associated changes. For example, women participating in a weight training program 3x per week for 16 weeks were able to increase their muscle mass and increase metabolic rate by nearly 10% (5). Preserving and if possible increasing your muscle mass is one of the key components of physical training and an adequate nutritional program. 

3. Clinical Conditions

Unfortunately, some medical conditions if left untreated can also affect body weight regulation. Hypothyroidism for example can cause a decrease in thermogenesis and resting energy expenditure (6). Mood disorders, including depression and anxiety, can influence the responsiveness of your reward and pleasure system in the brain, altering eating behaviors and increasing hunger (7). 

Other conditions such as rheumatoid arthritis that can influence your ability to participate in regular exercise training can also be a contributing factor for weight gain. Fibromyalgia, PCOS and Cushing’s disease have also been commonly listed as causes of weight gain. 

4. Stress & Sleep

Once used as survival mechanism, the flight or fight response is in constant activation in modern days. Chronic stress affects the rewarding system in the brain and influence hunger levels and food preferences (8). it also induces metabolic changes that contribute to weight gain. Abdominal fat storage pattern influenced by the hormone cortisol, ultimately leads to more metabolic dysfunction and the perpetuation of the fat gain scenario (9). 

Sleep deprivation is a very prevalent modern day ailment and has been shown in multiple studies to be associated with obesity. Sleep-deprived individuals show increased hunger likely caused by alterations in the levels of the satiating hormone, Leptin, as well as the hunger hormone, Ghrelin (10).  The fatigue generated by sleep deprivation also reduced the motivation and ability to participate in physical training (Fig 1).

5. Gut Health

This is perhaps the most exciting area of research in medicine at the moment. Several discoveries have been made in the past few decades showing the influence of the gut microbiota (the collection of bacteria living in the gut) in several aspects of health. 

We now have some emerging discoveries linking the composition of bugs living in your gut with obesity. 

One landmark study in this field, demonstrated that the gut microbiota of obese mice has the capacity to extract more energy from food (11). It also showed that when the gut microbiota of obese mice is transferred to germ-free mice, it results in greater weight gain than when the germ-free mice receives bacteria from lean mice (11). In another study, colonization of germ-free mice with human microbiota from an obese twin, resulted in an obese mice (12). When the germ-free mice is colonized by the microbiota of the lean twin, the mice remains lean (Fig 2).

Although in its infancy, discoveries like this demonstrate the importance of a holistic approach when it comes to fat loss. 

Fig 2. Germ-Free mice shows increase adiposity when receives the microbiota of an obese human adult

Bottom line: Control the variables that you can control and accept those that you can’t.


  1. Wharton, S., Raiber, L., Serodio, K. J., Lee, J., & Christensen, R. A. (2018). Medications that cause weight gain and alternatives in Canada: a narrative review. Diabetes, metabolic syndrome and obesity: targets and therapy11, 427.
  2. Davis, S. R., Castelo-Branco, C., Chedraui, P., Lumsden, M. A., Nappi, R. E., Shah, D., … & Writing Group of the International Menopause Society for World Menopause Day 2012. (2012). Understanding weight gain at menopause. Climacteric15(5), 419-429.
  3. Poehlman, E. (2002). Menopause, energy expenditure, and body composition. Acta obstetricia et gynecologica Scandinavica81(7), 603-611.
  4. St-Onge, M. P., & Gallagher, D. (2010). Body composition changes with aging: the cause or the result of alterations in metabolic rate and macronutrient oxidation?. Nutrition26(2), 152-155.
  5. Trevisan, M. C., & Burini, R. C. (2007). Resting metabolism of post-menopause women submitted to a training program with weights (hypertrophy). Revista Brasileira de Medicina do Esporte13, 133-137.
  6. Sanyal, D., & Raychaudhuri, M. (2016). Hypothyroidism and obesity: An intriguing link. Indian journal of endocrinology and metabolism20(4), 554.
  7. Singh, M. (2014). Mood, food, and obesity. Frontiers in psychology5, 925.
  8. Marks, R., & Landaira, M. Sleep, Disturbances of Sleep, Stress and Obesity: A Narrative Review. J Obes Eat Disord [Internet]. 2016 [cited 2018 Oct 5]; 1 (2).
  9. Rosmond, R., Dallman, M. F., & Björntorp, P. (1998). Stress-related cortisol secretion in men: relationships with abdominal obesity and endocrine, metabolic and hemodynamic abnormalities. The Journal of Clinical Endocrinology & Metabolism83(6), 1853-1859.
  10. Koren, D., Dumin, M., & Gozal, D. (2016). Role of sleep quality in the metabolic syndrome. Diabetes, metabolic syndrome and obesity: targets and therapy9, 281.
  11. Turnbaugh, P. J., Ley, R. E., Mahowald, M. A., Magrini, V., Mardis, E. R., & Gordon, J. I. (2006). An obesity-associated gut microbiome with increased capacity for energy harvest. nature444(7122), 1027-1031.
  12. Walker, A. W., & Parkhill, J. (2013). Fighting obesity with bacteria. Science341(6150), 1069-1070.

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