Your waistline has been gradually expanding through the years. You feel sluggish and tired, sometimes even on waking up. What could be the problem?

A slow metabolism could be the culprit. But while gaining weight and feeling fatigued could be signs of slow metabolism, it’s important to know that medically induced slow metabolism is quite rare. In most cases, slow metabolism is caused by poor diet, lack of sleep, or lack of physical exercise.

What is metabolism?

Simply put, metabolism is how your body turns calories into energy. When someone says they have a slow metabolism, they mean that their body holds on to calories, causing unwanted weight gain. Often, when people talk about metabolism, they’re referring to their metabolic rate -- which is the number of calories your body burns in a period of time.

People have different metabolic rates depending on factors such as sex, genetics, age, diet and physical activity. You can get a resting metabolic rate (RMR) test at a specialised clinic to accurately determine your metabolic rate.

Here are some hidden medical reasons for slow metabolism:

1. You have too much cortisol

One of the most common medical causes of slow metabolism is when your body produces too much cortisol, otherwise known as “the stress hormone.”

Normal levels of cortisol in the body work together with other hormones and chemicals to help you burn fat. However, if you have excess cortisol levels, which can happen when you’re stressed over a long time period, it might slow down your metabolism. You might also have excess cortisol if you have a medical condition called Cushing’s syndrome, a disorder which causes the adrenal glands to release too much cortisol into your bloodstream.

Excess cortisol often slows down your metabolism, leading to weight gain and other health issues. For instance, according to two separate studies published in the journal Science in 2013, elevated cortisol levels can trigger mental health issues and decreased resilience. Excess cortisol circulating in your bloodstream can also cause symptoms such as fragile skin and stretch marks.

2. You have high insulin levels

Insulin is mostly implicated in blood sugar management but it also affects lipid, carbohydrate and protein metabolism. Having high insulin levels has been linked to health problems such as obesity, heart disease, and even cancer.

High insulin levels in your blood can also lead to insulin resistance, signalling the pancreas to produce even more insulin. This creates a vicious cycle which may progress to Type 2 diabetes.

Insulin resistance is closely associated with obesity -- the two have a bit chicken-and-egg relationship. High insulin levels can lead to obesity, while obesity can lead to insulin resistance. However, it’s also possible to have insulin resistance without being overweight or obese.

Studies have found that high insulin levels can be triggered by stress. A study from Ohio State University’s Wexner Medical Centre found that women who reported being stressed had higher levels. The subjects were given the same high-calorie meal, after which their insulin levels were measured by the researchers. The women who reported being stressed burned 104 calories less than their counterparts – which could add up to 11 pounds of weight gain in a year.

You can reduce insulin levels through low-carbohydrate or ketogenic diets, eating low-calorie meals, having weight loss surgery, and a regular exercise routine.

3. You Have Thyroid Problems

The thyroid in an endocrine gland which plays a vital role in metabolism, growth, and development of the human body. When the thyroid gland fails in its functions, various body functions including metabolism are affected. If you don’t make enough thyroid hormones, a condition known as hypothyroidism, your metabolism will slow down.

The most common type of hypothyroidism is Hashimoto’s disease, an autoimmune disease that affects the thyroid gland. People who have Hashimoto’s disease produce antibodies which attack the thyroid gland. Experts are not very clear on how the thyroid affects metabolism. However, they know that thyroid hormones increase the body’s oxygen consumption, which affects the basal metabolic rate. Therefore, patients with hypothyroidism consume less oxygen which in turn means they have lower basal metabolic rates.

4. You’re taking medication

Certain types of medication can also cause your metabolism to slow down. For instance, if you have been taking anti-inflammatory steroids, they can elevate your appetite, cause insulin resistance, leading to consumption of more calories, slower metabolism, and weight gain.

Antidepressants can also slow down metabolism and cause weight gain. In fact, research has shown that excessive weight gain made patients to quit their antidepressant medication. Beta-blockers, which are prescribed to treat high blood pressure, can slow your heart rate and make you feel less energetic which ultimately leads to slower metabolism and weight gain. To offset the effects of medication on your metabolism, eat a healthy diet and exercise regularly.

5. Unbalanced Hormones

a.) For the women, you have low oestrogen levels

If you’re a menopausal or post-menopausal woman, you might notice weight gain or feel constantly fatigued- which could be symptoms of slow metabolism. In a 2011 study published in Cell Metabolism, researchers traced the effects of hormonal changes during menopause on different parts of the brain.

“When women approach menopause, they gain weight in fat and their energy expenditure goes down,” explained one of the researchers. Oestrogen acts on receptors found throughout the body- in fat, ovaries, and muscle tissue. When it comes to metabolism, research found that oestrogen affects receptors in the brain. To test this theory, the researchers deleted those particular oestrogen receptors from brains of mice. The rats with the deleted receptors consumed more food and got fat.

Just like the mice, it’s no secret that menopausal women are prone to weight gain even without necessarily consuming more food. Lack of oestrogen typically boosts weight gain while decreasing muscle mass.

b.) For the men, you have low testosterone levels

Men aren’t off the hook when it comes to slow metabolism caused by sex hormones going haywire. As men grow older, their testosterone levels decline, which consequently leads to decrease in muscle mass and weight gain. In a study published in the Journal of Endocrinology in 2013, researchers found that low testosterone levels had major influence on body fat composition, reduced insulin sensitivity, and impaired glucose tolerance.

Building muscles isn’t the only way testosterone interacts with the body’s metabolic system. The male sex hormone also plays a key role in bode growth and development during childhood. In adulthood, testosterone helps maintain bone density. In fact, studies have shown that lower testosterone level in older men is responsible for the increased risk of fractures.

To boost your testosterone levels, your doctor might suggest some lifestyle changes such as a healthier diet and a regular exercise routine. They might also, as a last resort, prescribe replacement therapies.