It is well known that constipation is a common problem. It means either going to the toilet less often than usual to empty the bowels,or passing hard or painful stools. To answer your question, it must be first established what the term weight gain implies.
Weight gain is an increase in body weight.It can involve an increase in
→ Muscle mass
→ Fat deposits
→ Excess fluids such as water or other factors.
A person generally gains weight by increasing food consumption, becoming physically inactive, or both.
Basically When energy intake exceeds energy expenditure the body stores the excess energy as fat. However, the physiology of weight gain is complex involving numerous hormones, body systems and environmental factors. Hence the term weight gain involves a very broad spectrum of causation. At present research has shown that constipation can cause weight gain.and there are several theories regarding the same.
Constipation can be a sign of an imbalance in gut flora and topping up levels of friendly bacteria has a positive effect on the breakdown of food and absorption of nutrients.Gut flora is basically the bacteria which populate one's digestive system and may play a key role in weight management. There is evidence that weight gain
and the composition of a person’s gut bacteria are tied. These bacteria are said to reduce the feelings of bloating and heaviness in the digestive system and can give a lighter feeling and reduce the appearance of stomach bloating.This is altered in constipation and is said to be one of the reasons of a feeling of weight gain.
Another obvious reason of weight gain in this condition is that in this condition, the individual may not be able to completely clear the faeces from the rectum. So,in chronic constipation, there is more chance of accumulation of faeces in the rectum and bowels.Technically, it can increase your body weight.For example,if 200 g of stools accumulate each day, one will gain one kg of weight in 5 days. That is a sudden weight gain.However, the increase in body weight is temporary, not permanent.To be clear,this theory suggests that one may have some short term weight gain due to severe constipation. However, it doesn’t mean that the person is getting fatter.it is not a fat accumulation,just a faecal weight.These two theories provide a direct link as to how constipation could cause weight gain.
There are other conditions which are associated with weight gain and have constipation as a symptom and hence could be misunderstood.One of which is an underactive thyroid. The thyroid gland secretes a hormone called thyroxine. This thyroid hormone controls many metabolic activities of the body.Low levels of thyroid hormone,slow down metabolic rate.This could lead to gain in weight. Constipation is a cardinal symptom of underactive thyroid.The second could be eating lots of junk food. These unhealthy foods have lots of calories which are usually loaded with fat or refined sugar. Eating these fast foods will make one put on weight. In addition, these foods don’t normally have enough fibre which can lead to constipation.Not enough physical activity and lack of exercise makes a person fat. In addition,it makes the bowels lazy. Slow bowel movements make the stools hard and difficult to pass.
Hence to answer your question more specifically we can say constipation can cause weight gain to a small extent. Most of the time, it is an association rather than a causation.