Disease and medication can make you fat, and there’s nothing you can do about it. (My doctor said so.)

I understand that doctors don’t always have the time to advise patients in great depth about the effects of the medication they are taking. But oversimplified statements like ‘may lead to weight gain’ are read by the non-specialist public as ‘You might put on weight from taking this. Don’t worry about it if you do, and definitely don’t bother trying to lose weight by dieting.’ Propagating this kind of false information is irresponsible — especially when it’s spread by the medical profession. In fact, medications usually only lead to weight gain in combination with other pre-existing causes, and those causes should be identified wherever possible because once they are known, patients certainly can do something to combat true weight gain.

The most serious illness which causes obesity is Prader-Willi Syndrome (PWS). The website of the German Prader-Willi Association (prader-willi.de) describes the condition as follows:

People with PWS are usually small in stature. Their muscles tend to be very weak, especially in early childhood, which often makes physical activity difficult and tiring for them … People with PWS are unable to feel ‘full’. A defect in the thalamus means they have no feeling of satiety, leaving them unable to regulate their eating behaviour.

This means that people with PWS have both a permanent uncontrollable appetite for food and also reduced muscle mass, meaning their food requirements are actually lower than normal. Combined, these are the worst possible conditions for avoiding obesity. Nevertheless, in a controlled environment, even people with PWS can achieve and maintain normal weight, as long at their energy intake is carefully monitored. Once again, it comes down to the balance of energy intake and output, although in people with PWS this balance is extremely difficult to maintain.

Another medical condition which can cause weight gain in women is polycystic ovary syndrome (PCOS), which causes hormonal imbalances. Even so, a survey by Norman et al. (2004) of past research showed that there was no relevant difference in rates of success in weight-loss programs between women with PCOS and women without it. The difference in their energy requirements was also very slight or non-existent. In one study which showed small differences in energy requirement, the possible weight gain due to metabolic rate was calculated to be less than 2 kg within a year — and even then, only if caloric intake was not altered to take the metabolic difference into account.

Then there is the always-mentioned hypothyroidism (underactive thyroid), which really does cause the body’s metabolic rate to slow down. I am always embarrassed when people try to praise me by saying things like, ‘It would have been so difficult for you to lose weight with an underactive thyroid condition. It’s incredible that you did it!’ It’s embarrassing because it isn’t extra-hard for me to lose weight. As long as my hypothyroidism is treated with hormone replacement tablets, there’s no fundamental difference between me and a healthy dieter. Also, the metabolic deceleration is not all that big; at 10 to 15 per cent, you can certainly feel it in energy levels, but it can be compensated for by dietary changes or by 20 to 30 minutes of physical activity. Nonetheless, the condition does initially result in uncontrolled weight gain, which has nothing to do with food intake. On its website, the American Thyroid Association states that hypothyroid patients may experience fluid retention resulting in weight gain of approximately 2.5 to 5 kg. This is obviously a large initial increase, but it isn’t real weight gain, as it can quickly be lost again with the right treatment. It will not cause obesity in anyone.

Fluid retention is often the cause of illness- or medication-related weight gain, especially if the medication contains hormones or cortisone. There’s not much that can be done to counteract this kind of weight gain, so the best approach is to endure the effects patiently, secure in the knowledge that they will disappear again once the course of medical treatment is over. Although you can also talk to your doctor about tackling the fluid retention with treatments like lymphatic drainage or diuretic tablets.

I remember watching a daytime talk show some years ago in which an incredibly arrogant thin woman said that she’d gained weight in the past due to the medication she was taking, and that she simply didn’t understand why fat people couldn’t lose weight. She had been able to lose 7 kg in two weeks without even trying. It is highly likely that this woman didn’t have to try very hard to lose 7 kg — of water. It isn’t comparable with the feat of losing 7 kg of fat (i.e., 49,000 kcal). To lose 7 kg of fat in two weeks, the talk-show lady would have had to shed half a kilo per day. If her daily energy consumption is estimated at a rather generous 2000 kcal, that would mean doing about three hours of intensive physical exercise every day while eating absolutely nothing. Although with that regime the chances are high of losing not only 7 kg of fat, but also her life — or at least of achieving total physical collapse.

Fluid retention is not the only way in which medication and illness can affect a person’s weight. Some medicines make patients more energetic, others make them lethargic. The patient might not even be conscious of the effect, but these kinds of drug can lead to changes in your daily habits, reducing your general level of activity, and causing you to opt for the elevator rather than the stairs, choose to lie down rather than do some gardening, or to put your gym visit off till tomorrow. Over time, this can lead to a drop in energy consumption of a few hundred to a thousand calories per month.

A medicine’s effect on the appetite should not be underestimated either. Very few people count every single calorie precisely or weigh out every single gram of food before they eat it. As we have already seen, we tend to be very bad at judging the amount we actually eat, and a change in appetite can also cause a change in this ability to perceive how much we are eating. It can lead people to pile a little more food on their plate or to reach for the snacks drawer in their desk more often, without giving them the feeling of having eaten more than usual. Their minimum for feeling that they have eaten enough increases along with their increasing feelings of hunger. What we normally notice is changes away from our baseline behaviour. If an altered appetite makes us feel permanently hungry, we may feel like we’ve eaten less than usual when in fact we have eaten more — simply because we feel hungry more frequently or we feel we have to fight feelings of hunger more often.

The bottom line is that medicine and illness can make it more difficult to achieve a balance between energy intake and expenditure. But they do not make you fat per se, nor do they remove your ability to lose weight.