As far back as I can remember, I was overweight. My parents were morbidly obese, just like three of my grandparents (my maternal grandmother was “only” obese – in other words, just fat). In my teenage years, I tried every diet going, and would sometimes lose weight, only to put it all on again. Eventually I concluded that so-called “normal” weight just wasn’t realistic for me. It would mean a life of permanent hunger.

I also decided such a life wasn’t necessary: excess weight was demonised without reason. I might be fat, but I didn’t smoke, drink, consume fast food or red meat. And I was physically fit. I decided to set other priorities in my life; I trained as a psychotherapist, got married and started renovating an old house.

At the age of 30, I tipped the scales at 150kg (23st 8lb). If asked, I told people I was comfortable with my weight and that my only wish, if anything, was to be a bit fitter. But at that same time, I was secretly visiting an obesity clinic as an outpatient and thinking about a stomach-stapling operation. I eventually decided not to have the operation, and buried myself even deeper in studies that showed that being overweight was not really harmful.

My doctor never mentioned my weight. Every time I went, my blood pressure was enormously high, but I dismissed it as “white coat hypertension”. I managed to ignore the fact that I was suffering frequent back pain and that I was having trouble sleeping, in part because of my heavy snoring.

This went on until, one day, I slipped while doing housework and injured my knee. I know now that I tore my cruciate ligament; but at the time my doctor said it was probably nothing to worry about and prescribed me ibuprofen. Then I had another accident while renovating our house. I walked with a limp for months, until it happened again.

After more than a year of pain and restricted mobility, I had a breakdown. I realised that, over the previous few years, my health had decreased while my weight had increased. And I knew that if I kept going this way, within a few years I would be unable to walk – while still in my early 30s. Something had to change.

For the first time, I consciously started thinking about my eating behaviour and began reading up on genetics, metabolism, diets and obesity. Although I had read around the topic for a while, I had done so selectively. Now, I began to explore the 95% of the research to which I had turned a blind eye. I came across the term “fatlogic” on Reddit and it immediately resonated with me. The term doesn’t mean “fat people’s logic”, but refers to the complex grab bag of supposedly medical facts, well-meaning advice, homegrown ideas and fantasies that make losing weight not only difficult, but impossible.

I don’t consider myself stupid or naive. I have always been the kind of person who questions things; I have a doctorate, and an interest in science. But still I believed in so much fat logic, probably because I was always surrounded by it. I was told from an early age that our family had “fat genes” and that my metabolism was “broken” – which appeared to be corroborated by my own experiences.

Tearing down the fallacies I had believed for my whole life was a long and sometimes painful process. But in the following year, I began to put it to practical use. I restricted my calorie intake. I moved more. Within a year, I was in the normal weight range for my 175cm (5ft 9in) height; and a few months later I weighed 63kg (9st 13lb) – the least I had weighed since the age of 12 or 13.

Fat logic is not just a problem for fat people; I have never met a person who was completely free of it. Here are a handful of the most persistent myths, debunked.

Myth 1: ‘I eat only 1,000 kcal a day, but I don’t lose weight’

There’s one thing we can all agree on: everybody needs energy. A widespread fallacy is that there is a huge range of difference in people’s metabolic rates. The amount of energy we need is influenced by various factors, but the main ones are body mass, and what that mass is made up of. A person’s energy consumption can actually be calculated relatively precisely using certain formulae. The only information you need is height, weight, sex and approximate daily activity levels. You can find plenty of online calculators; just search “basal metabolic rate calculator” (this is the number of calories you would require if you were resting all day).

There’s a high probability that your BMR will lie somewhere between 1,400 and 2,000 kcal a day – unless you happen to fall into one of the two extremes of very high or very low body mass. The bottom line is that most people use far more than 1,500 kcal a day, but even people with extremely low consumption still need significantly more energy than 1,000 kcal. Which means it’s practically impossible not to lose weight on a daily calorie intake of 1,000 kcal.

So the question is, are you eating as little as you think you are?

Overweight people have a strong tendency to underestimate the calorie content of their food

Despite the common cliche of the fast food-guzzling, fat person, my favourite meal used to be a large mixed salad with salmon. I ate it regularly, and in my mental calorie journal I would estimate it contained about 500 kcal. When, after many years, I finally weighed out all the ingredients and calculated the actual number of calories they contained, I discovered that the dressing alone, with three tablespoons of olive oil, contained about 300 kcal.

The number of calories in the salad itself – tomato, cucumber, red pepper and lettuce – was within reason. Mozzarella, though, added considerably more to the total, and the fact that the salmon was fried meant the final tally for this meal was 1,500 kcal – three times the amount I had estimated, and equivalent to the entire daily energy requirements for a small, slim woman.

People can hugely misjudge their calorie intake, and overweight people have a strong tendency to underestimate the calorie content of their food. A study carried out in 1992 investigated people described as “diet-resistant”. These people claimed not to be able to lose weight, despite restricting their calorie intake to fewer than 1,200 kcal a day. But it turned out that, in their nutrition journals, they underestimated their average calorie intake by 47% and overestimated their physical activity by 51%.

The hard truth is that anyone who believes they “don’t actually eat that much” and then still inexplicably puts on weight doesn’t have a problem with their metabolism, but with their perception of their own eating habits.

Myth 2: ‘Being overweight isn’t that bad for you’

This is the fat logic argument I encounter most often, and which I believed myself for many years. It is also the one I kick myself about the most, in retrospect. I always claimed to have made a rational decision about my weight, but I was labouring under two misapprehensions: that it is extremely difficult to achieve and maintain normal weight; and that it doesn’t have all that many advantages anyway. Now, I argue the opposite whenever I can.

I respect anyone’s decision to set other priorities and happily accept being overweight or obese. Just because you can change a situation, it doesn’t mean you must. That said, it’s important for that decision to be an informed one.

This is not about whether your bum looks better as a size 36 or a size 42. Rather, it’s about what goes on inside our bodies, and about how being overweight directly affects our quality of life. Obesity is a bit like smoking: the tumours don’t start growing right after the first cigarette. For someone who is naturally prone to lung problems, it might take five years. Another person’s lungs might be able to take 50 years of constant damage. But just because the damage isn’t visible, it doesn’t mean it isn’t there.

Analysis from 2013 investigated the long-term consequences of obesity with the specific aim of examining so-called “healthily obese” people. A comparison between healthy people of normal weight and healthy but obese subjects showed the latter group had a significantly higher risk of dying or developing cardiovascular disease. The scientists who carried out the study therefore came to the conclusion that the belief you can be “fat but fit” is just a myth.

A 2015 study confirmed those results. It followed supposedly healthily obese subjects over 20 years and found that more than half became unhealthily obese during that time. Their risk of becoming ill was eight times higher than that of the healthy group with normal weight. The risks include, but are not limited to: diabetes, cardiovascular disease, cancer, sleep apnoea, arthritis/joint problems, fertility problems, asthma, back pain, incontinence, gout and stroke.

Myth 3: ‘Being overweight doesn’t impede me’

For a long time, I convinced myself that being overweight didn’t impact particularly negatively on my life. I’d suppress the panting as I climbed the stairs, so I could tell myself I had no problem walking up three floors. Once at the top, I would sometimes pretend to cough or laugh to hide the fact that I was out of breath.

Our society makes it very easy for us to delude ourselves. People who exercise regularly are seen as “fitness freaks”, “sports fanatics” or similar, while “normal” people are the ones who lead physically inactive lives. Now that I can compare the abilities of my well-trained body (and I’m absolutely not athletic or super-fit) with my abilities before, I’ve come to realise how far below optimum my fitness level really was. I’m not saying there aren’t any overweight people who do a lot of exercise. But I can imagine lots of people fall prey to a similar kind of distorted thinking as I did: I used to consider even relatively normal things to be great sporting achievements.

The same applies to the achievement a US leader of the fat-acceptance movement, Ragen Chastain, claims makes her an “elite athlete” – with a morbidly obese BMI. In 2013, she ran a marathon and published an article about it with the title My Big Fat Finished Marathon. She wrote about how, after five months of training, she covered just over 40km in 12 hours and 20 minutes.

In the first few months, I was proud of reaching various milestones, such as walking for half an hour without stopping

It is an achievement for a severely obese person to walk the entire length of a marathon in one go. But Chastain’s average speed of less than 3.5km (2.2 miles) an hour is much slower than normal walking speed. The marathon had officially ended hours before she crossed the finishing line – the stands removed, the organisers gone. The last participant to complete the race, several hours before Chastain, was a woman in her 70s.

Of course, everyone has to start from their own fitness level. When I weighed 150kg and was more or less unable to move for six months, average sporting achievements were as likely for me as breaking Olympic records. In the first few months, I was proud of reaching various milestones, such as walking for half an hour without stopping, or spending 20 minutes on a bike for the first time in years.

It’s good to be proud of your own development and individual progress, even when it might not objectively seem that impressive. But declaring your own, below-average performance to be an objective record, and therefore to claim that any improvement is unnecessary, will only stop you – and others – from tackling the problem of excess weight.

Myth 4: ‘My family and friends don’t think I need to lose weight’

This statement is fat-logical only when referring to people who are not underweight or for whom losing weight would mean they would become underweight. Let me start with my own experience. When I weighed 150kg, there was no one who seriously claimed that losing weight would not be a good idea for me. But apart from my mother, as far as I can remember, in all those years nobody ever asked me about my weight. My weight was the elephant in the room, which no one mentioned – until I brought it up myself.

I lost my first 40kg (6st) in secret, without anyone noticing. When I reached about 105kg (16st 7lb), everyone around me suddenly noticed I’d lost weight. At over 100kg, I was still very much within the obese range, but others saw it quite differently. From all sides, I was asked, surely I didn’t want to lose any more weight? I must be done with my diet now, right? Yeah, that’s terrific losing so much weight, but you don’t need to lose any more – surely?

A neighbour who saw me gardening worriedly asked my husband how much I now weighed and asked him please to make sure I ate more. When I ran into a colleague on the street, she half-jokingly asked when I was going to be diagnosed with anorexia; another admitted he deliberately hadn’t reacted too enthusiastically to my new size for fear I might go to “the other extreme”.

It was ironic: when I was sick and almost bedridden at 150kg, no one ever expressed concern or commented on my weight in any way. And then, when I lost 40kg, was able to walk again and feeling better than I had for years, people started to get worried about my health. It was as if my body had suddenly become a public forum, after years of having been a taboo subject.

Why is it so socially acceptable to criticise someone for losing weight? Because most people don’t know what overweight looks like. In one British study, obese people were asked to assess themselves, and only 11% of women and 7% of men with a BMI of over 30 were aware they were obese. In a 2015 study, parents were asked about the weight of their children: 80% of parents of overweight children rated them as being of normal weight.

Myth 5: ‘Obesity is largely due to your genes’

Genes create a basic situation, but they don’t oblige anyone to be fat. Things that can genuinely be explained by genetics are appetite, preferences for certain flavours (such as sweet or fatty) and the natural urge to be physically active.

Several studies have shown that carriers of so-called obesity genes consume on average 125-280 kcal a day more and have no differences in their metabolic rates. To say that some children have a genetic propensity towards obesity means only that they have an inherently larger appetite than naturally slim children, who feel hungry less often.

But the deciding factor in whether children have a tendency towards being fat is the set of conditions created by their parents and the rest of their environment (such as school meals), which can serve either to encourage or discourage obesity. Living in a household where high-calorie food is constantly available won’t necessarily make children fat if their genetics mean they have a naturally small appetite. Children with naturally large appetites, by contrast, will pounce on the proffered fare.

However, studies have shown that food preferences are not an inescapable fate. In one experiment, the brains of obese and normal weight subjects were scanned to record their reactions to food. The reward centres in the obese subjects’ brains showed a strong reaction to high-fat foods (fast food, sweets). The test was repeated after the subjects had followed a dietary plan containing healthy, low-calorie foods for several months. The reward centres in the obese subjects’ brains reacted more strongly to these foods in the second test.

In the end, our genes just set out the path we will follow if we don’t actively strive to change its direction (which can take great effort). However, those efforts are only temporary: once we have become habituated to new behaviours, we no longer have to struggle to maintain them.

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So how is my life now that I’m maintaining my target weight? The actual weight loss hasn’t changed much, but the impact on my life has been great. The fact I now take pleasure in exercise has opened up an entire spectrum of new interests to me that would once have been out of the question. My husband and I went on a cycling holiday. I’ve discovered climbing as a new hobby – and pilates, too. My gym buddy and I now meet once a week for coffee and weight training. I’ve gained so much – and have come to see that being thin doesn’t have to mean a life of constant deprivation.

• This is an edited extract from Conquering Fat Logic by Nadja Hermann, published by Scribe on 10 January at £14.99. To order a copy for £11.99, go to guardianbookshop.com.

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