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Why do I gain weight?


As a doctor working to help people who want to lose weight, my initial duty is always to find out why weight gain occurred in the first place. This is one of the most important questions if we really want to resolve the issue.

1. “It’s in my genes, doctor”

Yes, genes do play a role in weight gain and in the difficulty to lose weight, but that is only a part of the story.

First, I want to say I am not going to talk here about the rare genetic disorders that directly provoke obesity. Rather, let’s look at how our genes might affect our risk of gaining weight in a certain environment.


Our genetic makeup has developed over hundreds of thousands of years. In prehistoric times, when famine and hunger were the greatest killers, those of our ancestors with genes that better enabled them to store fat were more likely to survive. So, the genes that favoured fat storage, the so-called “thrifty” genes, gradually became more widespread, as our ancestors who didn’t have these genes died. Those genetic variants, which took hundreds of thousands or millions of years to evolve, saving societies facing famine and hunger, are not going to readjust in the short time that it has taken for technological advances in agriculture to eliminate hunger from developed countries and provide us with a vast nutrient excess.

To date, more than 350 genes have been implicated in the control of body weight. Some will affect our ability to absorb and store fat, others might affect appetite, and still others might affect our ability to burn fats and carbohydrates. A large number of genes may affect each characteristic; each one will have a small effect, but together they produce a significant tendency. If this tendency is to weight gain, we need to be careful.

Here, let me make two important observations:

  • First, we each have our own, unalterable genetic makeup. We are unlikely to have all the variants that predispose to weight gain or all the ones that do the opposite; we will have a mix. This is what makes us different.
  • Second, these genes affect our susceptibility to gain weight, they don’t make us overweight or obese. Their effect on metabolism, on the body’s handling of fats and carbohydrates, on brain reward circuits, etc, will make us more or less likely to gain weight when faced with a calorie excess.

Epigenetics is the study of how our environment can affect the activity of our genes. Genes can be “turned on” or “turned off”. This starts in the womb, as environmental factors that affect the mother can also affect gene activation in the foetus, with the results sometimes only becoming visible later in life.

An understanding of the activity of genes means we should be able to undertake actions that will make those genes have less of an effect, either by making them less active, or by activating different pathways that bypass or counteract their negative effects. This is where genetic testing is leading, and the Human Genome Project really helped to get things moving, but the field is still in its infancy. The most recent profiling tests are beginning to show benefits that help people overcome weight issues.


As we age, we tend to reduce our spontaneous physical activity and structured exercise, burning fewer calories. This can lead to muscle loss, which can start as early as 30 years of age, and the proportion of body fat tends to increase. Other metabolically active tissues also shrink with age. For example, the brain accounts for about 25% of resting metabolic rate; brain size reduces with age, and so does its energy requirement.

Race and Ethnicity

A study of the resting metabolic rate found it to be lower in African-America than in Caucasian women.

2. Other factors affecting weight gain can be acted on

Willpower and lifestyle

Let’s get this out of the way right now: I don’t know anyone who actually wants to be obese. Eating behaviours and lifestyles that make us obese are determined by the interaction between our bodies (genetic makeup, metabolism, psyche) and the environment (food availability, social pressure, advertising, etc). While an occasional “lack of willpower” may mean we eat something we shouldn’t, persistent excessive calorie intake has far more complex origins.


Some drugs cause weight gain. Certain antidepressants and other drugs acting on the brain are the most common culprits. Also, hormones such as steroids and insulin. Obviously, the need for the treatment outweighs their side effects, or your doctor would not be prescribing them. However, sometimes we can remain on a treatment longer than strictly necessary, so regular reassessment should be performed, or alternatives may be found that remove or reduce the intensity of the effect.


  • The food environment: 

    Probably the most pervasive environmental factor affecting our weight. The food industry is, in some cases, more interested in profit than in our health. Studies by the Universities of Yale and Michigan have shown that the right combination of fats and sugars creates the greatest sensation of pleasure. Is it surprising that much fast food has “just the right balance”? In shops, junk foods are wrapped in bright packaging and are carefully placed on shelves known to attract most attention.

  • Advertising: 

    The marketing of unhealthy but highly pleasurable foods is often aggressive and frequently aimed at children. The money spent on advertising by companies far outweighs the amount spent on health campaigns.

  • False advertising:

    Special attention must be paid to false claims on food adverts. Certain breakfast cereals are particularly susceptible to this approach by food companies. Many of these cereals are so highly purified that they are no healthier than eating a bowl of sugar, but their manufacturers may make a minute change, such as adding a vitamin, that allows them legally to advertise the food as healthy, while it actually continues to be highly damaging.

  • Dubious information:

    In the same line, some claims that food preparations can help you lose weight or are health foods are, let’s say, misplaced. Fruit smoothies have gained great popularity recently but, before thinking that you have a health drink in your hand, you should look carefully at the sugar content and understand that turning fruit into a liquid simply releases its sugars for very rapid absorption, and may make them harmful.

  • Food availability: 

    Much in line with the above two factors, what we eat is also determined by what is accessible (within the distance we are able to travel to get food and within the price range we are able to pay). In the so-called “food deserts”, there are so few shops selling fresh foods that people can be forced to rely on processed and ultra-processed foods, which are often high in salt, sugars and fats. Food deserts are more prevalent in more deprived areas.Likewise, “healthy” food is often considerably more expensive than processed foods; and fast-food restaurants are certainly the cheapest food outlets.


  • Physical activity and exercise: 

    The “move more” part of the equation. Yes, it is very important to remain physically active to preserve good health. But it has been shown that exercise by itself will not achieve weight loss (just as a diet without exercise is ineffective – you might lose weight, but much of that weight may be muscle rather than fat).

    That said, sedentarism is harmful to health and to your waistline. Physical activity not only burns calories, it also maintains flexibility and provides the stimulus the body needs to ensure muscle tissue is preserved.
    With our ease of transport, entertainment piped into our homes 24 hours a day, food brought to the door, and even working from home with lockdown, the modern lifestyle is much less active than it used to be. We have to make a conscious decision to be active.

  • Yo-yo dieting:

    The lack of effective diets, with weight regain being the norm, drives us to repetitive diet attempts. If you use a diet that lacks specific nutrients or is not associated with muscle-preserving exercise, each diet attempt will provoke a loss of muscle as well as fat; but weight regain is typically just fat. So, after a few cycles you end up at pretty well the same weight but with much less muscle and a higher percentage body fat than before you started.

  • Sleep and stress: 

    Both of these factors affect weight. Stress is associated with elevated levels of cortisol, the “stress” hormone. This hormone can cause weight gain, especially if chronically high. Sleep is essential for many aspects of well-being. Insufficient sleep is closely associated with weight gain.

Chronic disease and disability

Here especially we must think of mental health and musculoskeletal issues, such as arthritis and trauma. Depression is a recognised cause of weight gain. Skeletal problems interfere with mobility and exercise. These conditions also often lead to multiple drug treatments, some of which can exacerbate weight issues. But all chronic diseases and disabilities will affect lifestyle and weight management to a greater or lesser extent.

Economic Status

Socioeconomic status is inversely related to weight for various reasons, including some of those described above: the affordability and availability of healthy foods, work-related stress and unemployment, and a higher prevalence of chronic diseases such as type 2 diabetes.

Given all that seems to be working against us, how can we control our weight?

The first task is to be aware of our situation and environment. Much is talked about mindfulness these days, and I believe it is an excellent path to a better and healthier lifestyle.

But there is more. All the different aspects that I discuss above make it very difficult to really understand what is happening in our bodies, and which particular aspects of our diet and lifestyle we can most beneficially change to improve our health and well-being.

For some people, awareness of what they eat and how they live will be sufficient to get back on track. For others, a more structured approach with professional guidance on specific dieting techniques and exercise is needed to resolve a situation that is out of control and putting them at risk of chronic disease.

If you have been trying to lose weight and get healthy but never quite seem to manage it, or the improvement is fleeting, seek help. This can range from self-help groups to high-level professional advice. I would say that concentrating on just one aspect of lifestyle, such as “just exercise” or “just diet” to lose weight, is likely to fail again. A holistic approach is needed, with a lifelong adaptation to a new way of life that will not only help you reach your goal, but keep you there in the long-term.


Get yourself on the right path with an individualised and personalised plan to achieve a healthy weight and lifestyle. Call Dr Bazire on 07702 737 367.