If our bodies are designed to regulate our weight, why are more people living with obesity than ever before?
Our genes haven’t changed significantly over recent decades, but our environment, lifestyles, and individual biology all influence how our bodies regulate hunger, burn energy, and store fat.
Understanding these factors can help explain why managing weight is more difficult for some people than others, even when they’re trying to make healthier choices.
How do our genes influence our weight?
Our genes influence how likely we are to gain weight by affecting factors such as appetite, metabolism, and fat storage. Research suggests they account for around 40-70% of the differences in BMI between people.1
Although hundreds of genetic variations have been linked to weight, our genes have changed little in recent decades, despite obesity becoming more common.
Our environment and lifestyle also matter. Factors such as nutrition, physical activity, and early-life experiences can affect how our genes work without changing our DNA. These changes, known as epigenetics, may influence weight regulation, although more research is needed.
Your genes can therefore make you more susceptible to weight gain, but they don’t determine your weight.
Why does our modern food environment make weight gain easier?
Convenient, inexpensive, and tempting foods are now widely available, making it easier to eat more than your body needs. But weight gain isn’t just about what you eat – how quickly you eat, how much you eat, and why you eat also matter.
Foods rich in protein and fibre tend to keep you feeling full for longer, while meals low in these nutrients may leave you hungry sooner. These foods can also produce a more rapid rise in blood sugar, particularly in people with insulin resistance.
Eating quickly or while distracted can also make it harder to recognise when you’re full. Stress, boredom, larger portions, and sugary drinks can further influence how much you eat.2
Do ultra-processed foods contribute to weight gain?
Ultra-processed foods do contribute to weight gain. One study found that people eating an ultra-processed food (UPF) diet consumed around 500 more calories per day than those eating minimally processed foods, despite meals being matched for key nutrients.3
Another study also found greater weight loss with a minimally processed diet than a UPF-rich diet.4
However, not all processed foods are unhealthy. Tinned beans, frozen vegetables, and natural yoghurt can all be nutritious choices.
Our food choices are also shaped by food costs, time, cooking skills, and access to nutritious options.
Understanding these influences helps explain why weight management isn’t simply about willpower – and why supportive eating habits and a healthier food environment matter.
How has modern life changed how much energy we burn?
Modern life can reduce how much energy we burn by limiting our everyday movement.
Our bodies use most of their daily energy to keep us alive, including breathing, circulating blood, and regulating body temperature.
Much of the energy we burn through movement comes from everyday activities such as walking, climbing stairs, doing housework, standing, and even fidgeting. However, sitting at work, driving, and spending leisure time on screens can reduce how much we move.
Our ability to stay active is also shaped by our health, work, finances, and access to safe places to walk or exercise.
Regular exercise is important, but so is moving throughout the day. Small changes, such as taking the stairs or breaking up long periods of sitting, can help increase daily movement.
How does poor sleep and stress affect your weight?
Sleep and stress are crucial factors when it comes to managing your weight. Both can influence the complex biological systems that regulate appetite, metabolism, and eating behaviours.
This is why a holistic approach to weight management should include an assessment of sleep quality and stress levels, alongside nutrition and physical activity.
How does poor sleep affect weight?
When you don’t get enough sleep, you often feel hungrier the next day due to the biological impact of sleep deprivation on your appetite.
Research found that sleep restriction increased daily calorie intake by almost 400 calories, while extending sleep reduced it by around 270 calories in adults who usually slept less than 6.5 hours a night.56
Poor sleep can also reduce insulin sensitivity affecting how our bodies regulate blood sugar and our wider metabolic health.
Sleep quality and timing also matter as much as duration. Shift work, disrupted body clocks, insomnia, and sleep apnoea can affect sleep and may contribute to weight gain.
How does stress affect weight gain?
Whilst acute stress can be a helpful response to a dangerous or frightening situation, keeping our bodies alert and responsive, chronic stress can affect our mental and physical health and in particular, our appetite.7
Cortisol, one of our main stress hormones, helps regulate our stress response but can also influence appetite and fat distribution. Whilst some find they lose their appetite when stressed, others find themselves searching for comforting, energy-dense foods.
Stress can also disrupt sleep, reduce motivation to be active, and make it harder to adopt and maintain healthy habits.
Mental health conditions such as depression, and the impact of weight-related stigma, can add to these challenges, creating a vicious cycle where stress and difficulties managing weight reinforce one another.
Recognising and addressing poor sleep and chronic stress can therefore be an important and often overlooked, aspect of managing weight.
What other factors can influence your weight?
Your weight is also influenced by hormones, medical conditions, and medications, which can affect your appetite, metabolism and how our bodies store fat.
Hormones
Hormonal changes during puberty, pregnancy, menopause, and ageing can affect your weight and body composition.
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Menopause – declining oestrogen levels are associated with increased abdominal fat, particularly visceral fat, which surrounds your internal organs and is linked to insulin resistance and inflammation.
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Ageing – muscle loss and changes in sleep and physical activity can affect your metabolism and body composition.
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Body fat and hormones – fat tissue produces hormones and converts testosterone into oestrogen. After menopause, it becomes a major lower testosterone levels
Medical conditions
Some medical conditions can also contribute to weight gain by affecting hormone levels, metabolism, or appetite.
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Underactive thyroid -low levels of thyroid hormones can slow down metabolism and contribute to weight gain.
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Polycystic ovary syndrome (PCOS) – hormonal imbalances and insulin resistance can make weight management more difficult for some people.
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Cushing’s syndrome– excess cortisol, can lead to weight gain, particularly around the abdomen, face, and upper back.
Medications
Some medications can affect appetite, metabolism, or fluid retention, potentially contributing to weight changes.
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Oral corticosteroids– can increase appetite and cause fluid retention, potentially leading to weight gain.
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Certainantidepressants– may affect appetite and weight, although the effects vary between medications and individuals.
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Antipsychotics– some can increase appetite and affect how your body regulates blood sugar and stores fat.
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Pain medications– some may contribute to weight gain through fluid retention, reduced activity or other effects, depending on the medication.
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Diabetes treatments – some can cause weight gain by changing how your body uses glucose or by reducing glucose lost through urine.
If you’re concerned about unexplained weight changes or think your medication may be affecting your weight, speak to your healthcare professional before making any changes to your treatment.
Can environmental chemicals affect weight?
There is also growing research into the role of environmental chemicals on weight, sometimes referred to as obesogens.
It is thought these may interfere with hormones that control fat storage and metabolism although the extent to which these exposures contribute to weight gain in humans remains the topic of ongoing research.
What should you do if you’re struggling to manage your weight?
If managing your weight feels difficult, there are some things you can do that may help.
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Look at your habits– consider whether changes to your diet, sleep, stress levels, or daily movement could help.
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Start small– focus on one or two realistic changes rather than trying to change everything at once.
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Consider your circumstances-your health, hormones, medications, and environment can all affect your weight. You may need to adapt your approach to suit your needs.
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Speak to your doctor– seek advice if you’re experiencing unexplained weight gain, new symptoms, difficulties with eating behaviours, or concerns about medication affecting your weight.
And remember, weight management isn’t simply about willpower. Understanding how your biology, environment, and circumstances affect your weight can help you find an approach that works for you in the long term.
- Loos RJF & Yeo GSH: The genetics of obesity: from discovery to biology.
- Gough et al: Eating while distracted: a systematic review and meta-analysis of the effect of distraction on concurrent and later energy intake in adults.
- Hall et al: Ultra-Processed Diets Cause Excess Calorie Intake and Weight Gain.
- Dicken et al: Ultraprocessed or minimally processed diets following healthy dietary guidelines on weight and cardiometabolic health.
- Al Khatib et al: The effects of partial sleep deprivation on energy balance: a systematic review and meta-analysis.
- Tasali et al: Effect of Sleep Extension on Objectively Assessed Energy Intake Among Adults With Overweight in Real-life Settings.
- Tomiyama AJ: Stress and Obesity.
Article history
The information on this page is peer reviewed by qualified clinicians.
Article also available inEnglish,German,Spanish,French,Italian,Portuguese,Hindi,Hebrew,Arabic, andSwedish.
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Next review due: 4 Nov 2029
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4 Nov 2026 | Originally published
Dr Caroline Butler
Dr Colin Tidy, MRCGP
Disclaimer: The views expressed in the community do not represent medical advice. For any health concerns, consult your GP or healthcare provider.
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