⚖️ Why do we gain weight? : The many causes of overweight and obesity 🧬🍔🏙️

This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

How strong is the evidence? ⭐ = a summary of many studies (systematic review or meta-analysis); 🔹 = a single study; 🐭 = animal research only.

⚡ The short version

⚖️ Weight gain happens when we take in more energy (kilojoules) than we use, but why that happens is far more complicated than "eating too much and moving too little".

🧩 Obesity has many causes that work together: biology and genes, what and how we eat, sleep, stress, family life, where we live, how much money we have, and even pollution (1).

🤰 Risk starts early, even before birth. A mother's weight before and during pregnancy, smoking in pregnancy, high birth weight, rapid infant weight gain and not breastfeeding are among the most consistent early-life risk factors (2, 3).

🥤 Some eating habits have strong evidence: sugar-sweetened drinks, large portions, eating quickly and eating while distracted all increase how much we eat or weigh (4–7).

😴 Sleep, screens and stress matter: short or irregular sleep, light at night, long screen time and emotional stress are all linked to weight gain (8–10).

🏙️ Our surroundings shape our choices: poverty, food insecurity, poor access to healthy food, unsafe neighbourhoods, car dependence and food-delivery apps all raise risk (11–13).

💡 Many of the strongest causes lie outside a person's control, which is why people living with obesity deserve understanding, not blame.

🧐 It's not just "eat less, move more"

⚙️ Energy or insulin? Probably both. One explanation, the energy balance model, says weight gain comes from regularly eating more kilojoules than the body uses. Another, the carbohydrate–insulin model, says that refined, quickly digested carbohydrates raise insulin, which encourages fat storage and makes us hungrier, so overeating is partly a result of the hormonal response (14, 15). These ideas aren't mutually exclusive: energy balance decides whether we gain or lose fat, but hormones, food quality and how processed our food is influence how hungry we feel, how much we eat and how the energy is stored (15). People also differ in how much insulin they release, so the balance between the two varies from person to person (16, 17).

🌍 ⭐ A review of more than 46,000 teenagers in low- and middle-income countries found obesity was driven by many overlapping factors: genes, poverty, unhealthy diets, the environment, low physical activity and psychological stress (1).

🤰 It starts early: from before birth to the toddler years

• ⭐ A review of 177 studies with nearly 1.9 million children found seven early-life factors most consistently linked to childhood obesity: a mother's higher weight before pregnancy, gaining too much weight during pregnancy, smoking in pregnancy, higher birth weight, being large for gestational age, not breastfeeding and rapid infant weight gain (2)
• ⭐ Across 39 studies, high pregnancy weight gain, caesarean birth, a large baby, antibiotic use and the mother's obesity raised the risk of obesity by age 5, while exclusive breastfeeding protected against it (3)
• 🔹 Children given antibiotics in their first two years, especially three or more courses of broad-spectrum antibiotics, were more likely to follow a high or rising weight path up to age 18 (18)
• 🔹 Teenagers born with a high birth weight (but not a low one) had about double the odds of obesity and fatty liver (19)
• 🔹 Longer breastfeeding was linked to lower odds of severe obesity (20), and what infants eat in their first year was linked to their weight later in childhood (21)
• 🌱 Stunting (being too short for one's age because of long-term undernutrition) can raise the risk of overweight later, especially when a child later has access to energy-dense food and is inactive. This "double burden" of under- and overnutrition is common in countries going through rapid change, including South Africa (22)

🎯 Takeaway: prevention should start before and during pregnancy, not only once a child is overweight.

👨‍👩‍👧 Family matters

• 🧬 🔹 In a Norwegian study of up to 85,866 children, children of heavier parents were heavier themselves, but shared genes explained most of this link (about 79% for mothers and 94% for fathers at age 8), rather than parents' habits alone (23)
• 😟 🔹 Parental stress matters: in a 12-week trial with 114 parent–child pairs, helping parents manage stress (alongside nutrition education) improved parenting and children's eating, and prevented short-term weight gain in the children (24)
• 😔 🔹 In 2,206 Chinese families followed for 8 years, a mother's depression (but not a father's) was linked to a higher risk of overweight in her teenage children (25)
• 🍲 🔹 In South Africa and elsewhere, some families see a larger body as a sign of health and prosperity, which can make weight gain seem less of a concern (26). On the other hand, pressuring children to lose weight can lead to body dissatisfaction, unhealthy dieting and later weight gain
• 👨 ⭐ Fathers influence their children's risk too, from before conception onwards (27), and family eating and activity habits, parents' education, jobs and weight all shape children's risk (28)
• 🎓 🔹 Surprisingly, in China, children of more educated mothers were more likely to be overweight, possibly because of more eating out, more kilojoules and more sitting (29)

🍽️ What and how we eat

• 🥤 ⭐ Sugar-sweetened drinks have some of the strongest evidence of any risk factor: across 85 studies, each extra daily serving was linked to higher weight in children and adults, and trials showed people gained weight when sugary drinks were added and lost weight when they were removed (4)
• 🍝 ⭐ Bigger portions made adults eat about 1,230 kJ (295 kcal) more a day in feeding trials, although long-term effects on weight still need study (5)
• ⏩ ⭐ Eating quickly was linked to about twice the odds of obesity (6, 30)
• 📺 ⭐ Eating while distracted, for example in front of the TV, increased how much people ate at that meal and later on (7)
• 🌙 🔹 Eating the same meals four hours later in the day increased hunger, reduced energy use and shifted fat tissue towards storing more fat (31)
• 🥡 ⭐ Eating out and takeaways were linked to weight gain (32), and 🔹 in China, the arrival of a food-delivery app in a city increased BMI and overweight among young urban adults (13)
• 🧪 🔹 Children eating more food additives had poorer diets, ate more ultra-processed food and, for some additive patterns, had a higher BMI (33). One new (not yet proven) idea is that the body may treat highly processed, unfamiliar foods as a signal of scarcity and store more fat in response (34)
• 🍺 🔹 In 5,761 adults, more alcohol meant more harmful belly (visceral) fat; the heaviest drinkers had over 10% more than the next group (35)
• 🔁 Yo-yo dieting: repeated cycles of losing and regaining weight can disrupt hunger hormones and shift regained weight towards more fat and less muscle (36)

😴 Sleep, light and the body clock

• 🛌 🔹 Six weeks of mildly shortened sleep increased body weight in adults (8) and short sleep affects hunger hormones and energy balance (37)
• 🔄 ⭐ Irregular sleep (going to bed much later on weekends) was linked to higher BMI and body fat in children (38) and 🔹 teenagers with obesity who slept less were less likely to reach a healthy weight (39)
• 💡 🔹 More light in the bedroom at night was linked to higher BMI in children, but not in those who were active for at least an hour a day (40), and 🔹 long-term night-shift work was linked to metabolic problems (41)
• 🕰️ Modelling in the USA suggests that the twice-yearly clock change disrupts the body clock, and that permanent standard time might reduce obesity (42)

📱 Screens, sitting and moving less

• 📱 ⭐ In nearly 400,000 women from 36 low- and middle-income countries, too much screen time, especially mobile phones and TV, was linked to overweight, with African women particularly affected (9). Greater smartphone use in teenagers was linked to higher odds of obesity, partly through too little sleep, and smartphone addiction to a higher long-term risk (43, 44)
• 🪑 Sitting for long periods is linked to more body fat, especially in children, and the WHO recommends limiting sedentary time, particularly recreational screen time (45)
• 🏠 🔹 Working from home was linked to higher obesity risk over time, mainly through less physical activity (46), and 🚗 long hours in cars due to urban sprawl reduce everyday movement (47)
• 🦠 🔹 Children's weight rose after COVID-19 lockdowns in the UK and Slovakia (48, 49)

🧠 Mind and emotions

• 💔 🔹 Childhood trauma, especially physical abuse and multiple adverse experiences, was linked to higher obesity risk in young adulthood, more so in women (10). A South African study found that the link between childhood trauma and adult BMI differed between Black and white women (50)
• 🎭 🔹 Women with obesity used fewer helpful ways of managing emotions, and, interestingly, positive emotions were linked to eating more unhealthy food (51)
• 🧘 🔹 In teenagers with obesity, low mindfulness and difficulty managing emotions were linked to emotional eating (52)
• 📚 🔹 In over 12,000 Swedish teenagers, schoolwork pressure was linked to overweight, especially in poorer families (53)
• 🍭 Children's brains develop the drive for rewards (tasty food, screens) faster than the ability to plan for the long term, which may make unhealthy choices harder to resist (54)
• 🩺 Depression and binge eating disorder often occur alongside obesity, but people with these conditions responded as well as others to a structured treatment programme (55)

🏙️ Where we live

• 🛒 🔹 Children living in low-income areas with poor access to healthy food had higher BMI as they grew up (11), although ⭐ evidence that "food deserts" directly cause obesity is mixed (56)
• 🍞 Food insecurity (not always having enough food) was linked to higher odds of obesity in 🔹 37,425 US teenagers (12) and to 🔹 higher weight and more pressured feeding in toddlers (57)
• 🌳 Neighbourhoods without parks, pavements and safe places to play limit activity (58, 59), and 🔹 poor housing and unsafe neighbourhoods raised children's risk (60)
• 🏘️ ⭐ Racial and ethnic segregation was linked to obesity among Black and Hispanic adults in the US, likely through poverty and the environment (61)
• 🇮🇳 🔹 In India, obesity in men more than doubled between 2005 and 2021, rising fastest among urban, wealthier and more educated men, a pattern of the nutrition transition (62)

🚚 Air pollution and "obesogenic" chemicals

• 🐭 In mice, exposure to diesel exhaust during pregnancy made male offspring heavier and less active as adults (63)
• 🔹 In 4,550 Californian children, living near busy traffic was linked to faster weight gain (64), and ⭐ a review of 15 studies linked fine-particle pollution (PM2.5) to about 28% higher odds of childhood obesity (65)
• ⭐ However, the largest European analysis (about 37,000 mother–child pairs) found a link only for pollution during pregnancy (66) and effects seem weaker where overall pollution is low. Traffic pollution also goes hand in hand with poverty, noise and less green space, so a direct cause is not yet proven
• 🧴 Endocrine-disrupting chemicals ("obesogens"), such as some plastics (BPA and BPS), phthalates, PFAS "forever chemicals", flame retardants and certain pesticides, can interfere with hormones and fat-cell development; much of the evidence comes from animal studies (67)

💰 Money and inequality

• 🔹 In 3,328 Australian children, those whose early life combined a poorer diet and social disadvantage had up to 5.6 times higher odds of obesity throughout childhood (68) and socioeconomic disadvantage raised risk regardless of where parents were born (69)
• 🔹 Rising income inequality increased obesity in countries that were previously more equal (70) and in Spain, walkable neighbourhoods alone did not close the gap between rich and poor (71)
• 🔹 In Pakistan, delaying marriage lowered women's obesity risk, possibly through more education, fewer children and more personal agency (72)

🇿🇦 Closer to home

• 🏥 🔹 At a hospital in KwaZulu-Natal, nearly half of children with obesity under 12 were barely active and spent a lot of time on screens, many drank soft drinks daily and many already had high blood pressure, prediabetes or abnormal blood fats, often with a family history of these conditions (73)
• 🧍 🔹 Many Black South African adults see a larger body as healthy and attractive, which can reduce concern about weight gain (26)
• 💔 🔹 Childhood trauma relates to adult weight differently in Black and white South African women (50)
• 🌱 Stunting and overweight often exist side by side in the same communities, and even in the same child (22)

🧬 Other biological clues

• 🔹 In Filipino women, high blood levels of certain amino acids (branched-chain amino acids) were linked to obesity, even though eating more of them was linked to lower risk, suggesting they are a marker of disturbed metabolism rather than a cause (74)
• 🔹 People with schizophrenia often have obesity, but genetic analysis suggests this is due to lifestyle, environment and medication, not the genes for schizophrenia itself (75)
• 🧬 Genes play a big role in who gains weight in a given environment; see the separate Q&A "Is There an Obesity Gene?" for more.

🎯 The bottom line

⚖️ Overweight and obesity are not simply the result of eating too much or moving too little. They grow out of a web of biological, behavioural, psychological, social, economic and environmental factors.

🤰 These influences begin before conception and add up over a lifetime, often clustering in the same families and communities.

🥤 Some factors you can change: sugary drinks, portion size, eating speed, distracted and late eating, alcohol, sleep and screen time. But even these are shaped by the environment we live in.

🏙️ Many of the most powerful influences, such as poverty, food insecurity, childhood adversity, neighbourhood design and pollution, are largely outside individual control. Prevention must support families and communities, not only individuals.

⚠️ Much of the evidence is recent and observational and comes from high-income countries, so it may not apply equally in African settings.

💚 Recognising this complexity is a reminder to approach people living with obesity, including ourselves, with understanding rather than blame.

💭 Questions to ask yourself

🛒 What does my everyday food environment make easy? What one change, such as keeping fruit visible, planning home-cooked meals or limiting delivery apps, would make the healthier choice the easier one?

😴 Am I getting enough regular sleep? Could I keep a more consistent bedtime, dim the lights and put screens away before bed?

🚶 How much of my day do I spend sitting? Where could I add a little movement, such as walking part of a journey or taking active breaks?

😟 How do stress and emotions affect what and how much I eat? What other ways of coping could I try: talking to someone, being active, mindfulness or professional support?

👨‍👩‍👧 What patterns did I inherit, and what am I passing on? Can I model healthy habits without pressure or shame? If I have risk factors I can't change, would regular check-ups and early support from a dietitian help?

Always seek guidance from dietitians for personalised diets and weight management plans.

💌 Love what you're reading? Share the knowledge! Forward this link to a friend who might also enjoy it. 📨

📚 References

Want to go deeper? Here are all the studies behind this summary. 🔍

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