Obesity (adiposity): causes, weight-loss injection & treatment
At a glance
Affected in Germany
~13 million adults (approx. 19 %); approx. 6 % of children/adolescents
Definition
Usually a BMI ≥ 30 kg/m² — grade I (30–34.9), grade II (35–39.9), grade III (≥ 40)
Other names
Adiposity, severe overweight
Secondary diseases
e.g. type 2 diabetes, heart attack, stroke, sleep apnoea, cancer, osteoarthritis
Medications (selection)
Semaglutide (Wegovy), tirzepatide (Mounjaro/Zepbound), orlistat
ICD-10
E66
1. What is obesity?
Obesity (adiposity, severe overweight) is a chronic disease — not a weakness of will and not a free choice. It usually arises through a complex interplay of genetics, hormones, mental health, environment and lifestyle. The German Bundestag (parliament) officially recognized obesity as a disease in 2020.
In Germany, according to RKI (Robert Koch Institute) data, around 13 million adults are affected — about one in five.¹ The prevalence has increased over the past decades, especially in younger age groups and in education groups with a lower socioeconomic status.
Even a 5–10 % weight loss helps markedly
Even a weight loss of about 5–10 % of body weight can markedly improve blood sugar, blood pressure and blood lipids.² With modern medications (semaglutide, tirzepatide), average weight losses of about 10–20 % have been described in studies.⁴˒⁵
2. BMI, waist circumference and new diagnostic criteria (Lancet 2025)
BMI classification (WHO)
| Category | BMI (kg/m²) | Meaning |
|---|
| Normal weight | 18.5–24.9 | No increased weight risk |
| Overweight (pre-obesity) | 25.0–29.9 | Increased risk, not yet a disease |
| Obesity grade I | 30.0–34.9 | Markedly increased risk |
| Obesity grade II | 35.0–39.9 | Strongly increased risk |
| Obesity grade III (morbid) | ≥ 40.0 | Very strongly increased risk |
Waist circumference — often more meaningful than BMI alone
BMI does not capture where the fat is located. Visceral abdominal fat in particular (around the internal organs) is regarded as a risk factor for metabolic and cardiovascular diseases.
Waist circumference risk thresholds (orientation values per WHO/IDF⁹)
Women: > 88 cm → increased cardiometabolic risk — even with a normal BMI.
Men: > 102 cm → increased cardiometabolic risk — even with a normal BMI.
Lancet Commission 2025: a new diagnostic logic
An international expert commission (Lancet Diabetes & Endocrinology, January 2025), supported by more than 75 international professional societies, recommends a fundamental reassessment: BMI alone is often not enough. In addition, waist circumference, direct fat measurement and organ function should be taken into account.
- Clinical obesity: excess body fat has already led to chronic organ damage (e.g. diabetes, heart failure, sleep apnoea, osteoarthritis). According to the commission, prompt, intensive therapy is indicated.³
- Preclinical obesity: increased body fat without detectable organ damage. An increased-risk state, but not yet a disease in the narrower sense. Preventive strategies are the priority.
A note on the Lancet Commission
The new categorization is partly disputed among experts — some professional societies criticize that the subdivision could delay access to therapies. Regardless of that, it remains undisputed that BMI alone is an incomplete measure.
3. Symptoms and health effects
Obesity itself often does not cause acute pain — but the body frequently sends warning signs that many people do not connect with their weight:
- Shortness of breath on exertion — e.g. when climbing stairs
- Joint pain — especially the knees, hips, lower back
- Tiredness during the day, reduced resilience
- Increased sweating, heat intolerance
- Heartburn — increased pressure in the abdomen can promote reflux
- Back pain from the additional load on the spine
- Skin problems: fungal infections in skin folds, acanthosis nigricans (can indicate insulin resistance)
- Sleep disorders, low mood
4. Causes — why does obesity arise?
Obesity is usually NOT simply "eating too much, moving too little"
The science shows a complex picture. Twin studies suggest that a substantial proportion of weight variation is genetically determined — estimates range from 40 to 70 %.²
Genetics
More than 300 gene loci influence hunger, satiety, metabolism and fat distribution. The satiety hormone leptin and the hunger hormone ghrelin can become out of balance in obesity (leptin resistance). Genetics does not determine whether someone becomes obese — but often how easily one gains weight and how difficult losing weight is.
Environment and lifestyle
- Diet: often high in calories and low in nutrients, large portions, highly processed foods (fast food, ready meals, sugary drinks)
- Lack of exercise: a sedentary job, the car instead of the bike, little everyday activity
- Lack of sleep: persistently less than about 7 hours per night can increase the obesity risk
- Stress: chronic stress promotes cortisol release → fat storage, emotional eating
Psychological factors
- Emotional eating — eating as a coping strategy with stress, grief, boredom
- Binge eating disorder — repeated binge episodes with a loss of control (a distinct mental health condition)
- Depression and anxiety disorders — can be both a cause and a consequence of obesity
Medical factors
- Hormonal disorders: e.g. hypothyroidism, PCOS, Cushing's syndrome
- Medications: e.g. certain antidepressants, antipsychotics, cortisone, beta blockers, insulin, antiepileptics can promote weight gain
- Gut microbiome: the composition of the gut bacteria can influence energy metabolism
Socioeconomic factors
According to RKI data, obesity is considerably more common in education groups with a lower socioeconomic status.¹ A healthy diet is often more expensive and more time-consuming. Social isolation and stigmatization can intensify the vicious circle.
5. Secondary diseases
Obesity is regarded as a risk factor for a multitude of accompanying and secondary diseases. As a rule: the higher the BMI and the longer the obesity persists, the higher the risk.
- Type 2 diabetes — obesity is one of the most important risk factors²
- Cardiovascular: high blood pressure, heart attack, stroke, heart failure
- Fatty liver (MASLD) — common in people with obesity
- Heartburn/GERD — among other things due to increased pressure in the abdomen
- Sleep apnoea — pauses in breathing during sleep, daytime tiredness
- Osteoarthritis — from overloading of the joints (e.g. the knees, hips)
- Increased cancer risk for various tumour entities (e.g. bowel, kidney, oesophageal, uterine and postmenopausal breast cancer)⁹
- Depression, anxiety disorders, social isolation
- Gallstones, gout, venous thromboses
6. Diagnosis
The diagnosis usually goes beyond the mere calculation of BMI:
- Calculate the BMI and determine the obesity grade
- Measure the waist circumference (to assess visceral fat)
- Blood test: e.g. blood sugar, HbA1c, blood lipids, liver values, thyroid (TSH)
- Measure blood pressure
- Sleep apnoea screening (e.g. with snoring, daytime tiredness)
- Record accompanying mental health conditions: depression, binge eating, other eating disorders
- Medication history: are weight-promoting medications being taken?
More: Preparing for a doctor's appointment.
7. Step-wise treatment (S3 guideline 2024)
Treatment goal: approx. 5–10 % weight loss — not necessarily a normal weight
This is often enough for marked improvements in the accompanying diseases. Depending on the starting situation, larger weight losses can also be sensible.²
Step 1
Lifestyle intervention — the basis for everyone
Diet
- A moderate calorie reduction as part of an adjustment accompanied by a doctor or a nutritional therapist — no starving, no crash diets
- Various evidence-based dietary patterns can be effective: e.g. the Mediterranean diet (well studied), low-carb, low-fat, intermittent fasting — the best dietary pattern is mostly the one that can be sustained in the long term²
- Reduce highly processed foods (fast food, ready meals, sugary drinks) where possible
- Formula diets can be an option as part of a medically supervised programme
- Regular meals, mindful portion sizes
Exercise
- Usually recommended: about 150–300 minutes per week of moderate physical activity (e.g. brisk walking, swimming, cycling)⁹
- A combination of endurance and strength training (building muscle can raise the basal metabolic rate)
- Increase everyday activity: e.g. the stairs instead of the lift, on foot instead of the car, interrupt sitting time
- Every bit of movement counts — even short sessions are usually better than none at all
Behavioural therapy
- Keep a food diary — to become aware of eating behaviour
- Identify triggers for emotional eating
- Stress management (e.g. relaxation techniques, sleep hygiene)
- Structured, multimodal programmes (e.g. the DMP obesity, a German disease management programme) often show better results than purely individual attempts²
