The complications of obesity, beyond weight itself

Obesity isn't just a matter of weight on a scale. It's a chronic disease that increases the risk of many complications, affecting nearly every major system in the body. Knowing them isn't meant to worry you. It lets you know what to watch for, and helps explain why even a modest weight loss already brings real benefits.

A point of vocabulary to start: these complications are statistical risks, not certainties. Many people with obesity will develop none of them, and all of them can be screened for, prevented, or treated.

An issue affecting a large share of the population

In France, 18.1% of adults have obesity according to the OFÉO 2024 study, or nearly 10 million people. In the United States, the share reaches 40.3% according to the CDC. Worldwide, the World Health Organization (WHO) counts 890 million affected adults in 2022.

The effect on life expectancy depends on the degree of obesity. A large analysis published in 2009 in The Lancet (Prospective Studies Collaboration), covering 900,000 adults, estimated that a BMI between 30 and 35 reduced life expectancy by two to four years on average, and a BMI between 40 and 45 by eight to ten years. These figures are averages: they vary a lot depending on whether complications are present.

Why excess fat affects the whole body

Two main mechanisms explain these complications.

The first is metabolic. Fat tissue isn't just a storage depot. It's an active organ that releases hormones and inflammatory substances. When it's in excess, especially around the belly, it sustains a low-grade but permanent inflammation and makes cells less sensitive to insulin. Fat then builds up in organs that don't normally contain it, such as the liver, pancreas, or muscle.

The second is mechanical. The extra weight puts pressure on joints, hampers breathing, and increases pressure in the abdomen.

Metabolic syndrome, a cascade of linked factors

Obesity, particularly abdominal obesity, promotes the joint appearance of several abnormalities: insulin resistance that can progress to type 2 diabetes, high blood pressure, and abnormal blood fats. This combination in the same person is called metabolic syndrome. It multiplies overall risk well beyond what each factor would represent on its own.

The link with type 2 diabetes is the strongest. A review of cohort studies published in 2010 (Abdullah et al.) estimated that diabetes risk was multiplied by about three in people who are overweight, and by seven in people with obesity. The Am I at risk of diabetes? tool can assess this risk.

The heart and blood vessels are affected too: heart attack, stroke, heart failure, and rhythm disorders are more common. This topic is covered in detail in the article on cardiovascular risk.

The liver, often overlooked

Excess fat also builds up in the liver. Fatty liver disease affects the vast majority of people with obesity. It's most often benign, but in some people it can cause inflammation and then scarring of the liver, fibrosis, which sometimes progresses to cirrhosis. It causes no symptoms for a long time. The Do I have fatty liver disease? and Is my liver damaged? tools can help assess this.

Gallstones and gastroesophageal reflux are also more common.

An impact on the joints

Excess weight puts additional pressure on the joints that carry the body: knees, hips, lower back. It speeds up cartilage wear and the risk of osteoarthritis. Inflammation linked to fat tissue also plays a role, which is why hand osteoarthritis is also more common.

Osteoarthritis can limit the ability to move, which makes weight loss harder. But this cycle also runs the other way. A study published in 2005 in Arthritis & Rheumatism (Messier et al.) showed that each kilogram lost reduced the load on the knee by about four kilograms with every step. So a modest weight loss genuinely eases pain.

The link with sleep apnea and breathing

Obesity is one of the main risk factors for sleep apnea. Fat that accumulates around the neck and throat promotes airway collapse during sleep. This link is strong enough that the STOP-BANG score, one of the most widely used sleep apnea screening tools and available on Hygietest, directly includes BMI among its criteria.

The relationship works both ways: fragmented sleep disrupts appetite hormones, and fatigue reduces physical activity, which promotes weight gain. Shortness of breath on exertion and asthma are also more common.

A recognized risk factor for certain cancers

This link is less well known to the public. In 2016, the International Agency for Research on Cancer (IARC) concluded that excess body fat increases the risk of thirteen cancers, including breast cancer after menopause, colon and rectal cancer, endometrial cancer, kidney cancer, liver cancer, pancreatic cancer, and esophageal cancer. In France, the IARC estimated that about 5.4% of new cancer cases, or nearly 19,000 cases in 2015, were attributable to overweight and obesity.

Several mechanisms are involved: chronic inflammation, excess insulin, which stimulates cell multiplication, and estrogen production by fat tissue. Organized screening for breast, colon, and cervical cancer is therefore particularly useful, and it's important not to skip it.

Fertility, pregnancy, and hormonal health

Obesity can disrupt ovulation and reduce fertility, in both women and men. It is often associated with polycystic ovary syndrome. During pregnancy, it increases the risk of gestational diabetes, hypertension, and cesarean delivery. These pregnancies go well in the vast majority of cases, with appropriate monitoring.

An impact beyond the purely physical

Obesity and depression are linked in both directions. A review published in 2010 in the Archives of General Psychiatry (Luppino et al.) showed that obesity increased the risk of developing depression by about 55%, and that depression increased the risk of developing obesity by about 58%.

Part of this link runs through stigma. People with obesity face remarks, hiring discrimination, and sometimes judgment within the medical field itself. This weight stigma erodes self-esteem, promotes isolation, and can lead to delaying care out of fear of being judged. It isn't a medical complication in the strict sense, but it has a real effect on health. Everyone has the right to respectful care and appropriately sized equipment, and it is legitimate to change providers if that isn't the case. The Am I depressed? tool can help identify distress that deserves support.

What can be done: screen and act early

The HAS recommends, for people with obesity, regular check-ups with the primary care doctor: blood pressure, blood sugar, blood fat levels, liver function, and screening for sleep apnea in case of snoring or daytime sleepiness. These simple tests make it possible to catch a complication at a stage when it responds well to treatment.

Above all, the risk of most of these complications decreases with moderate weight loss. Losing 5 to 10% of body weight improves blood pressure, blood sugar, the liver, sleep, and joint pain. In the American Diabetes Prevention Program study, published in 2002 in the New England Journal of Medicine, a 7% weight loss combined with 150 minutes of physical activity a week reduced diabetes risk by 58% in high-risk individuals.

What to remember

These complications aren't an inevitable fate tied to a number on a scale. They are statistically associated with obesity, but they can be screened for and treated, and their risk decreases with even moderate weight loss, as the last article on this blog explains.

To calculate your own BMI and waist circumference, take the “Am I overweight or obese?” test.