Obesity is on the rise worldwide, but researchers still don’t fully know why. We look at how weight gain sneaks up on us, if our genes are to blame after all, and why our parents leave us a lifelong legacy if they are obese.

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Why are some people prone to excessive weight gain, and how does our behavior affect our children’s genes?

In simple terms, being obese means that a person carries too much weight, specifically fat, around their body. To gain this extra fat, a person needs to eat more calories than they burn, which is generally thought to be the result of a sedentary lifestyle and the consumption of high-energy foods.

As researchers are beginning to get a deeper understanding of obesity, we start to appreciate that this simplistic view does not tell the full story.

We probably don’t need to convince you that obesity is a major health risk, because that fact is pretty well-established. However, while keeping our fingers on the pulse of the latest medical research, we came across five obesity facts this year that surprised us.

With more than 1 in 3 individuals in the United States obese and global childhood obesity rates 10-fold higher than they were in the 1970s, our five surprising facts point to the reasons behind why we are piling on the pounds and how this affects our health long-term.

Nobody is born obese. Instead, weight gain has a tendency to sneak up on us. And, as Prof. Claude Bouchard — from the Human Genomics Laboratory at Pennington Biomedical Research Center in Baton Rouge, LA — explains in the journal Nature Reviews Genetics:

The common form of human obesity is typically characterized by a weight gain of about 1–2 kg per year over a period of 15–25 years, depending on the individual. This rate of yearly weight gain is quite small when spread out over 365 days.”

This makes it difficult to pinpoint the precise drivers of obesity in individual cases, he adds. It seems to be a balance between nature and nurture.

The ability of a particular trait to contribute to obesity is called obesogenic potential. Many factors are said to harbor obesogenic potential, including excessive eating, food preference, and a sedentary lifestyle.

But these don’t work in isolation. They are intrinsically linked to our social environment and living spaces.

Here is the really interesting part: the same amount of calories and physical exercise have different effects on different people. It’s all about the combination of obesogenic factors.

Nurture certainly has a role to play. However, up to 70 percent of the variation in our body weight is linked to genetic factors, explains Prof. J. Alfredo Martinez — from the Center for Nutrition Research at the University of Navarra in Pamplona, Spain — in the journal Nature Reviews Disease Primers.

For a small number of obese individuals, there is a clear genetic cause for their weight. Up to 5 percent of extreme obesity starting in childhood can be traced back to mutations in the gene encoding the melanocortin 4 receptor.

Another culprit is the fat mass and obesity-associated gene. But aside from the small proportion of people carrying mutations in these genes, how can we explain the large number of obese individuals worldwide?

Many researchers think that a large number of genes contribute a small amount of risk each — which, together with lifestyle, cause obesity.

Prof. Bouchard found 118 such mutations in a large-scale meta-analysis of several genome-wide association studies.

A new study now published in Proceedings of the National Academy of Sciences points the finger squarely at a gene called ankyrin-B. The team was led by Dr. Vann Bennett — who is a professor of biochemistry at Duke University School of Medicine in Durham, NC — and it found that variations in this ankyrin-B cause excessive amounts of glucose to enter fat cells, causing them to double in size in return.

“We found that mice [with the mutated gene] can become obese without eating more, and that there is an underlying cellular mechanism to explain that weight gain,” Prof. Bennett explains. “We call it fault-free obesity.”

Whether or not these findings hold up in humans remains to be seen.

So, weight gain tends to sneak up on us and our genes are partly to blame. But the buck doesn’t stop with us; our weight has a direct effect on the next generation.

Roughly half of pregnant mothers in the U.S. are overweight or obese when they attend their first antenatal appointment, explains Dr. Martina Persson — from the Department of Medicine at the Karolinska Institute in Stockholm, Sweden — in an article in The BMJ.

In a study of more than 1 million births, which ran in Sweden from 2001 to 2014, the overall rate of major congenital malformations, or birth defects, was 3.5 percent. But the risk of such malformations increased proportionately with maternal weight.

This large population-based study found that overall risks of major congenital malformations and risks of several organ-specific groups of malformations progressively increase with maternal overweight and severity of obesity.”

Dr. Martina Persson

Mothers who have a body mass index (BMI) greater than 35 had a 23 percent greater risk of having a baby with malformations than those with a normal BMI. For those with a BMI greater than 40, this risk was 37 percent higher.

In addition to a higher risk of birth defects, babies born to obese mothers are more likely to be large at birth — a phenomenon called macrosomia.

Macrosomia puts babies at risk of bone breaks during birth and is associated with higher rates of cesarian delivery. It also increases the mother’s risk of extensive bleeding during birth.

Research by Dr. Cuilin Zhang — of the epidemiology branch at the Eunice Kennedy Shriver National Institute of Child Health and Human Development in Bethesda, MD — published this week in JAMA Pediatrics sheds light on how macrosomia develops during pregnancy.

The team found that babies with obese mothers developed longer bones and larger heads. The team could see this on ultrasound examinations as early as 21 weeks of pregnancy. The babies’ bellies were also larger from around 32 weeks of pregnancy.

If you are wondering why obesity would have such an effect, Dr. Zhang explains the team’s theory:

Women who are obese at conception are more insulin resistant, which may lead to overnutrition of the fetus and overgrowth at birth.”

But mothers’ weight during pregnancy doesn’t just affect the baby during development and birth; it leaves a lifelong trace.

A mother’s weight and diet during the pregnancy and breast-feeding phases can have a lasting effect on her children.

“In the prenatal period,” Prof. Martinez says, “excessive maternal gestational weight gain, especially in early pregnancy (first 20 weeks), is a risk factor for the development of overweight in children.”

This is down to the fact that the metabolic environment in which the growing baby finds itself causes permanent changes to the genetic code. These epigenetic changes affect the way that certain genes work.

For instance, “[O]vernutrition during lactation can result in epigenetic modifications in key genes that are known to be involved in the insulin signaling pathway in skeletal muscle that can manifest as impaired insulin sensitivity in later life,” Prof. Mark H. Vickers — from the Liggins Institute at the University of Auckland in New Zealand — explains in Frontiers in Endocrinology.

But fathers are not without blame. Epigenetic changes are passed on in sperm and carry with them a significant risk of developing obesity.

So, obesity turns out to be much more complex than simply eating more calories than we burn every day.

If you are looking for tips on how to manage your weight or the latest news on obesity, weight loss, and fitness research, look no further than our dedicated section on the Medical News Today website.