In prehistoric times, natural selection favored genotypes

that could endure harsh conditions by stocking the most fat.

With chronic malnutrition being the norm for most of human history,

genetics evolved to favor fat storage.

So when did body fat become problematic?

The negative impacts of being overweight were not even noted in medical literature

until as late as the 18th century.

Then, technological advances coupled with public health measures

resulted in the betterment of the quantity, quality, and variety of food.

Sustained abundance of good food enabled a healthier population

to boom economically.

Output increased, and with it, leisure time

and waistlines.

By the mid 19th century, being excessively overweight, or obese,

was recognized as a cause of ill health,

and another century later, declared deadly.

What is the distinction between being overweight and being obese?

A calculation called the BMI breaks it down for us.

For example, if someone weighs 65 kilgorams

and is 1.5 meters tall,

they have a BMI of about 29.

Obesity is a condition of excess body fat

that occurs when a person's BMI is above 30,

just over the overweight range of 25 to 29.9.

While BMI can be a helpful estimate of healthy weight,

actual body fat percentage can only really be determined

by also considering information like waist circumference

and muscle mass.

Athletes, for instance, have a naturally higher BMI.

So how does a person become obese?

At its most basic, obesity is caused by energy imbalance.

If the energy input from calories

is greater than the energy output from physical activity,

the body stores the extra calories as fat.

In most cases, this imbalance comes from a combination of circumstances

and choices.

Adults should be getting at least 2.5 hours of exercise each week,

and children a whole hour per day.

But globally, one in four adults and eight out of ten adolescents

aren't active enough.

Calorie-dense processed foods and growing portion sizes

coupled with pervasive marketing

lead to passive overeating.

And scarce resources,

and a lack of access to healthy, affordable foods

creates an even greater risk in disadvantaged communities.

Yet, our genetic makeup also plays a part.

Studies on families and on separated twins

have shown a clear causal hereditary relationship to weight gain.

Recent studies have also found a link between obesity

and variations in the bacteria species that live in our digestive systems.

No matter the cause, obesity is an escalating global epidemic.

It substantially raises the probability of diseases,

like diabetes,

heart disease,

stroke,

high blood pressure,

and cancer.

It affects virtually all ages, genders, and socioeconomic groups

in both developed and developing countries.

With a 60% rise in child obesity globally over just two decades,

the problem is too significant to ignore.

Once a person is obese, the climb to recovery becomes progressively steeper.

Hormonal and metabolic changes reduce the body's response to overeating.

After losing weight, a formerly overweight person burns less calories

doing the same exercises

as a person who is naturally the same weight,

making it much more difficult to shed the excess fat.

And as people gain weight,

damaged to signaling pathways makes it increasingly difficult

for the brain to measure food intake and fat storage.

There is, however, some evidence

that well-monitored, long-term changes in behavior

can lead to improvements in obesity-related health issues.

And weight loss from sustained lifestyle changes,

or invasive treatments like bariatric surgery,

can improve insulin resistance and decrease inflammation.

What was once an advantage for survival is now working against us.

As the world's population continues to slow down and get bigger,

moving and consciously eating our way towards a healthier weight

is essential to our overall well-being.

And with the epidemic affecting every country in the world

for different socioeconomic reasons,

obesity cannot be seen as an isolated issue.

More global measures for prevention

are essential to manage the weight of the world.