I'm an expert in obesity, a condition that is rampantly misunderstood. Here is why I'm very comfortable with a patient weighing 300 pounds.

• A leading obesity doctor and scientist says she's comfortable with her patient being 300 pounds.
• Obesity is much more complex than bad diet and exercise, but many doctors are still poorly informed.

This as-told-to essay is based on a conversation with Fatima Cody Stanford, a leading expert in obesity medicine and an associate professor at Harvard Medical School. It has been edited for length and clarity.

When I first saw this patient, I was working on my fellowship in obesity medicine in Boston. As soon as I walked into my waiting room, I recognized him.

He was a train conductor on the T, a commuter train that I took every morning.

"I see you moving up and down the aisles, taking the tickets, climbing the rails...." I told him.

This gentleman carried excess weight — he weighed 550 pounds when I met him. But from his work, I could see that he had a very active lifestyle.

Clearly, it was the way his body worked that was interfering with his ability to keep the weight off.

When I conveyed this to my patient, his mother, who was accompanying him, started crying.

"You're the first doctor in his entire life that's not just blamed him for his weight," she said.

Obesity is a chronic disease

The biggest misconception about obesity is that people have just let themselves go.

But research actually shows us that obesity is a disease caused by one's biology.

There are two pathways of the brain that regulate our weight, called the pro-opiomelanocortin and agouti-related peptide pathway (AgRP).

These pathways interact with different things within our environment, our genetics, our development, and our behavior, to determine what our weight status will be.

For my patient, like many other people with obesity, that excess weight is likely because that AgRP pathway tells him to store and hold onto more fat tissue, or adipose, than is typically necessary for someone to function.

There are, of course, changes people can make to alter their weight, like optimizing exercise, food, and sleep. But, for people predisposed to obesity, these changes are often not enough, through no fault of their own.

In fact, if you have parents that have obesity, there is a 50 to 85% likelihood you will have obesity, even with optimal behaviors.

So my patient was doing the right things — he had an active lifestyle, exercised, and ate well. But his body stored more adipose tissue than most.

Weight bias makes obesity worse

When my patient first came to the office, he had several weight-related diseases. These included obstructive sleep apnea, pre-diabetes, high blood pressure, and several other issues.

With a variety of treatments, including surgery and medications, we were able to get my patient to a weight of 300 pounds.

People may believe that all it takes to lose weight is to encourage people with obesity.

If my patient just needed encouragement, I would've done that. But that goes against the biology of the disease.

That misunderstanding just makes the disease worse.

The two most common forms of bias in the US currently are race bias and weight bias, which is discrimination against people with excess weight.

It is a particularly big problem because weight bias actually worsens weight-related diseases.

Data shows experiencing weight bias increases blood pressure, hemoglobin A1C, which is average blood sugar, and stress hormones. This makes the disease related to obesity worse. When you have increased stress, you have increased storage of adipose tissue.

Because of weight bias, people with obesity are not valued or are made to think they are not worthy of being respected.

From a psychological perspective, you can see increased anxiety, depression, and risk of suicide.

Physicians are a big source of bias — research shows between 79 to 90% of physicians have an implicit or explicit bias towards those with excess weight. Medical schools still don't teach much about the biology of the disease and many physicians still only focus on diet and exercise.

That is what the mom had always seen for her son. She had seen how he was working, how from very early in life, he was judged and his value and work as a human was dictated by his size.

Optimized weight at 300 pounds

I never give my patients a target weight or BMI, much to their dismay. It gives you information about just one key factor that plays into a whole recipe of things that you really need to look at the person's overall health.

Obesity can be life-threatening. We do see a shorter life expectancy for people with a high degree of excess weight, particularly in the category of severe obesity.

It causes over 200 diseases and is the precursor for 15 different types of cancers, osteoarthritis, reflux, and more.

But a person's weight and height don't mean much without other information suggesting poor health, such as blood pressure, cholesterol values, insulin levels, testosterone levels, fasting blood sugar, or hemoglobin A1C.

All of these measurements give me a full picture of the person's health.

At 300 pounds, these measurements showed that my patient's health has been optimized.

Now, when we hear 300 pounds, that sounds like a large number. And it is, even for him — he still has obesity. But 250 pounds down from his original weight, he no longer has blood sugar, sleep, or cholesterol issues. These weight-related diseases are all in remission.

So I am comfortable with him being at that weight.