Why Is It So Difficult to Lose Weight?

Understanding the science behind sugar intake and body weight in teens

Did you know that the body’s response to certain sugars, like fructose, may make weight loss especially challenging for teens with higher body weight? Each of us has a weight that allows us to function at our best. Getting to this weight can feel frustrating, particularly for teens and families working hard to eat well and stay active.

We discovered hidden biological reasons behind this struggle. Thanks to the participation of many teens in the studies of Sonia Caprio, MD!

The Brain Doesn’t Get the “I’m Full” Message

Teens with higher body weights tell us that it’s hard to feel full. So, we measured this! Teens volunteered to have a brain scan before and after drinking two different types of sugar, glucose or fructose.

When teens with higher body weight drank these sugars, the part of their brain that signals “I’m full!” didn’t work properly. Meaning, their brains did not recognize the calories that they drank and did not alert them that their body had enough. The protective signals were missing.

If you’ve ever felt like food has power over you, you are right. In those moments, your brain wires itself differently and does not signal you to stop eating.

Hunger Hormones Don’t Shut Off Properly

Normally, after eating, our body tells us we have had enough food by changing the amount of important hormones, like ghrelin. When ghrelin is high, we feel hungry. After we eat, ghrelin goes down and we no longer feel hungry.

But we found that in teens with higher body weight, this response is different when they consume fructose, a sugar found in soda and processed foods. Instead, after drinking or eating something with fructose, their ghrelin stays high and they experience hunger. In other words, even when their body receives a lot of calories, they stay hungry. It is easier to eat more when we are hungry, and eating more leads to gaining weight.

Too Much Fructose = Too Much Insulin = Too Much Fat Storage

The damage of fructose continues. We found that when teens with higher body weight consume fructose, their bodies release two to three times more insulin than when they consume glucose. Insulin is a hormone that stores sugars (like fructose) as fat. So, instead of using fructose for energy, the body saves it - on your belly, hips, thighs, wherever it can.

And that’s how a glass of soda becomes more than just a drink. It becomes a message to your body to store sugars and gain weight.

So, What Can You Do?

Take a break and pace your meals. You may be full and not getting the message. After eating a portion of food, take 20 to 30 minutes, then ask yourself – “Do I need more?”
Cut back on sugar. The type of food matters. Reduce intake of soft drinks, juices, candy, and processed foods.
Focus on whole foods. Many foods keep insulin at healthy levels. Veggies, whole grains, and lean proteins have better effects on your body’s hormones and fat storage.
Move your muscles after a meal. Even a walk uses up the sugars you ate and lowers insulin, so the body does not store the sugars as fat.
Be patient and informed. Biology can be stronger than willpower, especially in people with higher body weight. Know yourself and what you need. Find the people who are supportive and consider whether you want medications to help change the biology.

Final Takeaway

Losing weight is more than desire, discipline, and eating less. Losing weight is about understanding and working with your biology, hormones, and brain responses. This can be challenging, so don’t be hard on yourself.

We thank all the teens who participated in Dr. Caprio’s studies. Their help allowed us to uncover this hidden biology and generate evidence-based steps toward health.

This report was written by Abigail Kelley and Clare Flannery, MD

Scientific Studies

Here are all the studies behind these takeaways:

Blunted suppression of acyl-ghrelin in response to fructose ingestion in obese adolescents: the role of insulin resistance. Van Name M, Giannini C, Santoro N, Jastreboff AM, Kubat J, Li F, Kursawe R, Savoye M, Duran E, Dziura J, Sinha R, Sherwin RS, Cline G, Caprio S. Obesity (Silver Spring). 2015 Mar;23(3):653-61. doi: 10.1002/oby.21019. Epub 2015 Feb 3.

Fructose Consumption Contributes to Hyperinsulinemia in Adolescents With Obesity Through a GLP-1-Mediated Mechanism. Galderisi A, Giannini C, Van Name M, Caprio S. J Clin Endocrinol Metab. 2019 Aug 1;104(8):3481-3490. doi: 10.1210/jc.2019-00161. PMID: 30938760.

Altered Brain Response to Drinking Glucose and Fructose in Obese Adolescents. Jastreboff AM, Sinha R, Arora J, Giannini C, Kubat J, Malik S, Van Name MA, Santoro N, Savoye M, Duran EJ, Pierpont B, Cline G, Constable RT, Sherwin RS, Caprio S. Diabetes. 2016 Jul;65(7):1929-39. doi: 10.2337/db15-1216. Epub 2016 Apr 5.