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Fructose contributes to obesity by slowing down metabolism, study suggests. Image credit: Catherine Falls Commercial/Getty Images.
  • New research suggests that fructose — a simple sugar found in many foods — may drive obesity and related health conditions.
  • However, it may not be just the amount of calories but the source of those calories that can lead to obesity.
  • The fundamental cause of obesity is an energy imbalance between calories consumed and calories expended.

According to the World Health Organization (WHO), around 13% of adults worldwide have obesity.

New research suggests that at the core of obesity may not be just an energy imbalance, but where that energy comes from may drive the condition.

The study, published in Philosophical Transactions of the Royal Society B, suggests that fructose may drive obesity due to an evolutionary “survival switch” that causes people to store energy from fructose, rather than using it.

Dr. Eamon Laird, a postdoctoral research fellow at the University of Limerick, Ireland, not involved in the research, commented on the study for Medical News Today. He noted:

“This is a very interesting hypothesis — though it’s a narrative review and not a systematic or meta-analysis of the current evidence. It is a plausible idea that millions of years of evolution have resulted in a pathway that was beneficial but now is adverse due to our current energy-dense diets.”

The new study proposes that obesity and metabolic disorders may have developed from over-stimulation of an evolutionary-based biological response (survival switch) that aims to protect animals before a crisis, such as hibernation.

The researchers suggest that, unlike glucose which is used as immediate fuel, fructose triggers the body to store fuel.

This is ideal for an animal that is going to hibernate for several months — less so for a person who has continuous access to high-sugar foods.

Where people have plenty of access to food, this ‘survival switch’ may do more harm than good. Constant supplies of high-fructose foods lead people to lay down fat stores leading to obesity and related health problems.

So, how does fructose induce the body to store energy rather than use it?

Usually, adenosine triphosphate (ATP), the molecule that provides energy to power all cell processes, is used and rapidly replaced from nutrient intake or fat stores.

However, fructose lowers ATP concentration in cells and reduces the ability to make more ATP.

When ATP levels drop enough, this sets off a series of chemical reactions that stop the mitochondria of the cell from producing more ATP, and causes them oxidative stress.

As well as causing ATP levels to drop, ingestion of fructose stimulates further food intake. These extra calories are then stored as fat. Eventually, the ATP levels increase again, but the stored fat remains.

Over time, repeated oxidative stress leads to permanent mitochondrial dysfunction. In a hibernating mammal, the body adapts to the low ATP levels by reducing the resting metabolic rate.

In people who still have plenty of food available, unless intake of calories is reduced, this lower energy usage results in weight gain.

Dr. Laird agreed that this hypothesis might go some way toward explaining the rise in obesity.

“I agree it could be one component,“ he told us. “However, obesity and metabolic syndrome is a multifactorial condition — it is never just one variable. Important risk factors include lack of physical activity, poor lifestyle (smoking, alcohol, addictions), poor nutrition intakes, vitamin deficiencies, socio-economic causes, and even genetic and ethnicity risk factors.”

“So even if fructose did influence obesity it would be a small contribution in the grand scheme of things,” he added.

Although fructose is the natural sugar that makes fruit sweet, a typical Western diet contains many other sources of fructose.

The majority comes from table sugar, sucrose — a molecule made up of glucose and fructose chemically bonded together — and high fructose corn syrup (HFCS), a sweetener made from cornstarch.

HFCS contains up to 55% fructose. Manufacturers make it by adding enzymes to corn syrup to convert the glucose to the sweeter-tasting fructose.

The main difference between HFCS and table sugar is that the fructose in HFCS is free molecules, so is absorbed rapidly.

Many foods, and almost all processed foods, contain HFCS. They include:

  • sodas
  • sweetened fruit juices
  • crackers
  • pre-prepared meals
  • condiments and salad dressings
  • some breads and pastries.

The authors state that the global epidemics of obesity and diabetes correlate with the rise in sugar intake, particularly in the form of fructose-sweetened drinks, processed foods, and high glycemic index (GI) carbohydrates.

Dr. Mir Ali, bariatric surgeon and medical director of MemorialCare Surgical Weight Loss Center at Orange Coast Medical Center in Fountain Valley, CA, not involved in the research, told MNT that for those with overweight or obesity “[a]ny source of sugar, including non-processed sugars, such as found in fruits can have a similar effect on the body.“

“We advise our patients to minimize all sources of sugar, including fruits,” he noted.

But Dr. Laird advised that most people should not be too concerned about the fructose content of fruit: “Most of us don’t eat enough fruits and we should be eating more for general health for fiber, vitamins and micronutrients. The small amounts that we do eat from fruit would probably give little or zero risk.”

“However,” he added, “the biggest risk probably comes when the fructose is highly concentrated and added to other foods (these foods often contain high fat, high sugar, low nutrients), which could lead to an increased risk of obesity.“

So, to help reduce your risk of developing obesity, perhaps avoid that processed snack — it is likely to be loaded with fructose.

In the United States, more than 40% of adults have obesity.

Obesity is a risk factor for many health conditions, including cardiovascular diseases, type 2 diabetes, and some cancers.

Although obesity rates are increasing in low-income countries, most are in higher-income countries.

In the U.S., data from the National Institutes of Health (NIH) show that 42.4% of adults and 19.3% of children and adolescents had obesity from 2017 to 2018. And these numbers are increasing.

Obesity, which is related to poor diet quality and unbalanced energy intake, but may also have a genetic component, increases the risk of many health conditions. According to the NIH, these include:

Researchers and health experts are still exploring how to safely and effectively treat obesity. Understanding the mechanisms driving the condition — such as the role played by fructose intake — may lead to better therapies and treatments.