For decades, BMI (body mass index) has been the go-to number doctors use to decide whether someone is underweight, “normal,” overweight or obese. 

But new global research suggests it’s possible to have a normal BMI, look thin in clothes and still meet the medical definition of obesity.

A large study published in JAMA Network Open based on World Health Organization (WHO) survey data examined health data from more than 471,000 adults across 91 countries. 

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The researchers looked at people whose BMI fell in the normal range (18.5–24.9) but who carried extra fat around their midsection, measured by waist circumference.

The results showed that roughly 1 in 5 people with a normal BMI had abdominal obesity, a large waistline linked to serious health risks.

These so-called “skinny fat” individuals weren’t necessarily healthy just because the scale said they were. 

Compared to people of normal BMI and smaller waists, those with belly fat had 29% higher odds of high blood pressure and 81% higher odds of diabetes. They were also significantly more likely to have unhealthy cholesterol and triglyceride levels.

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In short, their metabolism looked more like that of people with obesity, even if their weight didn’t.

The findings seem to expose a blind spot in how obesity is measured. BMI only considers weight and height but says nothing about where fat is stored.

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Fat that sits deep in the abdomen and surrounds vital organs is particularly dangerous, fueling inflammation, insulin resistance and cardiovascular problems, according to experts. Someone can be “thin outside, fat inside,” a concept the researchers call TOFI.

Compared to people of normal BMI and smaller waists, those with belly fat had 29% higher odds of high blood pressure and 81% greater risk of diabetes. (iStock)

Lifestyle also plays a role, the researchers noted. People with normal BMI who had belly fat were more likely to be inactive, eat fewer fruits and vegetables and have desk jobs or unemployment. 

That combination of little muscle and lack of movement makes it easier to accumulate visceral fat even without gaining much overall weight.

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The latest research suggests that waist size may be just as important to track as weight. 

For most adults, a waist above roughly 32 inches for women or 37 inches for men signals excess abdominal fat and higher health risks.

People with normal BMI who had belly fat were more likely to be inactive, eat fewer fruits and vegetables, and have desk jobs or unemployment.  (iStock)

The researchers noted several limitations. Because the study was cross-sectional, it can’t prove cause and effect, only associations. 

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Data also came from many countries and time periods, and high-income nations were underrepresented, which may affect how widely the results apply. 

The study didn’t include actual body fat scans and instead depended on people’s own reports about their habits, which can make the results less reliable. However, the WHO’s survey methods are generally well-validated.

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A small molecule has been hailed as potentially promoting longevity and combating certain diseases — but experts are also calling for more research.

NAD+ (Nicotinamide adenine dinucleotide) — often nicknamed the “fountain of youth” — is a coenzyme that is essential for cellular survival, which naturally declines with age. In recent years, supplements designed to boost NAD+ levels have grown in popularity.

Last month, researchers led by the University of Oslo reviewed preclinical and clinical studies investigating the link between NAD+ and aging.

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The review, which was published in the journal Nature Aging, concluded that restoring NAD+ levels could help to promote healthy aging and potentially slow or treat neurodegenerative diseases.

Every cell in the body contains NAD+, which plays a “critical role” in body functions that are essential to survival.

Previous research has shown that patients with Alzheimer’s disease exhibit “notably low levels” of NAD+, the researchers noted — which aligns with the fact that NAD+ declines with age.

“NAD+ may hold the key to healthier aging and could protect us against diseases like Alzheimer’s disease and Parkinson’s disease,” said Evandro Fei Fang-Stavem, an associate professor at the University of Oslo, in a press release. “However, we need further research on how to effectively utilize NAD+ in patient treatment.”

Benefits of NAD+

Research has shown that boosting NAD+ can improve mitochondrial and stem cell function, improve energy metabolism, enhance focus and mood, and support tissue repair, according to Dr. Amanda Kahn, a board-certified internist and longevity expert in New York City.

“From a longevity standpoint, NAD+ is one of the most central molecules in maintaining youthful cellular function,” she told Fox News Digital. “It activates … longevity pathways that protect against DNA damage and inflammation.”

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Boosting NAD+ also supports neuroprotection from Alzheimer’s and Parkinson’s, enhances insulin sensitivity, reduces inflammation, repairs skin, boosts immune defense and improves symptoms in long COVID patients, Kahn added.

While NAD+ itself is a large molecule that can’t cross into cells, NAD+ “precursors” can enter cells and then work to create NAD+. (iStock)

Dr. Halland Chen, a double board-certified physician in New York who specializes in longevity, reiterated that studies in both animals and early human models show that maintaining healthy NAD+ levels can improve energy metabolism, cognitive function and overall vitality.

“Restoring NAD+ is ultimately about cellular repair and mitochondrial optimization, not just an energy boost,” he told Fox News Digital. “Clinical experience shows that NAD+ therapy enhances energy, focus and endurance, while supporting detoxification and reducing oxidative stress from daily life, travel or poor sleep.”

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NAD+ also activates sirtuins, a group of longevity-related proteins that regulate DNA repair, cellular stress responses and gene expression, Chen said, although he noted that larger human studies are still needed.

Methods of boosting NAD+

In clinical studies, patients with age-related diseases have taken compounds designed to boost NAD+.

While NAD+ itself is a large molecule that can’t cross into cells, NAD+ “precursors” can enter cells and then work to create NAD+, experts confirm.

These precursors include nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN). Most people take these NAD+ precursors in the form of pills, capsules or powders.

“Clinical experience shows that NAD+ therapy enhances energy, focus and endurance while supporting detoxification.”

“Oral NAD precursors can modestly raise NAD+ levels and are convenient for daily maintenance,” Khan said. “In contrast, injections or IV infusions bypass the gut and deliver NAD+ directly into circulation, leading to more rapid and noticeable effects on energy, focus and recovery.”

Khan also prescribes a weaker NAD+ nasal spray for patients who can’t tolerate needles, though she said the benefits are typically less pronounced.

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“In general, I find that combining daily oral precursors with weekly NAD+ injections offers the most comprehensive support for both intracellular and extracellular NAD stores,” she advised.

Chen noted that oral supplements are best for maintenance, while IV therapy is often chosen for a “rapid cellular reboot, post-travel recovery or enhanced performance.”

“Sessions typically last two to four hours and are administered as a series of three infusions within seven to 10 days, followed by monthly maintenance.”

A doctor noted that oral supplements are best for maintenance, while IV therapy is often chosen for a “rapid cellular reboot, post-travel recovery or enhanced performance.” (iStock)

NAD+ therapy tends to have the most benefit for those managing fatigue, metabolic dysfunction or neurological decline, as well as anyone seeking to enhance energy, focus and recovery, Chen said.

“In healthy adults, benefits are typically modest, but can be amplified when combined with optimized nutrition, exercise and sleep,” he added.

Risks and caveats

While NAD+ therapy is generally safe and well-tolerated, it can cause some mild side effects, according to Chen.

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“Headache, nausea or chest tightness can occur, especially during faster IV infusions, and typically resolve when the rate is slowed,” Chen, who is also chief medical innovation officer at the digital wellness platform Eden, told Fox News Digital.

“Injections or IV infusions bypass the gut and deliver NAD+ directly into circulation, leading to more rapid and noticeable effects on energy, focus and recovery,” a doctor said. (iStock)

“Most reactions are transient and self-limited, and therapy is considered safe under medical supervision,” the doctor added.

He advises caution for people with active liver or kidney disease, those with a personal or family history of cancer, and women who are pregnant or breastfeeding.

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Kahn noted that common side effects include mild redness, itching or tenderness at the injection site, “which is harmless and dissipates quickly.”

“Some patients notice a transient rush, lasting no more than 10 minutes, after taking an NAD+ injection due to a brief release of adrenaline and mitochondrial ATP,” she went on. “While not dangerous, it can feel uncomfortable if not anticipated.”

“If the core pillars — sleep, movement, nutrition, metabolic balance and stress regulation — are not in place, NAD+ therapy won’t deliver its full potential.”

When choosing a NAD+ product, quality matters more than quantity, according to multiple experts.

“Patients should look for clinically validated, third-party-tested formulations and start with lower doses to gauge tolerance,” Chen said.

NAD+ therapy isn’t a shortcut to health and longevity in and of itself, but one layer of a much larger foundation, according to the expert.

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“If the core pillars — sleep, movement, nutrition, metabolic balance and stress regulation — are not in place, NAD+ therapy won’t deliver its full potential,” he said. “It’s like pouring premium fuel into an engine that hasn’t been maintained — the energy source is there, but the system isn’t optimized to use it.”

NAD+ therapy isn’t a shortcut to health and longevity in and of itself, but one layer of a much larger foundation, according to experts. (iStock)

It’s important to speak with a doctor before starting a NAD+ regimen, experts agree.

“Your doctor needs to know about all supplements you plan to take, including NAD+, because it has strong biological effects,” Chen said. “Your doctor will evaluate your personal health risks and check your medications for potential interactions with NAD+ supplements.”

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Khan noted that because NAD+ interacts with metabolic and cellular pathways, dosing and timing should be individualized — “especially for those with cancer history, metabolic conditions or multiple medications.”

“A physician should help determine the appropriate delivery method, duration and monitor for response and tolerance,” she said.