Sugar or sweetener: Is diet soda safer for your liver?

If you’ve been swapping regular soda for the “diet” version to help your liver, new research suggests that may not be the win you hoped for.

In a large study of more than 120,000 adults in the U.K., those who drank at least one can a day of either sugar-sweetened or low- or no-sugar-sweetened beverages were more likely to develop metabolic dysfunction-associated steatotic liver disease (MASLD).

This condition, formerly known as non-alcoholic fatty liver disease (NAFLD), occurs when excess fat builds up in the liver and can lead to serious health problems over time.

Tracking liver disease development

The project, led by the Inova Health System, was focused on data from the UK Biobank and followed 123,788 participants who started out without liver disease.

Instead of a one-off food questionnaire, researchers used repeated 24-hour dietary recalls to get a better picture of how often people actually drank sweetened beverages.

The experts monitored participants over a median of 10.3 years, tracking new MASLD diagnoses, measured liver fat, and liver-related deaths.

Daily drinks, higher damage

The results showed that drinking more than about one standard can per day (>250 grams) of diet-style drinks was linked to a 60 percent higher risk of MASLD, and the same amount of sugary drinks to a 50 percent higher risk.

Across the follow-up, 1,178 people developed MASLD, and 108 died from liver-related causes. Both drink types also marched in step with higher liver fat on imaging.

A further twist: while sugary drinks didn’t show a statistically clear link to liver-related mortality in this dataset, diet drinks did.

That result doesn’t mean “diet” soda is somehow worse than regular across the board. It does challenge the comfy idea that swapping sugar for sweeteners automatically neutralizes metabolic risk. The liver seems to care about more than just calories.

How soda stresses the liver

The biology isn’t mysterious for sugary beverages. Big, rapid spikes in glucose and insulin drive the liver to make and store fat.

Fructose-heavy formulas can also raise uric acid and nudge weight gain – two more strikes against a healthy liver. The pathways for artificially sweetened drinks are trickier.

Scientists point to several plausible routes: changes in the gut microbiome, blunted satiety signaling, stronger cravings for sweetness later in the day, and, in some cases, a small insulin response even without sugar. Any one of those, repeated daily for years, could add up to more fat in the liver.

Drinking water wins every time

There was at least one practical bright spot. When the team modeled simple swaps, replacing one daily serving of sugary soda with water was associated with about a 12.8 percent lower MASLD risk.

Making the same swap from a diet drink to water looked even better at roughly 15.2 percent. Trading a regular soda for a diet soda, though, didn’t move the needle in a meaningful way.

In other words: the direction of travel that helps is away from sweetened beverages altogether.

When quiet damage turns deadly

It’s worth pausing on why this matters. MASLD is now the most common chronic liver disease globally. It starts as fat buildup in the liver and can smolder for years before causing inflammation, scarring, cirrhosis, or liver cancer.

Estimates suggest that more than 30 percent of people worldwide have some degree of steatotic liver disease, and liver-related deaths tied to metabolic dysfunction are climbing.

Because it’s often silent until late, prevention – diet, movement, weight management – does most of the heavy lifting.

One can may cross the line

As with all observational research, there are caveats. This study can’t prove that the drinks caused disease on their own.

Diets were self-reported (better than nothing, but imperfect), and heavy soda drinkers may differ in other ways that also matter – sleep, alcohol patterns, overall diet quality, medications.

The cohort also skews a bit healthier than the general population. Still, the associations were consistent across outcomes and showed a meaningful dose threshold right around a can a day. That’s hard to shrug off.

Drink smarter, live longer

So where does that leave you if you enjoy a daily fizz? You don’t have to live like a monk, but it’s smart to rethink the routine.

Make water your default. Keep unsweetened tea or coffee in the rotation. If bubbles are non-negotiable, try plain or naturally flavored seltzer. And if you do have a soda, aim for “now and then,” not “every day.”

The researchers say the next step is digging into mechanisms with randomized trials and genetics- or microbiome-based studies, to separate the effects of sugar from those of sweeteners and figure out who’s most vulnerable.

Until then, the simplest, safest guidance is also the oldest: drink more water. Your liver will appreciate the break – whether your usual choice comes in blue cans or silver ones.

The study is published in the journal Hepatology.

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