The Wellness Brew - Issue 35

Ultra-processed food makes up about 55% of the calories the average American eats. A reader asked me about one specific item in that category. He eats well, trains, doesn’t drink, but has a Diet Coke every day, which he called his vice. He wanted to know how bad it really is and if he needed to consider dialing it back. It’s a fair question and a common situation that’s worth digging into.

But first, we filtered the noise — here's what's worth knowing this week.

Nineteen states have approved measures to make daylight saving time permanent, and the federal Sunshine Protection Act would do it nationwide. Researchers modeled what that would mean for schoolchildren, particularly how many commutes would begin before sunrise. The answer is at least two billion additional dark-morning commutes a year. In collision records, fatal crashes involving school-aged pedestrians ran about twice as likely in the dark as after sunrise, though that’s a comparison of records rather than a controlled analysis. So while adjusting clocks may have a health impact by disrupting your circadian rhythm, there are other factors we need to consider. — Rodríguez Ferrante G, Dasika NS, Nam A, et al., Current Biology, 2026.

A smartwatch study on biological aging, and what the headline skips

Researchers compared Garmin data from 621 adults aged 50 to 79 against DunedinPACE, a blood test estimating how fast a person is biologically aging. Lower resting heart rate and higher daily step counts both tracked with slower aging. The correlations were 0.29 and negative 0.25, real but modest, accounting for under a tenth of the variation between people. Three things worth knowing. Everyone was measured at a single point in time, so nothing was followed year over year. The authors state outright that they applied no correction for multiple comparisons given how many they ran, which means some of these associations are probably noise. And the abstract and the results section report different numbers for resting heart rate. The underlying idea holds up elsewhere, but this particular paper is thinner than its title. — Khandan A, Sun J, Majidi S, et al., Digital Health and Innovation, 2026.

Sprinting produces a large growth hormone response

A single 30-second treadmill sprint pushes peak growth hormone to roughly 18 to 20 micrograms per liter in untrained men, and as high as 44 in trained sprinters. Duration does a lot of the work: a 6-second cycle sprint gets you to about 4. Running appears to beat cycling, probably because it recruits the arms along with the legs. Researchers report peak concentrations rather than percentage increases, because resting growth hormone arrives in pulses and sits near undetectable between them, which is why the percentages floating around vary so wildly. Two findings complicate the obvious plan of simply doing more sprints. Growth hormone suppresses its own release, and when men sprinted a second time an hour later, the response was blunted in six of ten whose levels were still elevated. Six weeks of sprint training also lowered the overall response, even as power output rose 5%.— Stokes K., Growth Hormone & IGF Research, 2003.

In July 2023 the World Health Organization's cancer research agency put aspartame in the category labeled "possibly carcinogenic to humans." The headlines wrote themselves. What got lost was what that category means.

The agency uses four levels, and "possibly carcinogenic" is the third of them. It describes how strong the evidence is that something can cause cancer under some circumstances, not how much risk it carries at the amount a person actually consumes. The evidence behind the aspartame decision was described as limited in humans, limited in animals, and limited on biological plausibility. The human studies concerned liver cancer. [1]

That same week, a separate World Health Organization committee, the one that sets safe intake limits for food additives, reviewed the same evidence to decide whether the limit for aspartame should change. It didn't. The committee called the human cancer evidence "not convincing" and left the number where it had been, at 40 milligrams per kilogram of body weight. Translated into cans, a 170-pound adult would have to drink more than ten cans a day to cross it. [1]

The population research is largely one study. French researchers followed 102,865 adults for nearly eight years and found that the people consuming the most aspartame had roughly a 15% higher rate of cancer diagnosis. That's an association found in observational data, not a demonstration that one caused the other, and the authors state as much. Their participants skewed female, more educated and more health-conscious than France as a whole. [2]

So the cancer question is unresolved, and it will probably stay that way, because the study that would settle it — randomly assigning thousands of people to drink aspartame for twenty years — is never going to be run.

Most of the discussion around Diet Coke focuses on aspartame, but there are other ingredients worth looking at. 

Here is what's in a can of Diet Coke: carbonated water, caramel color, aspartame, phosphoric acid, potassium benzoate, natural flavors, citric acid, caffeine. [3]

The most cited study on processed food came out of the National Institutes of Health in 2019. Twenty adults lived on a metabolic ward for a month and ate as much or as little as they wanted from two menus offering similar calories, fat, sugar, salt and fiber. Two weeks on ultra-processed food, two weeks on whole food. On the processed menu they ate about 500 more calories a day and gained two pounds. On the whole-food menu they lost two. [4]

The food industry's reading of that result is that processed food is a calorie problem, and that making it less calorie-dense solves it. À la Diet Coke. 

However, the controlled-feeding research says otherwise. In a double-blind trial, researchers fed one group a diet containing no food additives and fed a second group the identical diet with one emulsifier added: carboxymethylcellulose, a thickener used in ice cream, sauces and bread. The calories were the same. After eleven days the additive group had less diverse gut bacteria, lower production of short-chain fatty acids, which are the compounds gut bacteria make to feed the cells lining the colon, and two of them had bacteria pushing into the mucus layer that normally keeps them at a distance. [5]

Sixteen people, eleven days, and measurements of biology rather than of disease. Small and short. Worth knowing anyway, because the calories didn't move and something else did.

The population data points in the same direction. A French cohort of more than 104,000 adults found that for every additional 10% of the diet made up of ultra-processed food, type 2 diabetes risk rose about 15%, and that this held after accounting for the overall nutritional quality of what people were eating. [6] Something about processing appeared to carry a cost the nutrition label wasn't capturing.

Which raises the question nobody usually asks: which parts exactly of ultra-processed food are driving the downstream health impacts?

One study has looked. Harvard researchers followed 31,712 women for fourteen years, recording what they ate every four years and tracking new diagnoses of depression. The women eating the most ultra-processed food had roughly 50% higher odds of being diagnosed than the women eating the least. Total calories were already accounted for. Adjusting for overall diet quality barely moved the number. And when the researchers looked only at cases diagnosed at least four years after the diet was recorded, to reduce the chance that early symptoms were shaping food choices, the association held. [7]

Then they broke ultra-processed food into its components: grain products, sweet snacks, savory snacks, ready meals, processed meat, dairy, fats and sauces. Two components carried the association. Artificially sweetened beverages, and artificial sweeteners themselves. [7]

That finding deserves a hard look rather than a victory lap. Eight independent researchers reviewed the study for the Science Media Centre, and every one of them said the same thing: this is an association, not a cause. The objection that came up most often was direction. People at higher risk of depression may be drinking more diet soda, for weight reasons or convenience or because low mood changes what you reach for, and a four-year gap doesn't rule that out when depression can build slowly over years. [8]

Where that leaves things: two large studies, both accounting for calories, both landing on the same ingredient category.

Go back to the ingredient list and look at two entries. Caramel color, and natural flavors.

"Natural flavors" is a legal category rather than a description and there’s nothing “natural” about it. Federal regulation permits a manufacturer to print those two words instead of naming the substances the flavor is built from. The same regulation goes further: an incidental additive that arrives inside a flavor doesn't have to be declared at all. A single flavor can contain dozens of compounds, and the label is under no obligation to name one of them. [9]

"Caramel color" is four different things. There are four classes, produced by different processes, and the label never says which one you're drinking. There's also no federal limit on how much caramel color a food or beverage may contain. When Consumer Reports tested 110 cans and bottles across twelve brands, every sample listing caramel color contained 4-methylimidazole, a compound formed during the production of two of those four classes and listed by the state of California as a carcinogen. Coca-Cola's products came in lowest of every brand tested, averaging 4.3 micrograms or less per serving against California's 29-microgram warning threshold, which is worth saying plainly. Consumers Union petitioned the Food and Drug Administration to require labels to disclose which class is used. That was 2014. It hasn't happened. [10]

There's a second layer to this. Additives are evaluated for safety one substance at a time, and people swallow them dozens at a time. One large French cohort tried studying them the way they're actually consumed, grouping additives by which ones tend to be eaten together, and found two of the five most common combinations associated with higher type 2 diabetes risk. [11]

And then the thing that makes all of this genuinely hard to settle: there's no blood or urine test for additive intake. Every study in this area estimates exposure by matching what people report eating against product databases. The opacity isn't only a problem for the person reading the label. It's a problem for anyone trying to measure what the label leaves out.

This is where context matters and how you’re consuming Diet Coke impacts your decision.

Comparing a Diet Coke to alcohol isn't close. Alcohol has been classified as a confirmed human carcinogen since 1988, the top category, the same one that holds tobacco and asbestos. It causes at least seven cancers: mouth, throat, voice box, esophagus, breast, liver and colon. The World Health Organization's stated position is that no level of drinking is safe where cancer risk is concerned, and in 2020 nearly a third of the breast cancers attributable to alcohol across Europe occurred in people drinking no more than one or two a day. [12] Aspartame sits two categories below that, on evidence its own reviewers called limited.

So anyone who swapped drinking alcohol for Diet Coke has made a good trade.

What about against regular Coke? Again, Diet Coke is likely better. Thirty-nine grams of sugar in one can versus none in the other. Calories still matter, even if they aren’t the only thing that does.

Against water? Not even close. In the best-designed trial on sweeteners and metabolism, 120 adults took one of four sweeteners for two weeks at matched calories. Saccharin and sucralose worsened blood sugar control. Aspartame and stevia didn't, which means the sweetener in Diet Coke came out cleanest of the four on that measure. But all four changed the bacteria in the gut and the mouth and the compounds circulating in the blood, so aspartame isn't doing nothing. [13]

Then there is the acid. The Framingham Osteoporosis Study found cola specifically, and not other carbonated drinks, linked to lower hip bone density in women, with diet cola showing the same pattern. Decaffeinated cola showed it too, so caffeine doesn't explain it, and the researchers pointed at phosphoric acid, one of the ingredients in Diet Coke. [14] It’s a snapshot rather than a study that followed people forward, and the cola drinkers ate less calcium, so treat it as an open question. 

The acid does another thing. When researchers measured the pH of 379 beverages sold in the United States, 93% of them came in under 4. [15] That's acidic enough that the American Dental Association names frequent consumption of low-pH drinks as an erosion risk, pointing to a review that found soft drinks more than double it. [16] Sugar-free doesn't exempt a can, because it's the acid doing that work rather than the sugar. Worth knowing if you care about your tooth enamel. 

Diet Coke is better than alcohol by a wide margin, and that margin is documented rather than assumed. However, it’s also worth noting that it isn’t a free pass. 

What the evidence supports, stated plainly: processing and additives do things that calories alone don't explain. Both studies that have broken ultra-processed food into its parts landed on sweetened drinks. Nobody has tested these additives in the combinations people actually consume them in. And the label is legally permitted to withhold what's inside two of the eight ingredients.

None of that proves a can of Diet Coke will hurt you. All of it argues against drinking more of them than you need to.

If it's genuinely standing in for alcohol, that's a trade worth keeping. If it's four a day out of habit, or even one every evening, it’s worth considering cutting back and treating Diet Coke like all ultra-processed foods and consuming in moderation.

Three things you can do this week:

1. If it took the place of alcohol, keep the trade. Just don’t make it daily.

The comparison isn’t close. Alcohol has been a confirmed human carcinogen since 1988, it causes at least seven cancers, and the World Health Organization’s position is that no amount is safe where cancer risk is concerned. Swapping a nightly drink for a Diet Coke is a genuine upgrade. But clearing that bar doesn’t mean much on its own, and nobody should be drinking alcohol every night either. Treat it the way you’d treat any ultra-processed food: once or twice a week, not a fixture.

2. Protect your enamel on the days you do drink one.

This is the part nobody disputes. The American Dental Association’s guidance for acidic drinks is specific: use a straw, don’t hold or swish the liquid in your mouth, and afterward rinse with water or follow it with milk or a piece of cheese, both of which help neutralize the acid. Brushing immediately is the common instinct and the wrong one, since enamel is temporarily soft right after exposure, and the Association’s advice following acidic foods is to wait an hour. One more thing that matters more than people expect: nursing a can across an entire afternoon is worse for your teeth than drinking it in ten minutes, because the damage tracks how long the acid sits there.

3. The evening can is a caffeine decision.

A 12-ounce Diet Coke carries 46 milligrams of caffeine, roughly half what’s in a cup of coffee. If the habit is one with dinner or one in front of the television, that’s a dose arriving inside the window where it can degrade sleep quality even when it doesn’t keep you awake. If you’re going to keep one a day, move it to the morning.

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Wellness, filtered.

The Wellness Brew

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Disclaimer: The Wellness Brew is for informational purposes only and does not constitute medical advice. The content published here is not intended to diagnose, treat, cure, or prevent any disease or health condition. Always consult a qualified healthcare professional before making any changes to your diet, supplement routine, or lifestyle.