Welcome to the second installment of Brass Tacks, where I finish the arguments I start online. Last time it was the ritual shaming of an ingredient list. This time it is the opposite fight: not an ingredient people are wrong to fear, but an ingredient people are wrong to dismiss.
The post that set me off made a claim that is very popular right now and half correct. It said that brands are slapping a single isolated prebiotic fiber onto sodas and ice cream to borrow the "gut healthy" halo of foods that earned it honestly, and that this is marketing, not gut health.
I agree with the narrow version. A soda with a gram of inulin is still a soda. An ingredient can support gut health without redeeming the entire product it lives in. Fine. Said it myself.
But that is not the claim most people in those threads are actually making. The claim they are making, underneath the reasonable one, is broader and wrong: that adding fiber to a processed food is inherently a trick, that the functional ingredient is by definition a costume. And that is where I will plant my flag, because I have been making the opposite case for a decade and I have the papers to back it.
So here is the provocation I actually believe, stated as plainly as I can: I could take a Twinkie and make it modulate your gut microbiome in a direction a tomato, a banana, an avocado, and a mango could not. I am not saying a Twinkie is health food. I am saying that on the specific axis of feeding your microbiome the high-molecular-weight soluble fiber it is starving for, a properly formulated processed food can beat a whole food, and that almost nobody can stomach that sentence, for reasons that have nothing to do with the science.
I would bet a million dollars on it.
Start with the deficiency, because it is staggering
Less than five percent of American adults meet the daily fiber target. Depending on which dataset you use, modeling of national survey data puts it around 3.8% for adults, and the American Society for Nutrition describes mean intakes of roughly 15 to 18 grams a day against targets in the 25-to-38-gram range. More than ninety percent of the population falls short of a nutrient that affects glucose control, cholesterol, the gut barrier, and the entire ecology of the microbiome.
We have been telling people to "just eat more vegetables" for fifty years and the number has not moved. When a deficiency is this universal and this stubborn, the behavioral lever is exhausted, and the realistic intervention is the food supply itself. That is arithmetic, and I made the full version of the argument in Beyond "Just Eat Real Food," Revisited, which is the load-bearing premise here too.
"Fiber" is not one thing, and that is the whole argument
The people calling added fiber a gimmick are treating "fiber" as a single undifferentiated substance, as if a gram is a gram. It is not. Fiber is a sprawling category of non-digestible carbohydrates that differ by solubility, by chain length, by branching, and, decisively, by molecular weight and the viscosity that comes with it. Those differences are the difference between a fiber that changes your physiology and a fiber that mostly just adds a number to the label.
The viscous, high-molecular-weight soluble fibers are the ones that earn their keep. A 2023 systematic review and meta-analysis found that viscous soluble fiber meaningfully improves glucose and lipid metabolism, with the effect tracking viscosity. A separate meta-analysis of psyllium found glycemic improvements that scaled with how poor a person's control was to begin with. Highly soluble β-glucan modulates both blood-glucose regulation and intestinal permeability. These fibers form a gel that physically slows the absorption of glucose and bile. That is a mechanism, measured in clinical trials.
And here is the part that genuinely offends people: by the only metrics that matter for that mechanism, the concentrated functional fiber in a formulated product can do more than the equivalent gram of whole-food fiber, because you can deliver the right fiber, at the right molecular weight, at a dose a human will actually eat. Take gum arabic, the acacia fiber that turns up in exactly the kinds of products people sneer at. It is 85 to 90% soluble fiber by weight, it is bifidogenic (it selectively grows your beneficial bacteria), it ferments cleanly to short-chain fatty acids, and it is tolerated up to about 30 grams a day. A single gram of gum arabic in a can gets you closer to your soluble-fiber target than half a loaf of whole-wheat bread will. I wrote that down in a paper seven years ago, before the trend, and it is still true.
Humans, for the record, do not make the enzymes to break most of these fibers down. Our beneficial bacteria do. That is the literal definition of a prebiotic, and it is why I can stand in front of a Keto certification panel, read an ingredient label, and call whether a product will pass the lab test for net carbs before the assay runs. The fiber is invisible to your digestion and visible to your microbiome. Shape, branching, polysaccharide class, and molecular weight decide what it does. If you do not know those variables, every fiber looks like a gimmick, because you cannot see the thing that separates them.
The bias nobody wants to name
Now the part that is really a piece about psychology wearing a piece about fiber.
The stated objection is that processed foods are unfairly borrowing a health halo. But watch what happens when you propose the reverse: a processed food that, mechanistically, does something good for you. The same crowd cannot accept it. A food they have already filed under "junk" cannot be redeemed by any amount of evidence, because the category is doing the thinking, not the data. That is not the health halo. That is the health halo running in reverse, and it carries a quietly cruel message: don't bother improving anything, because if it started out impure it can never count.
That is the appeal-to-nature fallacy in a lab coat: the unexamined assumption that natural is good and processed is bad, full stop, regardless of what the molecule actually does in the body. It feels like rigor. It is the opposite of rigor. Rigor is being willing to follow the mechanism even when it leads somewhere your aesthetics hate, which is to a fortified Twinkie outperforming a tomato on the one specific axis we are measuring.
Receipts
I am aware of how this sounds, so let me be clear that I am not theorizing from the sidelines. I called this trend emerging ten years ago. I wrote the paper on high-molecular-weight prebiotic fiber. I consulted on prebiotic formulation for companies in this exact space years before "gut healthy" was a marketing category to be cynical about. Being early and on the record is the through-line of everything I do, and this is one more entry in it.
None of which makes a soda health food. It makes the lazy dismissal of functional fiber wrong, which is a narrower and more defensible claim, and the one I will actually die on. The full mechanistic and regulatory case is in the companion essay. This piece is just me, refusing to let "added fiber is a trick" stand as the last word in another thread.
Brass tacks. The fiber is real. The bias is the gimmick.