Fiber passes through your system unchanged, triggering fullness without the calories
In the ongoing human struggle to understand what nourishes us versus what merely satisfies us momentarily, researchers at Mexico's National Autonomous University have offered a clarifying lens: not all carbohydrates are equal in the eyes of the body, and this distinction may be among the most consequential choices we make at the table. By mapping how fiber, resistant starch, and simple sugars each travel through human physiology, the scientists illuminate why modern diets—engineered for speed and pleasure—have quietly accelerated the rise of obesity and metabolic disease. The wisdom embedded in this research is ancient in spirit: that slowness, in digestion as in life, often signals something worth keeping.
- Obesity and metabolic disease are rising across all age groups, and the foods dominating modern diets—refined carbohydrates and simple sugars—are directly implicated in that trend.
- Simple sugars and rapidly digestible starches cause blood glucose to spike and crash, triggering insulin surges that, over time, erode the body's ability to regulate itself.
- Industrial food processing has systematically stripped away fiber and resistant starch—the very components that slow digestion, create fullness, and protect metabolic health.
- UNAM researchers are drawing a clear line between carbohydrates that the body processes slowly and protectively versus those it converts to glucose almost instantly, offering a practical framework for better dietary decisions.
- The path forward is not carbohydrate elimination but carbohydrate discernment—choosing whole grains, legumes, and fiber-rich foods that sustain rather than destabilize.
Your body does not treat all carbohydrates the same way, and that single fact sits at the heart of new research from Mexico's National Autonomous University. Three UNAM scientists examined how different carbohydrates function in the body, cutting through diet culture noise to focus on a distinction with real consequences for weight and metabolic health.
Digestible carbohydrates—simple sugars and starch—account for up to 80 percent of most people's daily energy intake. The issue is not that carbohydrates are inherently harmful, but that modern food systems have been engineered to deliver the kinds the body processes fastest and most completely, which are precisely the ones most linked to weight gain and disease.
Fiber tells a different story. Found in fruits, vegetables, legumes, and whole grains, fiber cannot be broken down by the body's digestive enzymes. This is a feature, not a flaw: fiber passes through the digestive tract intact, creating fullness, helping expel fats rather than absorb them, and preventing constipation. Certain compounds in vegetables and legumes—raffinose and stachyose among them—work the same way, traveling through the system unchanged and producing the same protective effects.
Resistant starch occupies a middle ground, behaving like fiber once it reaches the colon. The researchers distinguish three starch types by digestion speed. Rapidly digestible starch, common in pastries and ultra-processed foods, spikes blood glucose immediately and triggers insulin surges linked to diabetes and obesity. Slowly digestible starch, found in whole grains and intact seeds, keeps blood sugar stable and prevents the insulin resistance that builds when the body is chronically flooded with glucose.
Industrial refining has engineered away much of this protection, stripping fiber from flour and making starch more accessible to enzymes. Simple sugars compound the problem by triggering pleasure responses that encourage overconsumption even as they destabilize blood chemistry.
The practical implication is straightforward: choosing whole grain bread over white is not a matter of willpower or aesthetics—it is a choice between two different metabolic outcomes. As sedentary lifestyles become more common and metabolic disease touches more lives, the researchers argue that the road to better health runs not through eliminating carbohydrates, but through choosing the ones the body was built to process slowly.
Your body does not treat all carbohydrates the same way. This simple fact, explored in recent research from Mexico's National Autonomous University (UNAM), holds the key to understanding why some foods leave you hungry an hour later while others keep you satisfied, and why obesity and metabolic disease have become so difficult to reverse.
Three researchers—Edgar Alejandro Esquivel Fajardo, Héctor Emmanuel Cortés Ferré, and Tere Arredondo Ochoa—examined how different carbohydrates function in the body and what that means for weight and health. Their work, published through UNAM Global, cuts through the noise of diet culture to focus on a distinction that matters: digestible carbohydrates, particularly simple sugars and starch, account for up to 80 percent of the daily energy requirements most people consume. When paired with fats, these two categories dominate the modern diet. The problem is not that carbohydrates are inherently harmful. The problem is that we have engineered our food system to deliver the kinds that your body processes fastest and most completely—and those are precisely the ones most likely to contribute to weight gain and metabolic disease.
Fiber tells a different story. Found in fruits, vegetables, legumes, and whole grains, fiber is technically a carbohydrate, but your digestive system cannot break it down. The chemical bonds that hold fiber together—called beta bonds—resist the enzymes your body produces to extract energy from food. This limitation is actually a feature, not a flaw. When fiber passes through your digestive tract intact, it creates a sensation of fullness that makes you eat less. It also drags fats along with it, helping your body expel them rather than absorb them. Diets rich in fiber tend to be lower in calories overall, they stimulate the movement of food through your intestines, and they prevent constipation. Certain oligosaccharides found in vegetables and legumes—compounds with names like raffinose and stachyose—work the same way, traveling through your system unchanged and triggering the same protective effects.
Resistant starch occupies a middle ground. Not all starch gets converted to glucose. A meaningful fraction, called resistant starch, behaves like fiber once it reaches your colon, arriving undigested and producing the same physiological benefits. The researchers distinguish between three types of starch based on how quickly your body breaks them down. Rapidly digestible starch, common in ultra-processed foods like pastries and donuts, causes blood glucose to spike immediately after you eat it. This spike triggers a corresponding surge in insulin, a pattern linked directly to the development of diabetes and obesity. Slowly digestible starch, found in whole grains and intact seeds, maintains stable blood sugar levels, preventing the insulin resistance that develops when your body is constantly flooded with glucose.
Industrial processing has engineered away much of this protection. When manufacturers refine flour, they strip out most of the fiber and expose the starch to easier enzymatic access. The result is a product that delivers more calories and contributes to metabolic imbalance. Simple sugars—the kind in candy, ice cream, and processed foods—create not just a rapid glucose spike but also a pleasurable sensation that makes people want to eat more. The body registers these foods as rewarding even as they destabilize blood chemistry.
Understanding these distinctions offers a practical path forward. The choice between a white bread sandwich and one made from whole grain is not merely aesthetic or a matter of willpower. It is a choice between two different metabolic outcomes. One will leave you hungry again within hours. The other will sustain you. One will push your blood sugar toward the danger zone. The other will keep it stable. As obesity and metabolic disease continue to affect people across all age groups, and as sedentary lifestyles become more common, the ability to recognize which carbohydrates your body can actually use versus which ones it will simply store as fat becomes increasingly important. The research suggests that the path to better health runs not through eliminating carbohydrates entirely, but through choosing the ones your body was designed to process slowly.
Citas Notables
Diets rich in fiber have low caloric content, stimulate intestinal movement, and help prevent constipation— UNAM researchers Esquivel Fajardo, Cortés Ferré, and Arredondo Ochoa
Simple sugars create a pleasurable sensation when consumed, but their rapid absorption leads to glucose and insulin spikes linked to diabetes and obesity— UNAM research analysis
La Conversación del Hearth Otra perspectiva de la historia
Why does it matter that fiber can't be digested? Doesn't that mean your body gets nothing from it?
That's the counterintuitive part. Fiber doesn't give you calories, but it gives you something more useful—it makes you feel full without the energy cost. It also clears out fats your body would otherwise absorb. The benefit isn't in the calories; it's in what doesn't happen.
So when someone eats a donut versus an apple, what's actually different in their body?
The donut hits fast. Your blood sugar spikes, insulin floods in to manage it, and two hours later you're hungry again because the energy burned through so quickly. The apple delivers fiber that keeps you satisfied longer, and the carbohydrates it contains break down slowly enough that your blood sugar stays stable.
Is resistant starch something people should actively seek out, or does it just happen naturally in certain foods?
It happens naturally in whole grains, legumes, and certain vegetables. You don't need to hunt for it as a special ingredient. It's just what you get when you eat foods that haven't been heavily processed.
The research mentions that refined flour eliminates fiber. Can that fiber be added back in?
Technically yes, but it's not the same. When you refine flour, you're also changing the starch structure itself, making it digest faster. Adding fiber back in helps, but you've already lost the slow-digestion benefit of the whole grain.
What does this mean for someone who's already struggling with obesity or diabetes?
It means the food choices available to them matter enormously. If they're eating processed foods full of simple sugars and refined starch, they're fighting against their own blood chemistry. Shifting toward fiber-rich, whole foods gives their body a chance to stabilize.