One rarely hears about fiber in this age of high-protein, antioxidant, gluten-free, and lactose-free diets promoted by social media influencers. However, fiber is one of the food components with the most proven benefits, beyond facilitating intestinal transit. And this includes an effect with a mysterious name, the “second meal effect”.
But what exactly is fiber? The official definition tells us that it is the edible part of plant foods that is resistant to digestion and absorption in the small intestine, with complete or partial fermentation in the large intestine. That is, it is in foods of plant origin and we do not digest it in the small intestine (as it happens with nutrients), but it feeds our bacteria.
More than 40 years ago, British researchers observed that if healthy people were given bread or lentils for breakfast, blood glucose (sugar) levels rose less with lentils, which have more fiber. And if they were all given the same food afterwards, those who had lentils for breakfast had lower blood glucose. Since high glucose rises increase the risk of type 2 diabetes, this ability of fiber is very important: it is the “second meal effect”.
The next step was to find out why this was happening. At first, it was thought that it was only due to the glycemic index, which has to do with how quickly we absorb sugar from a food (lentils or bread, although they do not have sugar like the one in our sugar bowl, do contain starch, which in our body it is broken down into sugar). However, it was later found that the “second meal effect” had to do with our microbiota.
At the end of our large intestine, in the colon, lives a large population of bacteria: our colonic microbiota. These bacteria feed on remains of food that we have not digested, generating the compounds that we call fermentation metabolites. And as we have said before that fiber is not digested, when it reaches the colon intact it is used by the microbiota to generate these metabolites.
In 2006, Italian researchers were the first to study this fermentation and relate it to the second meal effect. They provided a group of people with three different breakfasts, which had fiber with different glycemic indices and the ability to be fermented by the microbiota. And what they observed was that the most important thing for the glucose to drop after lunch was not the glycemic index, but that the fiber could be fermented.
Eating fiber, therefore, provides food for our bacteria which, in return, give us back the fermentation metabolites. And these metabolites help keep glucose from getting too high, not only at that moment, but also at the next meal.
By the way, the fiber at dinner also makes glucose after breakfast not rise as much. Something that contradicts that widespread idea that it is convenient to exclude carbohydrates from dinner.
This regulation of blood sugar after several meals is very important for health, but it is not something we perceive. However, the second meal effect has another positive side that we can notice, and that is related to another property of the fiber. It is about its ability to increase satiety because, when we eat fiber, the stomach takes longer to empty.
In 1997, a group of US military received orange juice, half of them enriched with pectin (a type of fiber) and the other half without enrichment. At four hours, everyone was given an ice cream. And an hour after eating the ice cream, those who had drunk the juice with fiber in the morning felt much more satiated.
That is to say that taking fiber in a meal makes us arrive at the next one with less hunger. This is also part of the second meal effect.
We are going to include some new actors in this story: polyphenols. They are food compounds that travel with fiber and that also serve as food for our colonic microbiota. The new metabolites they generate have several beneficial effects, the best known being their antioxidant capacity.
Well, it is being studied whether polyphenols contribute to the second meal effect. Specifically, a study with the residue left in the industry after obtaining orange juice showed that, if it was given to overweight men for breakfast, they had less glucose and insulin in their blood after the meal. The most interesting thing is that this residue was a mixture of fiber and polyphenols.
Today we know that polyphenols and fiber travel together to the colon where, in addition, the presence of one stimulates the fermentation of the other, and vice versa. That is why for many of its effects it is difficult to know which one is due to the fiber and which to the polyphenols. But research may soon show us that polyphenols are part of the second meal effect.
So, although there are still aspects to be investigated, we know that fiber has many more functions than transit. Consuming enough (at least 30 grams a day) ensures that at the next meal we will be less hungry and our glucose rise will be less, which reduces the risk of being overweight and type 2 diabetes. Currently, most of us are very far from reaching those recommendations, so this is a good time to start increasing our fiber intake.
This article was originally published on The Conversation.