Dietary fiber: my best health allies

What if the most protective nutrient for my health was also the most neglected in my diet?

In France, the average fiber consumption is around 18 g per day in adults, while ANSES recommendations set the objective at 25-30 g minimum. This almost generalized deficit is not trivial: dietary fiber is among the nutrients whose health benefits are best scientifically documented, with demonstrated protective effects on cardiovascular risk, type 2 diabetes, obesity, colorectal cancer and the health of the intestinal microbiota. Understanding what fiber is, where to find it and why it is so important is a priority for any serious health prevention approach.

1. What is dietary fiber?

Dietary fibers are complex carbohydrates of plant origin that human digestive enzymes cannot break down. Unlike other carbohydrates, they are therefore not absorbed in the small intestine and reach the colon almost intact, where they exert most of their beneficial effects.

There are two main categories of fibers, with complementary properties:

A well-balanced fiber diet combines both types. Neither one nor the other is enough alone: ​​soluble fibers mainly act on blood sugar and cholesterol, while insoluble fibers are allies of transit and the prevention of colorectal cancer.

Type	Features	Main sources
Soluble fibers	Dissolve in water and form a viscous gel in the intestine. Fermented by colon bacteria. Slows down the digestion and absorption of carbohydrates and lipids.	Oats, barley, apple, pear, citrus fruits, legumes, chia and flax seeds, psyllium
Insoluble fibers	Do not dissolve in water. Increase stool volume and accelerate intestinal transit. Little fermented.	Wheat bran, whole grains, green vegetables, nuts, seeds, skin of fruits and vegetables

2. The effects of fiber on health: what science establishes

The level of scientific evidence in favor of dietary fiber is among the highest in nutrition. Here are the best documented effects:

• ●Cardiovascular protection: a meta-analysis published in The Lancet (2019) covering 185 prospective studies concluded that an increase of 8 g of fiber per day reduced the risk of coronary heart disease by 19%, the risk of stroke by 15% and the risk of type 2 diabetes by 16%. The dose-response relationship is linear: the more I consume, the stronger the protection.

• ●Reduction of the risk of colorectal cancer: the IARC (International Agency for Research on Cancer) classifies cereal fibers as probably protective against colorectal cancer. An increase of 10 g of fiber per day is associated with a 10% reduction in risk according to several meta-analyses.- Regulation of blood sugar: soluble fiber slows the absorption of carbohydrates, reducing post-prandial glycemic peaks. They improve insulin sensitivity, a particularly important benefit for the prevention of type 2 diabetes.

• ●Reduction of LDL-cholesterol: soluble fibers (notably beta-glucan from oats) bind to bile acids in the intestine and promote their fecal elimination. The liver must then draw on circulating cholesterol to make new ones, which reduces blood LDL.

• ●Satiety and weight management: fiber increases the volume of the food bolus, slows gastric emptying and stimulates the secretion of satiety hormones (GLP-1, PYY). High fiber intakes are associated with less weight gain in longitudinal studies.

• ●Microbiota health: prebiotic fibers selectively nourish beneficial bacteria in the colon, including Bifidobacterium and Lactobacillus. Their fermentation produces short-chain fatty acids (butyrate, propionate, acetate) with anti-inflammatory and protective effects of the intestinal barrier.

  Aliment (portion usuelle)	Fibres apportees	Categorie
  Haricots rouges cuits (150 g)	~10 g	Legumineuses
  Lentilles cuites (150 g)	~8 g	Legumineuses
  Pois chiches cuits (150 g)	~7,5 g	Legumineuses
  Avocat (1 moitie, 75 g)	~5 g	Fruits
  Framboises (100 g)	~6,5 g	Fruits
  Poire avec peau (1 moyenne)	~5,5 g	Fruits
  Flocons d'avoine (40 g, sec)	~4 g	Céréales completes
  Pain complet (2 tranches, 60 g)	~4 g	Céréales completes
  Brocoli cuit (150 g)	~4 g	Légumes
  Graines de chia (15 g)	~5 g	Graines
  Amandes (30 g)	~3,5 g	Oleagineux
  Patate douce avec peau (150 g)	~4 g	Légumes

  ## 3. The best sources of dietary fiber

4. Why I’m not getting enough – and how to fix it

The fiber deficit in the Western diet is mainly linked to three structural factors:

• ●The growing role of ultra-processed products: cereal refining and industrial processing eliminate almost all fibers. White bread provides 2 g of fiber per 100 g, compared to 7 g for quality wholemeal bread.

• ●The drop in consumption of legumes: lentils, chickpeas and beans are the foods richest in fiber but remain under-consumed in France. Anses recommends 2 servings of legumes per week; the majority of French people only consume one.

• ●Low consumption of fruits and vegetables: only 1 in 4 French people reach the recommended 5 servings per day. However, fruits and vegetables - especially eaten with their skins - are important sources of insoluble fiber.

How to increase my fiber intake without disrupting my transit:

• ●I increase gradually: too sudden an increase in fiber can cause bloating and digestive discomfort. I aim for +2 to 3 g per week until I reach the goal of 25-30 g.

• ●I replace refined cereals with their complete versions: this is the simplest and most impactful change. Wholemeal rice, wholemeal bread, wholemeal pasta.

• ●I include legumes at least twice a week: in salads, soups, curries or bowls.

• ●I eat fruits with their skin when possible: the skin of apples, pears or kiwis contains a high concentration of insoluble fiber.

• ●I add seeds to my dishes: a tablespoon of chia or flax seeds in my yogurt or salad adds 3 to 5 g of fiber without changing the taste.

• ●I drink enough water: fibers need water to exert their effects. Without sufficient hydration, an increase in fiber can instead cause constipation.

5. Fiber and prevention of chronic diseases: the evidence in figures

To quantify the impact of fiber on the prevention of chronic diseases, the most recent and exhaustive synthesis of the scientific literature is that published in The Lancet in 2019 by Reynolds et al., sponsored by the WHO:

• ●Each 8 g increase in fiber per day is associated with a 5 to 27% reduction in the risk of coronary heart disease, stroke, type 2 diabetes, colorectal cancer and all-cause mortality.

• ●People consuming 25-29 g of fiber per day have a significantly lower risk of chronic disease than those who consume less than 15 g.

• ●The benefits are even greater beyond 30 g per day, with no ceiling effect identified in the current data.

These figures make dietary fiber one of the nutrients with the most favorable benefit/cost ratio in health prevention: they are abundant in inexpensive foods (legumes, whole grains, seasonal fruits and vegetables), without side effects for a healthy person, and their benefits are multisystemic.