Fiber, fermented foods and probiotics, without the hype

The cheap, well-studied part: fiber

Fiber is a substance found in plants: in fruits, vegetables and grains.¹ It adds bulk to your diet, makes you feel full faster, aids digestion and helps prevent constipation.² There are two forms. Soluble fiber attracts water and turns to gel, which slows digestion; research has shown it lowers cholesterol.³ Insoluble fiber adds bulk to the stool and helps food move through the intestines.⁴

Diets higher in fiber can increase the frequency of bowel movements and can reduce the risk of cardiovascular disease.⁸ Yet most Americans do not get enough, which is why fiber is called a "dietary component of public health concern".⁷ On average Americans eat about 16 grams a day; the recommendation is 21 to 38.⁵

Getting more without the discomfort

Good sources include whole grains such as oatmeal and whole wheat bread; legumes such as lentils, black beans and chickpeas; fruits such as berries, apples with the skin on and oranges; and vegetables such as carrots, broccoli and green peas.¹² Peeling fruits and vegetables removes some of the fiber.¹³

The commonest reason people give up is going too fast. Extra fiber can bring gas and cramps; this often settles as your gut bacteria adjust, and adding fiber gradually over a few weeks helps.¹⁰ Water helps fiber pass through.¹¹ On a food label, 20% DV or more per serving counts as high.⁹

If you use a fiber supplement such as psyllium, drink at least 8 ounces of liquid with each dose, and keep digoxin, aspirin and nitrofurantoin at least 3 hours away from it.³⁵

Fermented foods: good food, modest promises

Your gut contains trillions of microorganisms, together called the microbiome. The current Dietary Guidelines say vegetables, fruits, fermented foods such as sauerkraut, kimchi, kefir and miso, and high-fiber foods support a diverse microbiome, which may be beneficial for health, while highly processed foods can disrupt it.¹⁴

Fermented foods are made with microorganisms: yogurt, cheese, sourdough bread, pickles, miso, kimchi and sauerkraut among them. But not all of them contain probiotics with proven health benefits.¹⁶ Eat them because they are good food, not as a treatment.

Probiotics: a specific tool, not a category

Probiotics are live microorganisms, such as bacteria and yeasts, that provide health benefits when taken in sufficient amounts.¹⁵ The catch is specificity. If one kind of Lactobacillus helps prevent an illness, that does not mean another kind, or any Bifidobacterium, will do the same.¹⁸

There is some evidence for acute diarrhea, antibiotic-associated diarrhea and atopic eczema; for most other conditions, strong evidence is lacking.²² In one review of people taking antibiotics, probiotics were linked to about half the likelihood of antibiotic-associated diarrhea, a conclusion considered tentative because the studies were of moderate quality.²³ There are no official recommendations for probiotic use by healthy people.²⁰

In healthy people, probiotics may cause gas but rarely cause infections.²⁸ The risk of harm is greater with severe illness or a weakened immune system,³⁰ and severe or fatal infections have been reported in premature infants.²⁹ Prebiotics are something different again: nondigestible food components that feed helpful microorganisms. Synbiotics combine the two.³³

How to read the label

Probiotics are named by genus, species and strain; in Lacticaseibacillus rhamnosus GG, GG is the strain.¹⁷ A label that stops at "Lactobacillus" does not tell you which one you are buying. A higher CFU count alone does not mean a bigger benefit, and counts can fall over time.¹⁹

Many probiotics are sold as dietary supplements, which do not need FDA approval before they are marketed. Their labels may claim effects on the structure or function of the body, but not that they lower the risk of a disease without the FDA's consent.²⁵ The FDA has not approved any health claims for probiotics,²⁶ and some products have contained microorganisms not on the label.²⁷

A historical perspective: Dr. Majid Ali

“Five days a week, my lunch is yogurt or kefir”

— Dr. Majid Ali, 25 August 2014

Dr. Ali wrote about fermentation for decades. On food, his view is consistent with current guidance: current guidance says fermented foods and high-fiber foods support a diverse microbiome.¹⁴

He also used "fermentation" as a name for disease, and attached bowel-detox, yeast-overgrowth and colon-cleanse programmes to it. The sources used here do not support those: a review found no compelling research for detox diets, and colon cleansing can have serious side effects.³⁴ Nor do they support his claims for probiotics after colonoscopy, during chemotherapy or for colds.

What to do this week

1. Add one high-fiber food you already like

   Beans in a soup, oatmeal at breakfast, an apple with the skin on.¹²¹³

2. Go slowly, and drink with it

   Add fiber over a few weeks;¹⁰ water helps it pass through.¹¹

3. Put a fermented food next to a habit

   Plain yogurt with breakfast, or a spoon of sauerkraut with dinner. Food, not a supplement.¹⁴

4. Read your probiotic's label

   Look for genus, species and strain, not just a CFU number.¹⁷¹⁹

5. Starting antibiotics? Ask first

   Some strains may help when started within 2 days of the first dose. Ask which one.²¹³¹

Every numbered marker links to the exact passage it rests on. Check the sources.