SIBO and Dairy: Lactose, Casein, Motility & Secondary Lactase Deficiency

In short: the problem with dairy in SIBO is mainly lactose, the sugar in milk. Bacteria in the small intestine can ferment lactose before it is digested, and SIBO can reduce lactase, the enzyme that breaks it down. In a 2010 study, SIBO was four times as common in people whose lactose malabsorption caused symptoms as in those whose did not². Low-lactose dairy such as hard cheese, butter and lactose-free milk is usually tolerated¹, and lactose tolerance can improve after SIBO is treated⁴.

This guide explains how SIBO affects lactose digestion, which dairy foods are low in lactose, and what is known about casein and A2 milk. For the wider diet, see our SIBO diet guide.

How SIBO affects lactose

Lactose is normally broken down by lactase, an enzyme on the lining of the small intestine. Lactose malabsorption has several causes: the common genetic decline of lactase after childhood, secondary loss of lactase when the lining is damaged by infection or other conditions, rapid transit, and SIBO¹. In SIBO, two things can happen:

• Bacteria ferment the lactose in the small intestine before it is absorbed, producing gas. In a 2010 study of people with chronic functional diarrhea and lactose malabsorption, SIBO was found in 64% of those who had symptoms after lactose, compared with 15% of those who did not. The authors concluded that SIBO makes lactose intolerance more likely through fermentation in the small intestine².
• Lactase activity can fall. SIBO reduces the activity of several digestive enzymes, including lactase⁴. SIBO practitioners remove lactose during treatment and for about a month afterwards to let the lining recover⁶.

Treating SIBO can help. In a 2025 uncontrolled pilot study of people with IBS, SIBO and self-reported lactose intolerance, two weeks of rifaximinRifaximin An antibiotic that is barely absorbed into the bloodstream, so it acts mainly inside the gut. It is a common prescription treatment for SIBO. reduced lactose malabsorption on breath testing from 88% to 52% of participants⁴.

SIBO can cause a false lactose intolerance result

A lactose breath test measures hydrogen from bacteria fermenting lactose. If bacteria are in the small intestine, the test can turn positive early even when lactase works normally. In a 2020 study of 264 people tested at a Hungarian center, the lactose breath test alone gave a false positive result in 9%, and 75% of those false positives were explained by SIBO³. If you have been told you are lactose intolerant, it may be worth asking whether SIBO was considered. See our breath test guide.

Lactose in common dairy foods

Representative values from a 2019 review¹. Lactose is also added to many processed foods, such as sausages, sauces, bread and ready meals.

Most people with lactose malabsorption tolerate at least 12 g of lactose, about 250 mL of milk, at one time, and more when it is taken with food. A strictly lactose-free diet is not usually needed¹. People with active SIBO may tolerate less, which is why SIBO diets limit lactose during treatment⁶. Fermented dairy such as kefir and yogurt contains less lactose than milk; a long-fermented 24-hour homemade yogurt is treated as lactose-free in SIBO diets based on the Specific Carbohydrate Diet⁶. Lactose is the “D” (disaccharide) in FODMAPFODMAPs Fermentable oligo-, di- and monosaccharides and polyols: short-chain carbohydrates that are poorly absorbed and quickly fermented by gut microbes.; see our low-FODMAP guide.

Lactase supplements

Lactase tablets taken with dairy improve lactose digestion and symptoms, but the effect is modest¹. They do not affect the bacteria themselves, so they help with lactose but not with other fermentable foods.

Casein and A2 milk

Most cow’s milk contains two forms of the protein beta-casein, A1 and A2. A2 milk contains only A2. A 2017 systematic review found that A1 slows gut transit in rodents. In humans, a limited number of studies linked A1 with slower transit (one study) and looser stools (two studies)⁵. Several of the review’s authors had worked as advisers to an A2 milk company. A2 milk still contains lactose, and there are no studies of A2 milk in SIBO.

Dairy fat

In SIBO, bacteria can break down bile acids, which are needed to absorb fat, leading to poor absorption of fat and fat-soluble vitamins⁸. If high-fat dairy causes greasy, loose stools, smaller portions may help; tell your doctor, because fat malabsorption can lead to vitamin deficiencies.

Histamine in aged cheese

Aged cheeses can contain histamine. In Spanish market data, raw-milk cheese averaged 59 mg/kg and reached up to 390 mg/kg, while no histamine was detected in fresh unripened cheese⁷. This matters for people with histamine intolerance, not for SIBO itself.

Do not cut out dairy for longer than you need to

Dairy is a valuable source of protein, calcium and vitamin D¹. If you remove it, replace these nutrients with lactose-free dairy or fortified alternatives, and reintroduce dairy after treatment to find your own tolerance. See our guide to eating after SIBO treatment.

Common questions

Does dairy feed SIBO?

Lactose can be fermented by bacteria in the small intestine, which causes gas and symptoms². Low-lactose dairy, such as hard cheese and butter, contains very little fermentable sugar¹.

Will my lactose intolerance go away after SIBO treatment?

It may, if SIBO was the cause. In the 2025 pilot study, lactose malabsorption fell from 88% to 52% of participants after rifaximin⁴. Genetic lactase decline in adults does not reverse¹.

Is goat’s milk better for SIBO?

Goat’s milk contains almost as much lactose as cow’s milk¹.