Galactose is a simple sugar (monosaccharide) chemically similar to glucose. It is rarely found in foods in its isolated form. The primary source of dietary galactose is lactose (milk sugar), a disaccharide formed when galactose combines with glucose. For individuals restricting their intake, understanding all dietary sources beyond dairy is necessary.
Galactose in Dairy Products
The vast majority of dietary galactose comes from consuming lactose-containing dairy products. In the small intestine, the enzyme lactase breaks down lactose into one molecule of glucose and one molecule of galactose. Both simple sugars are then absorbed into the bloodstream.
Fresh milk is the most common source of lactose, but unprocessed milk contains relatively low amounts of free galactose (around 7 mg per 100 grams). The concentration of free galactose changes dramatically when dairy products are fermented or processed. For example, bacterial cultures used to make yogurt actively break down lactose, meaning a 100-gram serving of yogurt can contain 60 to over 1,500 milligrams of free galactose.
Highly processed or fermented products also contain high amounts of free galactose. Cheeses labeled “lactose-free” can contain high levels of free galactose because the manufacturing process adds the lactase enzyme to pre-digest the lactose. This intentionally breaks the lactose bond, leaving behind high concentrations of both glucose and galactose. Some “lactose-free” cheeses contain an extremely high concentration of free galactose, sometimes exceeding 800 milligrams per 100 grams. Whey powder, derived from milk and used in protein supplements and processed foods, also contains residual lactose that yields galactose upon digestion.
Hidden Sources in Fruits and Vegetables
While dairy products contribute the most significant quantities, galactose is also present in a variety of non-dairy foods, either as a free sugar or bound within complex carbohydrates. In plants, galactose is often a component of larger molecules called galactans, found in cell walls and seeds. These plant-based sources contribute much smaller amounts compared to dairy, but they are relevant for those on a strict galactose-restricted diet.
Many common fruits contain measurable amounts of free galactose, including dates, papaya, and watermelon, often exceeding 10 milligrams per 100 grams. Certain vegetables, such as tomatoes and bell peppers, also contain detectable amounts of the free sugar. The free galactose content in most fruits and vegetables typically remains below 40 milligrams per 100 grams, which is minimal compared to the amounts found in dairy products.
Legumes are another source of galactose because they contain complex sugars known as raffinose-family oligosaccharides. These compounds consist of a sucrose molecule linked to one or more galactose units. Legumes such as soybeans, lentils, and chickpeas contain these compounds, which release galactose when broken down in the digestive tract. The bound galactose in these foods must be considered when strictly managing total dietary intake.
Medical Reasons for Tracking Galactose Intake
The primary medical reason for strictly restricting galactose intake is Galactosemia, a rare, inherited metabolic disorder. This condition is caused by a deficiency in one of the enzymes necessary to metabolize galactose, most commonly Galactose-1-phosphate uridyltransferase (GALT). Without a functional GALT enzyme, the body cannot convert galactose into glucose for energy.
When galactose cannot be metabolized, it and its toxic byproducts, such as galactitol and galactose-1-phosphate, accumulate in the blood and tissues. This buildup can rapidly cause severe, life-threatening complications in newborns, including liver damage, kidney failure, and brain damage. If the condition is not diagnosed and treated shortly after birth, it can lead to developmental delays, speech difficulties, and premature ovarian insufficiency in females.
The treatment for Galactosemia is a lifelong, strict avoidance of all galactose-containing foods, including nearly all dairy products. Even the small amounts found in certain fruits, vegetables, and legumes can be problematic for individuals with the most severe form of the disorder. Therefore, a restricted diet must go beyond simply eliminating milk and cheese. Early diagnosis through newborn screening and immediate dietary restriction are necessary to prevent devastating long-term effects.