The low‑FODMAP approach is built on the premise that certain short‑chain carbohydrates are poorly absorbed in the small intestine and become rapidly fermented by colonic bacteria, producing gas and drawing water into the lumen. When these compounds accumulate, they can trigger the hallmark symptoms of functional bowel disorders—bloating, abdominal pain, diarrhoea, and constipation. Understanding exactly which molecules belong to the FODMAP family, how they differ chemically, and why they behave the way they do is essential for anyone who wants to apply the diet intelligently, whether for personal health or professional guidance.
Below is a comprehensive breakdown of the five main FODMAP categories. Each section outlines the chemical nature of the group, typical dietary sources, the physiological reason for its low‑absorbability, and the practical implications for a low‑FODMAP regimen. The focus is on evergreen information that remains relevant regardless of the latest research nuances, while deliberately avoiding deep dives that belong to the neighboring articles on individual sub‑types.
Fructans (a type of oligosaccharide)
Chemical profile
Fructans are polymers of fructose molecules linked primarily by β‑(2→1) glycosidic bonds, sometimes terminated with a glucose unit. The degree of polymerisation (DP) can range from 2 to 60 units, with higher DP compounds being progressively more resistant to enzymatic breakdown in the human small intestine.
Why they are poorly absorbed
Humans lack the brush‑border enzyme *fructanase* (also called inulinase) needed to cleave the β‑linkages. Consequently, fructans travel intact to the colon where resident microbiota possess the requisite enzymes. The fermentation process releases short‑chain fatty acids (SCFAs) and gases (hydrogen, methane, carbon dioxide), while the osmotic activity of the intact polymer draws water into the lumen.
Common food sources
- Wheat‑based products (bread, pasta, cereals)
- Rye and barley
- Certain vegetables: onions, garlic, leeks, and shallots
- Some fruits: watermelon, cantaloupe (in modest amounts)
- Inulin‑type prebiotic supplements
Practical dietary tip
Because fructan content varies with plant part and processing, a “low‑fructan” version of a food can often be identified by choosing refined wheat products (where the bran, rich in fructans, is removed) or by opting for gluten‑free grains such as rice, quinoa, and oats (provided they are certified low‑FODMAP). Cooking does not significantly degrade fructans, so preparation methods alone are insufficient to reduce their load.
Galactans (another oligosaccharide)
Chemical profile
Galactans are polymers of the monosaccharide galactose, most commonly encountered as *galactooligosaccharides* (GOS). The typical structure consists of a terminal glucose linked to one or more galactose units via β‑(1→4) or β‑(1→6) bonds. Like fructans, the DP influences fermentability; shorter chains are more readily fermented.
Why they are poorly absorbed
The human small intestine lacks the enzyme *α‑galactosidase* required to hydrose the β‑galactosidic linkages. As a result, galactans reach the colon largely intact, where they become substrates for bifidobacteria and other saccharolytic microbes. Their fermentation yields gases and SCFAs, and the osmotic effect can exacerbate luminal distension.
Common food sources
- Legumes: beans (especially kidney, black, and pinto), lentils, chickpeas, and soybeans
- Certain nuts: cashews and pistachios (in moderate amounts)
- Some seeds: pumpkin seeds (small quantities)
Practical dietary tip
Soaking, sprouting, or fermenting legumes can partially reduce galactan content by activating endogenous α‑galactosidase, but the reduction is variable. For a low‑galactan approach, many individuals find it easier to replace high‑galactan legumes with canned lentils (which are often lower in GOS due to processing) or with low‑FODMAP protein sources such as firm tofu, tempeh, and eggs.
Lactose (the disaccharide)
Chemical profile
Lactose is a disaccharide composed of one glucose and one galactose unit linked by a β‑(1→4) glycosidic bond. It is the primary carbohydrate in mammalian milk and dairy products.
Why it can be poorly absorbed
Lactase, the brush‑border enzyme that hydrolyses lactose into its constituent monosaccharides, is expressed at high levels in infancy but declines in many individuals after weaning—a condition known as lactase non‑persistence. When lactase activity is insufficient, lactose remains in the small intestine, where it exerts an osmotic effect and is subsequently fermented by colonic bacteria.
Common food sources
- Milk (cow, goat, sheep) and cream
- Soft cheeses (ricotta, cottage cheese)
- Yogurt (especially unflavoured, though the live cultures can aid digestion)
- Ice cream and custard
- Processed foods containing milk solids, whey, or casein
Practical dietary tip
Lactose content varies widely among dairy products. Hard, aged cheeses (e.g., cheddar, parmesan) and butter contain minimal lactose and are generally tolerated even by lactase‑deficient individuals. Lactose‑free dairy products are produced by adding lactase during processing, effectively hydrolysing the sugar into glucose and galactose, which are readily absorbed.
Fructose (the monosaccharide)
Chemical profile
Fructose is a six‑carbon keto‑hexose that can exist as a free monosaccharide or as part of the disaccharide sucrose (glucose + fructose). In the context of FODMAPs, the focus is on *excess fructose*—that is, fructose present in a food in greater amounts than its accompanying glucose.
Why it can be poorly absorbed
Fructose absorption occurs via the GLUT5 transporter on the apical membrane of enterocytes, with facilitation by GLUT2 on the basolateral side. The process is relatively low‑capacity and is enhanced when glucose is present (via the GLUT2‑mediated co‑transport). When fructose exceeds glucose, the transporter becomes saturated, leaving unabsorbed fructose to travel to the colon for fermentation.
Common food sources
- High‑fructose fruits: apples, pears, mangoes, and cherries (especially when consumed in large portions)
- Sweeteners: honey, agave syrup, high‑fructose corn syrup (HFCS)
- Dried fruits: raisins, dates, apricots (concentrated sources)
- Certain vegetables: sweet corn, sugar snap peas (moderate amounts)
Practical dietary tip
A simple rule of thumb for low‑fructose eating is to pair a fruit or sweetener containing fructose with a source of glucose (e.g., a small amount of orange juice or a slice of bread) to improve absorption. However, many low‑FODMAP guides recommend limiting high‑fructose fruits to a single serving per meal and favouring low‑fructose options such as bananas, blueberries, strawberries, and kiwi.
Polyols (sugar alcohols)
Chemical profile
Polyols are a class of sugar alcohols that include sorbitol, mannitol, xylitol, maltitol, and isomalt. Their structure consists of a polyhydric alcohol derived from a corresponding monosaccharide, retaining the carbon backbone but replacing the aldehyde/ketone group with a hydroxyl group.
Why they are poorly absorbed
Polyols are absorbed via passive diffusion and, to a lesser extent, via specific transporters (e.g., SGLT1 for sorbitol). Their molecular size and polarity limit passive uptake, resulting in a substantial fraction reaching the colon unchanged. There, they are fermented by bacteria, producing gases and SCFAs, while their osmotic activity can draw water into the lumen.
Common food sources
- Sugar‑free or “diet” products: chewing gum, mints, and confectionery
- Certain fruits and vegetables: stone fruits (peaches, plums), apples, cauliflower, and mushrooms (contain sorbitol or mannitol naturally)
- Processed foods: sugar‑free baked goods, low‑calorie desserts, and some protein bars
- Medicinal syrups and cough drops (often contain sorbitol)
Practical dietary tip
Because polyol content can be hidden in “sugar‑free” or “reduced‑calorie” items, reading ingredient lists is crucial. When polyols are unavoidable, limiting portion size is effective; for example, a single piece of sugar‑free gum may be tolerated, whereas a whole pack is likely to provoke symptoms. Some individuals find that certain polyols (e.g., xylitol) are better tolerated than others, reflecting personal variations in intestinal transport capacity.
Integrating the Five Categories into a Low‑FODMAP Plan
- Identify the dominant FODMAP type in each meal – When constructing a meal, consider which of the five groups is most prevalent. A breakfast of wheat toast with honey and orange juice contains fructans (wheat), excess fructose (honey), and possibly polyols (if a sugar‑free spread is used). By swapping one component (e.g., using gluten‑free toast and a low‑fructose jam), the overall FODMAP load can be dramatically reduced.
- Mind the cumulative effect – Even if each individual food item stays within a “low” threshold, the total load across a meal can exceed tolerance. Keeping a simple log of servings per category helps prevent inadvertent over‑consumption.
- Use portion control as a safety net – Many foods contain a mix of FODMAPs (e.g., a serving of lentils provides both galactans and polyols). Reducing the portion size can bring the total amount of each category below the individual’s symptom‑triggering level.
- Re‑introduce systematically – After an initial elimination phase, re‑introduce each category one at a time, starting with the smallest serving. This process clarifies which specific groups are most problematic for the individual, allowing a personalized, less restrictive long‑term diet.
- Consider the role of the gut microbiota – While the article does not delve into the detailed interactions between FODMAPs and microbial composition (covered elsewhere), it is worth noting that regular, moderate exposure to certain FODMAPs can support beneficial bacteria. Therefore, a lifelong “zero‑FODMAP” diet is neither necessary nor advisable; the goal is to find a sustainable balance that minimizes symptoms while preserving microbial diversity.
By grasping the distinct chemical nature, absorption pathways, and typical food sources of fructans, galactans, lactose, fructose, and polyols, readers can make informed choices that align with the low‑FODMAP framework. This foundational knowledge empowers both patients and practitioners to tailor dietary strategies that are both effective and nutritionally sound, paving the way for better gut comfort and overall wellbeing.
