Fructans: Definition, Types, and Health Benefits

Fructans are a group of naturally occurring carbohydrates that have gained significant attention in nutrition science over recent years. These complex molecules, composed primarily of fructose units, serve as storage carbohydrates in many plants and offer numerous health benefits when included in our diet. From supporting gut health to potentially reducing the risk of certain diseases, fructans play a more important role in human nutrition than many people realize.

While you may not recognize the term "fructans," you've almost certainly consumed them regularly in foods like onions, garlic, wheat, and bananas. As interest in gut health and the microbiome continues to grow, understanding these functional carbohydrates becomes increasingly relevant for anyone looking to optimize their diet and overall wellbeing.

What Are Fructans?

Fructans are complex carbohydrates made up of chains of fructose molecules linked together, often with a glucose molecule at one end. Unlike simple sugars that are readily digested and absorbed in the small intestine, fructans resist digestion because humans lack the enzymes needed to break down the specific bonds between fructose molecules. This resistance to digestion is precisely what gives fructans many of their health benefits.

These carbohydrates serve as energy storage for many plants, particularly those that grow in cooler or seasonally dry climates. For the plants, fructans provide a stable form of energy storage that helps them survive challenging environmental conditions. For humans who consume these plants, fructans function as prebiotics—non-digestible food ingredients that beneficially affect the host by selectively stimulating the growth and activity of beneficial bacteria in the colon.

Chemical Structure

The chemical structure of fructans consists of fructose molecules joined by β(2→1) or β(2→6) glycosidic bonds. The specific arrangement of these bonds determines the type of fructan. Most fructans contain between 2 and 60 fructose units, though some can be much larger. The degree of polymerization (DP)—the number of fructose units in the chain—significantly influences how fructans function in both plants and the human body.

What makes fructans particularly interesting from a nutritional perspective is that their β-linkages cannot be hydrolyzed by human digestive enzymes. This structural feature allows fructans to pass through the upper gastrointestinal tract intact, reaching the colon where they serve as food for beneficial bacteria.

Natural Sources

Fructans occur naturally in a wide variety of plant foods. Some of the richest sources include chicory root, Jerusalem artichoke, garlic, onions, leeks, asparagus, and wheat. The concentration of fructans varies significantly among different plants and even within the same plant depending on growing conditions, harvest time, and storage methods.

Interestingly, many traditional diets around the world naturally incorporate fructan-rich foods. For example, the Mediterranean diet, often praised for its health benefits, includes generous amounts of garlic, onions, artichokes, and wheat—all significant sources of fructans. Similarly, traditional diets in parts of Asia and Latin America incorporate various fructan-containing plants that have been consumed for centuries.

Types of Fructans

Fructans come in several distinct types, each with unique structural characteristics and properties. Understanding these different types helps explain their varied effects on human health and their diverse applications in food science and nutrition.

Inulin

Inulin is perhaps the most well-known type of fructan. It consists of linear chains of fructose molecules linked by β(2→1) bonds, typically with a glucose molecule at one end. Inulin chains can vary considerably in length, with degrees of polymerization ranging from 2 to 60 or more. This variation in chain length affects how quickly and completely inulin is fermented in the colon.

Chicory root is the primary commercial source of inulin, containing up to 20% inulin by fresh weight. Jerusalem artichokes, also known as sunchokes, are another rich source. Inulin has gained popularity as a functional food ingredient due to its prebiotic properties and its ability to replace fat in food products while maintaining texture and mouthfeel. It's commonly added to yogurts, baked goods, and other processed foods to increase fiber content and improve gut health.

Oligofructose

Oligofructose, sometimes called fructooligosaccharides (FOS), consists of shorter fructan chains with a degree of polymerization typically between 2 and 10. These shorter chains make oligofructose slightly sweet (about 30-50% as sweet as table sugar) and more soluble than longer-chain inulin. Oligofructose can be naturally present in foods or produced commercially by partially hydrolyzing inulin or through enzymatic synthesis from sucrose.

Due to its shorter chain length, oligofructose is fermented more quickly in the colon than longer-chain inulin. This rapid fermentation can produce beneficial short-chain fatty acids more quickly, but it also means that oligofructose may be less effective at reaching the distal (far) portions of the colon compared to longer-chain fructans.

Levans

Levans are less common in the human diet but represent an interesting type of fructan with distinct properties. Unlike inulin, which has β(2→1) linkages, levans contain fructose units joined by β(2→6) bonds. These structural differences give levans unique properties, including higher water solubility and different fermentation patterns in the gut.

While not as prevalent in food sources as inulin and oligofructose, levans are found in some grasses and are also produced by certain bacteria. In the food industry, bacterial levans have potential applications as thickeners, stabilizers, and encapsulating agents due to their unique physical properties.

Health Benefits of Fructans

The resistance of fructans to digestion in the upper gastrointestinal tract is precisely what makes them beneficial for human health. As they pass undigested into the colon, they become available for fermentation by beneficial gut bacteria, triggering a cascade of positive health effects throughout the body.

Prebiotic Effects and Gut Health

The most well-established benefit of fructans is their prebiotic effect. By selectively nourishing beneficial bacteria in the gut—particularly Bifidobacteria and certain Lactobacillus species—fructans help maintain a healthy balance of gut microbiota. This selective stimulation of beneficial bacteria is often referred to as the "bifidogenic effect" and is considered a hallmark of prebiotic activity.

When gut bacteria ferment fructans, they produce short-chain fatty acids (SCFAs) such as acetate, propionate, and butyrate. These SCFAs serve as the primary energy source for colonocytes (cells lining the colon) and help maintain the integrity of the intestinal barrier. Butyrate, in particular, has been extensively studied for its anti-inflammatory properties and potential role in preventing colorectal cancer.

Regular consumption of fructans has been associated with improved bowel function, including increased stool frequency and improved stool consistency. This can be particularly beneficial for individuals suffering from constipation or irregular bowel movements.

Metabolic Health and Weight Management

Emerging research suggests that fructans may play a role in metabolic health and weight management. Several mechanisms appear to be involved, including increased satiety, reduced caloric intake, and improved glucose metabolism. When fructans are fermented in the colon, they stimulate the production of satiety hormones such as glucagon-like peptide-1 (GLP-1) and peptide YY (PYY), which may help reduce overall food intake.

Studies have shown that regular consumption of fructans may help improve insulin sensitivity and glucose metabolism. This effect appears to be mediated both by changes in gut hormone production and by beneficial shifts in the gut microbiota composition. Some research also suggests that fructans may help reduce triglyceride levels and improve lipid profiles, though more studies are needed to fully understand these effects.

Immune Function and Disease Prevention

The relationship between fructans, gut health, and immune function is an area of growing scientific interest. About 70% of the body's immune cells reside in the gut-associated lymphoid tissue (GALT), making the gut microbiota a key player in immune regulation. By promoting a healthy balance of gut bacteria, fructans may help support proper immune function and reduce inflammation.

Some studies suggest that regular consumption of fructans may help reduce the risk of certain diseases, including colorectal cancer, inflammatory bowel disease, and even certain infections. The mechanisms behind these protective effects likely involve multiple pathways, including the production of beneficial metabolites by gut bacteria, improved intestinal barrier function, and modulation of immune responses.

Fructans and FODMAPs: The Other Side of the Story

While fructans offer numerous health benefits for many people, they can cause digestive discomfort in certain individuals, particularly those with irritable bowel syndrome (IBS) or fructan sensitivity. This is why fructans are included in the group of fermentable carbohydrates known as FODMAPs (Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols).

Fructan Sensitivity and IBS

For individuals with IBS or fructan sensitivity, consuming fructan-rich foods can trigger symptoms such as bloating, gas, abdominal pain, and altered bowel habits. These symptoms occur because the rapid fermentation of fructans in the colon produces gases and draws water into the intestinal lumen, potentially triggering discomfort in sensitive individuals.

The low-FODMAP diet, which temporarily restricts fructans along with other fermentable carbohydrates, has shown effectiveness in managing IBS symptoms in many clinical studies. However, it's important to note that this diet is not meant to be followed long-term due to the potential negative impacts on gut microbiota diversity and the loss of the beneficial effects of fructans and other prebiotic fibers.

For those with fructan sensitivity, working with a healthcare provider or registered dietitian to determine personal tolerance levels is often more beneficial than complete avoidance. Many people find they can tolerate small amounts of fructans without symptoms, allowing them to still gain some of the health benefits these compounds offer.

Incorporating Fructans Into Your Diet

For most healthy individuals without specific sensitivities, incorporating fructan-rich foods into the diet can be a simple and delicious way to support gut health and overall wellbeing. Rather than focusing on supplements, emphasizing whole food sources of fructans provides additional nutrients and phytochemicals that work synergistically to promote health.

Some easy ways to increase dietary fructans include adding garlic and onions to savory dishes, incorporating Jerusalem artichokes or chicory root into salads, enjoying ripe bananas as snacks, and consuming whole grain wheat products in moderation. For those interested in a more targeted approach, inulin and FOS supplements are widely available, though it's generally best to start with small amounts and gradually increase intake to minimize potential digestive discomfort.

As with most aspects of nutrition, balance and individual tolerance are key. By mindfully incorporating a variety of fructan-containing foods into a diverse diet rich in other plant foods, most people can enjoy the health benefits these fascinating carbohydrates have to offer while supporting a thriving gut microbiome and overall health.