Is Mannitol a Stored Food? Understanding This Unique Carbohydrate

January 3, 2026 •

4 min read

While starch is the primary energy reserve for most plants, other organisms utilize different carbohydrates for storage. Mannitol, a type of sugar alcohol, serves as a crucial stored food and carbon compound in many fungi, brown algae, and some plants. Its biological function, however, is often more complex and multi-purpose than simple energy storage.

Quick Summary

Mannitol functions as a stored food and carbon reserve in specific organisms, most notably brown algae and fungi, for energy and stress resistance. Unlike starch, it has multiple physiological roles including osmoregulation and acting as an antioxidant.

Key Points

Specific Storage: Mannitol serves as a stored food primarily in brown algae (Phaeophyceae) and various fungi, unlike higher plants which rely mainly on starch.
Multi-functional Role: Beyond energy, mannitol acts as a compatible osmolyte, helping organisms regulate water balance and resist environmental stresses like high salinity and drought.
Antioxidant Properties: Mannitol can scavenge reactive oxygen species (ROS), providing protection against cellular damage from oxidative stress in both plants and fungi.
Fungal Metabolism: In fungi, stored mannitol is rapidly metabolized to provide a carbon source, particularly during important life cycle stages like spore germination.
Minor Role in Higher Plants: Although found in common vegetables like mushrooms and celery, mannitol is not a primary energy storage compound in these higher plants.
Commercial Applications: Due to its properties, mannitol is commercially produced as a sugar alcohol for use as a sweetener, bulking agent, and coating in foods and pharmaceuticals.

The Diverse Functions of Mannitol

Mannitol is a six-carbon sugar alcohol, or polyol, that is naturally produced by a wide range of organisms, including bacteria, fungi, algae, and plants. Its role varies significantly depending on the organism and environmental conditions. While it is a form of stored food, its utility extends far beyond a simple energy reserve. In many species, it functions as a compatible osmolyte, a substance that helps organisms tolerate stress from salinity or drought by regulating internal osmotic pressure. It also acts as a potent antioxidant, scavenging reactive oxygen species (ROS) that can damage cells.

The Role of Mannitol in Brown Algae

Brown algae, or Phaeophyceae, are a prime example of organisms that use mannitol as a major stored food. Genera such as Fucus store complex carbohydrates in the form of laminarin and mannitol, which are essential for fueling their metabolic processes. This is a key difference when comparing brown algae to green algae (Chlorophyceae), which predominantly store food as starch.

Photosynthetic Product: In brown algae, mannitol is a primary product of photosynthesis, synthesized in the leaves and translocated to other parts of the organism, where it can be stored.
Osmotic Adjustment: Beyond storage, brown algae in intertidal zones use mannitol for osmoregulation to cope with constant changes in salinity.

Mannitol as an Energy Reserve in Fungi

Fungi accumulate significant amounts of polyols, including mannitol, within their structures. In some fungal species, mannitol can constitute a large percentage of the dry weight of their hyphae and spores.

Spore Germination: For organisms like Aspergillus niger, mannitol stored in conidiospores is rapidly metabolized during germination, indicating its role as a key carbon source for this developmental stage.
Stress Resistance: Fungi also accumulate mannitol to enhance survival against environmental stresses such as heat, dryness, and oxidative damage.

Beyond Energy Storage: Mannitol's Multifunctional Roles

Mannitol's functions in nature are numerous and go beyond being a simple sugar reserve. These diverse capabilities make it a fascinating molecule in biological systems.

Osmoprotection: In higher plants like celery, mannitol is synthesized to act as an osmoprotectant, helping the plant tolerate conditions of high salinity or drought. It accumulates in response to water stress to help adjust the cellular osmotic balance.
Antioxidant: Research indicates that mannitol is an effective scavenger of harmful reactive oxygen species (ROS), protecting cells from oxidative stress. This role is crucial during periods of environmental stress or in host-pathogen interactions.
Pathogenicity Factor: Some plant pathogenic fungi secrete mannitol into host plant tissue. This allows the pathogen to quench the host's ROS-mediated defense responses, a key strategy for infection.
Industrial Applications: In its purified form, mannitol is used commercially as a food additive (E421) and a sugar substitute. Its low hygroscopicity makes it ideal for coatings, while its mild sweetness and cooling effect are valuable in confections and chewing gums.

Comparison: Mannitol vs. Starch as Stored Food


Characteristic	Mannitol	Starch
Organisms	Brown algae, fungi, and some plants (often secondary)	Most higher plants, green algae
Chemical Nature	Sugar alcohol (polyol) - C6H14O6	Polysaccharide (chain of glucose units)
Primary Function	Carbon storage, osmoregulation, antioxidant	Primary long-term energy storage
Stress Tolerance	Plays a key role in drought, salt, and oxidative stress tolerance	Generally does not serve a direct stress-tolerance role
Osmotic Role	Functions as a compatible solute to regulate cellular osmotic pressure	Does not significantly affect osmotic pressure due to high molecular weight

Mannitol in Higher Plants: A Minor Player

While mannitol is a significant storage compound in brown algae and fungi, its role in most higher plants is more limited. It is present in small quantities in various fruits and vegetables, including celery, onions, olives, and mushrooms. In these cases, it is typically a minor component and not the primary energy reserve, which remains starch. In fact, it is often noted in relation to the low FODMAP diet, where its malabsorption can cause gastrointestinal discomfort.

The Mannitol Cycle

In fungi, the metabolism of mannitol follows a cyclical pathway known as the mannitol cycle. Fructose-6-phosphate is converted to mannitol-1-phosphate, then dephosphorylated to mannitol. This mannitol is later oxidized back to fructose by the enzyme mannitol dehydrogenase (MTD), which is then re-phosphorylated back to fructose-6-phosphate to re-enter the pathway. This cycle not only stores carbon but also plays a role in regenerating reducing power (NADPH) and controlling cytoplasmic pH. Research on this topic has been ongoing for decades, highlighting the complexity of this metabolic process.(https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2015.01019/full)

Conclusion

So, is mannitol a stored food? The answer is a definitive yes, but with important qualifications. It is a critical energy reserve for specific organisms like brown algae and fungi, fulfilling a storage role similar to starch in plants. However, unlike starch, mannitol is a multi-functional molecule that also provides vital physiological benefits such as stress protection and osmoregulation. Its presence in higher plants is generally minor, where it does not serve as the primary stored food. This diversity in function underscores its importance across various biological kingdoms, making it more than just a simple source of energy.

Frequently Asked Questions

Mannitol is a type of sugar alcohol, or polyol, that occurs naturally in various plants, algae, and fungi. It is used as a sweetener and medication, but in biological terms, it serves multiple purposes beyond just providing energy.

Mannitol is a significant stored food in brown algae, such as Fucus, and in many fungal species. For brown algae, it is a primary product of photosynthesis, while in fungi, it is stored in spores and hyphae.

While starch is a large polysaccharide primarily used for long-term energy storage in most plants, mannitol is a smaller sugar alcohol with multiple functions. It not only stores carbon but also helps with osmoregulation and acts as an antioxidant, especially important in organisms subjected to environmental stresses.

Yes, mannitol functions as a compatible osmolyte, helping organisms resist osmotic stress caused by high salinity or drought. Its antioxidant properties also protect cells from damage caused by stress-induced reactive oxygen species.

Yes, mannitol can be found in small amounts in many fruits and vegetables, including cauliflower, celery, mushrooms, and pumpkins. It is not typically the primary storage carbohydrate in these higher plants.

In fungi, mannitol can be stored intracellularly and is metabolized rapidly during specific life stages, such as spore germination, to serve as a readily available source of carbon and energy.

The mannitol cycle is a metabolic pathway in fungi where fructose-6-phosphate is converted to mannitol-1-phosphate, then to mannitol. This mannitol is later recycled back into fructose, allowing for carbon storage, stress resistance, and regeneration of reducing power.