What foods contain D amino acids and why is it important?

October 7, 2025 •

4 min read

While proteins in living organisms are primarily built from L-amino acids, recent research confirms that D-amino acids are widely present in both natural and processed foods. Dietary sources containing D amino acids include fermented products, heat-treated items, and certain animal and plant-based foods. This surprising discovery is opening new avenues for understanding nutrition and its complex effects on the human body.

Quick Summary

D-amino acids are present in many foods, including fermented dairy like cheese and yogurt, processed items like instant coffee, and natural sources such as shellfish and certain vegetables. Their presence is influenced by microbial activity and food processing methods like heating and alkali treatment. Research is ongoing to understand their full impact on human health and physiology.

Key Points

Fermented foods are a primary source: Cheeses, yogurts, and certain alcoholic beverages are rich in D-amino acids produced by microbial activity during fermentation.
Food processing increases D-AAs: High heat and alkali treatments can cause L-amino acids to racemize into D-amino acids, increasing their content in foods like powdered milk and instant coffee.
Shellfish contain natural D-AAs: Marine invertebrates such as oysters and mussels naturally contain relatively high concentrations of D-amino acids.
D-AAs have distinct biological roles: Some D-amino acids, like D-serine and D-aspartate, act as signaling molecules in the brain and endocrine system, playing a role beyond standard protein synthesis.
Health impacts are still under investigation: Research is ongoing to understand the nutritional and physiological effects of dietary D-amino acids, especially from processed sources, with some findings linked to age-related changes and immune response.
Gut microbiota are a major contributor: The bacteria in our gut can also produce D-amino acids, which can interact with intestinal processes and immune responses.

What Are D-Amino Acids?

Amino acids are the building blocks of proteins, and with the exception of glycine, they exist in two mirror-image forms called enantiomers: L-amino acids and D-amino acids. For decades, it was assumed that only L-amino acids were biologically relevant for humans. However, sophisticated analytical techniques have revealed that D-amino acids are a common and natural part of our diet, with their presence stemming from several sources and processes.

Dietary Sources of D-Amino Acids

Dietary D-amino acids can originate from microorganisms, such as those used in fermentation, or through chemical changes like racemization during food processing. Their concentration and types vary significantly depending on the food's origin and preparation method.

Fermented and Aged Foods

Fermentation is one of the richest sources of naturally occurring D-amino acids due to the activity of bacteria and yeasts.

Cheese and Yogurt: Aged cheeses like Parmigiano Reggiano contain high concentrations of D-Asp, D-Glu, D-Pro, and D-Ala, with some varieties having up to 5 g/kg. Yogurt and kefir also contain significant levels, especially D-Ala, D-Asp, and D-Glu.
Fermented Beverages: Wine, beer, and sake contain D-amino acids, and the levels can vary depending on the microbial activity during fermentation.
Other Fermented Products: This includes soy sauce, vinegar (especially lactic fermented tomato vinegar), and fermented black beans, which all show increased levels of D-amino acids produced by microbial processes.

Processed Foods

High-temperature and alkali treatments used in food processing can cause the racemization of L-amino acids into their D-form, increasing their concentration.

Dairy: Infant formulas and milk powder show higher D-amino acid levels compared to raw milk due to processing.
Roasted Products: Instant coffee contains significant amounts of D-Asp, D-Glu, and D-Pro due to roasting. Roasted toast and meats also show increased levels.
Alkali-Treated Foods: Certain soy products treated with alkali have elevated D-amino acid content.

Natural and Animal-Based Foods

Some foods contain D-amino acids naturally, often in low quantities, and marine invertebrates are a notable exception.

Shellfish: Marine invertebrates such as oysters, mussels, and scallops have relatively high concentrations of D-amino acids, which are incorporated from their aquatic food chain.
Meats: Raw and cooked meats contain D-amino acids, with the levels increasing with processing. Parma ham, for example, develops higher concentrations during aging.
Milk: While levels are low in raw milk, small amounts of D-Ala, D-Asp, and D-Glu are still present.

Plant-Based Foods

Some plants and fungi contain small amounts of D-amino acids.

Fruits and Vegetables: Items like carrots, celery, beets, apples, and tomatoes contain various D-amino acids in low amounts.
Spirulina: This algae contains D-amino acids and is often consumed as a supplement.

Comparison: L-Amino Acids vs. D-Amino Acids in Food


Characteristic	L-Amino Acids	D-Amino Acids
Primary Source in Diet	Constituent of proteins in most living organisms	Microbial activity (fermentation), spontaneous racemization (aging), or industrial processing
Biological Role	Building blocks for proteins, enzymes, and hormones; standard for most biological functions	Act as neuromodulators (D-serine, D-aspartate), components of bacterial cell walls, and may influence host immune response
Dietary Absorption	Efficiently absorbed through specific active transport systems	Generally absorbed slower than their L-forms; transport mechanisms are less understood
Metabolism	Well-established metabolic pathways in humans	Metabolized by enzymes like D-amino acid oxidase (DAAO), mainly in the liver and kidneys
Taste	Often associated with bitter taste, but depends on the specific amino acid	Often have a sweeter taste than their L-enantiomers; D-Ala, D-Phe, and D-Trp are noted for sweetness
Typical Dietary Intake	Constitutes the vast majority of amino acids consumed in an average diet	Varies depending on food choices (more in Western diets rich in processed foods), potentially 50–100 mg per day

Health and Nutritional Implications

The presence of D-amino acids in our diet is a relatively new and evolving area of research. Their exact nutritional and physiological impact is not yet fully understood, and high levels in processed foods have raised some health concerns.

Nutritional Value

In some cases, such as with certain essential amino acids, the D-form may be poorly utilized by the human body compared to the L-form. High levels of D-amino acids resulting from harsh processing can reduce protein digestibility. The Food and Nutrition Board of the National Academy of Sciences has expressed caution regarding supplemental D-amino acids, noting a lack of evidence for nutritional benefits and potential toxicity risks.

Potential Biological Functions

Despite these concerns, D-amino acids are not always considered detrimental. Free D-serine and D-aspartate, for example, act as important signaling molecules in the brain and endocrine system, though their dietary contribution versus internal synthesis is complex. Research also indicates that some gut microbiota-derived D-amino acids can influence the immune system and gut health. The presence of D-amino acids in bacteria is an essential part of their cell wall, and dietary intake can influence host-microbe interactions.

Age-Related Changes

Research has linked the accumulation of D-aspartic acid in long-lived human proteins to age-related conditions such as cataracts and Alzheimer's disease. This accumulation is a non-enzymatic racemization that occurs over a lifespan. While this is a physiological process, it highlights the potential biological activity of D-amino acids that is still being investigated.

For more detailed information on D-amino acid functions and recent findings, the FEBS Journal offers a comprehensive review of their functional insights: d-amino acids: new functional insights.

Conclusion

D-amino acids are a natural part of our diet, arising from both microbial activity in fermented foods and chemical changes during processing. While they have distinct biological roles, including signaling in the nervous system and contributing to gut health, much is still unknown about their overall dietary impact on human physiology. As the consumption of processed and fermented foods continues, further research is needed to fully understand the long-term health implications of dietary D-amino acid intake. For now, understanding the sources allows for more informed dietary choices, particularly regarding highly processed items. Increased intake from processed foods is a trend that requires careful monitoring and further scientific investigation.

Frequently Asked Questions

The health effects of dietary D-amino acids are still being studied, and there is no simple answer. While high levels in processed foods can reduce protein digestibility, some naturally occurring D-amino acids have important signaling functions in the body. More research is needed to understand their overall impact.

D-amino acids enter food in two main ways: through microbial activity during fermentation, as seen in cheese and yogurt, or through food processing methods like heat treatment and alkali exposure, which cause L-amino acids to convert to the D-form.

L-amino acids and D-amino acids are mirror-image molecules called enantiomers. While proteins in living organisms are composed of L-amino acids, D-amino acids are found in some foods and play different roles, such as serving as signaling molecules or components of bacterial cell walls.

Most fermented foods contain D-amino acids, with lactic acid bacteria and yeast activity being primary drivers of their production. However, the specific types and amounts of D-amino acids can vary greatly depending on the fermentation process and ingredients used.

Yes, high heat and prolonged cooking times can cause a process called racemization, which converts L-amino acids to D-amino acids, increasing their levels in food. This is why processed and roasted products tend to have higher concentrations.

Some D-amino acids, particularly D-proline, have been used as an indicator for the age of wine. Increased levels of certain D-amino acids can also signal bacterial contamination or excessive thermal/alkali treatment during processing.

Expert panels have expressed caution regarding supplemental D-amino acid intake, stating that there is no established nutritional rationale and that such practices may pose risks. It is recommended to obtain amino acids from dietary protein rather than supplements.