DL-Phenylalanine vs L-Phenylalanine: Understanding the Key Differences

Phenylalanine, an essential amino acid, plays a large role in various physiological processes and is required for the synthesis of proteins and other important molecules in our body. 

In general, phenylalanine exists in three forms: L-phenylalanine (LPA), D-phenylalanine (DPA), and a mixture of these two forms, referred to as DL-phenylalanine (DLPA) [1]. 

This article aims to explain the key differences between LPA and DLPA for physicians and clinic owners, emphasizing the potential benefits of each form, which include their use as appetite suppressants and the scope for integration into clinical practice.

L-Phenylalanine: The Natural Form

L-phenylalanine is the natural form of phenylalanine, which is predominantly found in protein-rich foods, such as meat, fish, poultry, dairy, eggs, and soy products. The human body can only synthesize LPA from dietary sources, and its demand is more significant than DPA [2]. Several studies have reported the role of LPA in the production of neurotransmitters, such as norepinephrine, which is crucial for maintaining mood, improving memory, and helping signals travel between the brain and the body’s nerve cells [3].

LPA has shown potential as an appetite suppressant due to its involvement in the synthesis of cholecystokinin (CCK), a neuropeptide. CCK is released in the gastrointestinal tract in response to nutrients and functions as a satiety signal. 

Studies have demonstrated that enhanced CCK production instigated by LPA supplementation can suppress appetite leading to reduced food intake and potential weight loss [4]. This discovery opens up possibilities for incorporating LPA supplements and injectables into weight management programs in clinical practice.

DL-Phenylalanine: Combining the Advantages of L and D Forms

DL-phenylalanine (DLPA) is a mixture that contains equal parts of L and D phenylalanine. While LPA plays a role in neurotransmitter synthesis, DPA has distinct properties that make the combined form, DLPA, a unique therapeutic option [5].

One of the key advantages of DPA is that it inhibits the enzyme enkephalinase, which is responsible for breaking down endorphins and enkephalins – the body’s natural pain-relieving substances [6]. As a result, DPA administration has been found to increase the levels of these substances in the body, which offers an analgesic effect by adjusting the way the body perceives it [7].

Due to the combined properties of its LPA and DPA components, DLPA supplementation has been studied for its potential in managing various pain-related conditions, such as fibromyalgia, arthritis, and chronic pain. It has also been found to be beneficial in supporting mood, cognitive functions, and attentiveness, as LPA’s involvement in neurotransmitter synthesis is maintained in the mixture [8].

Applying Phenylalanine in Clinical Practice

For physicians and clinic owners, understanding the potential benefits and uses of LPA and DLPA can help provide personalized treatments for patients. Both LPA supplements and injectables can be considered for appetite suppression in weight management programs, while DLPA could be integrated into the management of pain-related conditions or administered to improve mood and cognitive functions.

To ensure the safety and efficacy of phenylalanine supplementation and injection, it’s important to follow dosage guidelines and monitor patients for possible side effects, such as hypertension, anxiety, restlessness, and gastrointestinal discomfort. Note that phenylalanine use is contraindicated in those with phenylketonuria (PKU), a congenital disorder where the body cannot metabolize phenylalanine [9].


L-phenylalanine and DL-phenylalanine have distinct advantages that can be harnessed to offer personalized treatment approaches for patients in clinical practice. While LPA’s role as an appetite suppressant may be beneficial in weight management, DLPA carries both analgesic and mood-enhancing properties. By understanding the key differences and potential applications, physicians and clinic owners can make informed decisions on integrating phenylalanine supplements and injectables in their practice.

Taylor Froiland is the president of Legere Pharmaceuticals in Scottsdale, Arizona and serves on the Board of Directors for RK Logistics Group. He also owns and operates Medmetrics Compounding Pharmacy in Chandler, Arizona, specializing in various pharmaceutical services. Taylor holds a PharmD and has expertise in compounding, medicinal chemistry, and quality control.
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