Why Sublingual Strips Are Replacing Injections: The Science of Smarter Peptide Delivery

Introduction: NAD+ and the Science of Cellular Energy

Nicotinamide adenine dinucleotide — better known as NAD+ — has become one of the most intensively studied coenzymes in modern biochemistry. Found in every living cell, NAD+ plays a central role in the energy metabolism reactions that power cellular function. Over the past two decades, research has illuminated its broader significance: as a signaling molecule, a regulator of gene expression, and a key mediator of cellular stress responses.

This guide covers what the science says about NAD+, why its decline with age has attracted research attention, what the supplementation literature shows, and why delivery format — including NAD+ sublingual strips — may matter for bioavailability.

What Is NAD+ and Why Does It Matter?

NAD+ is a dinucleotide found in all living cells, consisting of two nucleotides joined by a phosphate bond. It exists in two primary forms: NAD+ (oxidized) and NADH (reduced). The interconversion between these forms is central to cellular redox chemistry.

NAD+ serves several critical biochemical functions:

• Electron carrier in metabolism: In glycolysis and the citric acid cycle, NAD+ accepts electrons from metabolic substrates, becoming NADH. NADH then donates these electrons to the electron transport chain, driving ATP synthesis — the cell’s primary energy currency.
• Substrate for sirtuins: Sirtuins (SIRT1–SIRT7) are NAD+-dependent deacylases involved in regulating gene expression, DNA repair, and metabolic adaptation. Their activity is directly coupled to NAD+ availability.
• Substrate for PARPs: Poly(ADP-ribose) polymerases use NAD+ to detect and respond to DNA damage, consuming NAD+ in the process. During periods of high DNA damage stress, PARP activation can dramatically deplete cellular NAD+ pools.
• CD38 signaling: CD38 is a major NAD+-consuming enzyme involved in calcium signaling and immune function. CD38 expression increases with age and inflammation, contributing to age-related NAD+ decline.

NAD+ Decline with Age: The Research Landscape

One of the most consistent findings in NAD+ biology is that tissue NAD+ levels decline with age. Landmark studies — including work published in Cell by Yoshino et al. (2011) and subsequent research by Imai, Guarente, and colleagues — documented significant age-related NAD+ depletion in multiple tissues, including skeletal muscle, liver, and brain.

The mechanisms driving this decline are multifactorial:

• Increased PARP activation due to age-associated DNA damage accumulation
• Rising CD38 expression in aging tissues, particularly in macrophages
• Reduced expression of NAMPT (nicotinamide phosphoribosyltransferase), the rate-limiting enzyme in the NAD+ salvage pathway
• Declining efficiency of NAD+ biosynthesis from tryptophan via the kynurenine pathway

This age-related decline has prompted significant research interest in NAD+ precursor supplementation as a strategy to restore NAD+ levels.

NAD+ Precursor Supplementation Research

Several NAD+ precursors have been studied in human and animal research:

• Nicotinamide Riboside (NR): A form of vitamin B3 that enters the NAD+ salvage pathway via NRK enzymes. Multiple human clinical trials have demonstrated that oral NR supplementation raises blood NAD+ levels. A 2018 study in Nature Communications (Martens et al.) showed NR significantly elevated NAD+ metabolites in healthy older adults.
• Nicotinamide Mononucleotide (NMN): Another NAD+ precursor that has shown efficacy in raising NAD+ levels in human studies. A 2022 randomized controlled trial in NPJ Aging (Yi et al.) demonstrated that NMN supplementation increased blood NAD+ levels and showed effects on muscle insulin sensitivity in older women.
• Nicotinamide (NAM): The amide form of niacin, which enters the salvage pathway. High doses can inhibit sirtuins via product inhibition, a consideration in formulation design.
• Direct NAD+ administration: IV infusion of NAD+ has been studied in clinical settings, with IV delivery achieving rapid and complete systemic distribution. Oral bioavailability of intact NAD+ is limited by GI hydrolysis.

The Delivery Challenge: Why Oral NAD+ Bioavailability Is Limited

A key challenge with NAD+ supplementation is delivery. Intact NAD+ is poorly absorbed orally. The GI tract contains ectonucleotidases (including CD73 and CD38) that rapidly hydrolyze NAD+ to NMN and then to NR and nicotinamide before systemic absorption occurs. This means oral NAD+ effectively functions as a precursor delivery system rather than delivering intact NAD+.

This has driven interest in alternative delivery routes that bypass GI degradation:

• IV infusion: Most direct, achieves complete bioavailability, but requires clinical administration
• Intramuscular injection: Bypasses GI tract, achieves systemic distribution
• Sublingual delivery: Absorbs through mucosal tissue directly into systemic circulation, bypassing hepatic first-pass metabolism and GI enzymatic degradation

FitDaily’s NAD+ sublingual strip is formulated to deliver NAD+ via the sublingual route, designed to dissolve rapidly beneath the tongue and maximize mucosal absorption.

NAD+ and Cellular Wellness: What the Research Suggests

The breadth of NAD+’s cellular roles has generated a wide research literature exploring its potential connections to wellness markers. Key areas of ongoing investigation include:

• Mitochondrial function: NAD+ is essential for the electron transport chain. Research in aged mice has shown that restoring NAD+ levels can improve mitochondrial function markers, though translating these findings to humans requires further study.
• Sirtuin activation: As SIRT1 and other sirtuins require NAD+ as a substrate, raising NAD+ availability may enhance sirtuin-mediated regulation of metabolism, inflammation, and gene expression.
• DNA repair capacity: PARP-mediated DNA repair is NAD+-dependent. Research has explored whether maintaining NAD+ levels supports the capacity for efficient DNA damage response.
• Cognitive and neurological research: Preclinical studies have examined NAD+ supplementation in models of neurodegeneration, though human evidence remains early-stage.

Frequently Asked Questions

Q: What is the best form of NAD+ to supplement with?
A: Research has studied NR, NMN, and direct NAD+ administration. Each has distinct pharmacokinetics and entry points into the NAD+ biosynthesis network. Delivery route also significantly influences bioavailability — sublingual and IV routes bypass GI degradation that limits oral intact NAD+ absorption.

Q: Does NAD+ decline with age?
A: Multiple studies have documented significant age-related NAD+ decline in human and animal tissues. The mechanisms include increased NAD+-consuming enzyme activity (PARPs, CD38) and reduced biosynthesis pathway efficiency.

Q: What is a sublingual NAD+ strip?
A: A sublingual strip is an oral dissolving film designed to be placed beneath the tongue, where it dissolves and the active compounds are absorbed through the highly vascularized sublingual mucosa directly into systemic circulation. This bypasses GI degradation of NAD+.

Q: Is NAD+ supplementation safe?
A: NAD+ precursors like NR and NMN have shown favorable safety profiles in published clinical trials. Always consult a qualified healthcare provider before starting any supplementation protocol.

Explore FitDaily’s NAD+ sublingual strip and visit the FitDaily shop to see the full range of research-oriented sublingual formulations, including the Glow strip for cellular wellness support.