NAD+ is well recognized for its role in anti-aging, as it is crucial for maintaining metabolism and vitality. But when it comes to muscle anti-aging, is it dispensable or absolutely essential?
A previous counterintuitive study found that when NAD+ levels in the skeletal muscle of adult mice drop by 85%, their muscle strength and exercise capacity are no different from those of normal mice. This has raised many questions: Is the anti-aging myth of NAD+ really falling apart?
Although a compensatory mechanism was hypothesized to explain this phenomenon, a definitive answer was lacking. However, a recent study published in Cell Reports has solved this puzzle: NAD+ is not ineffective, but it has a golden window in early childhood. NAD+ deficiency during this period accelerates skeletal muscle aging in mice; in contrast, if NAD+ deficiency occurs only in adulthood, the muscles have already developed compensatory mechanisms to cope with it. Unfortunately, early-life NAD+ deficiency causes irreversible muscle damage, and supplementing NAD+ in adulthood can hardly reverse the aging process that began in childhood.
Early-Life NAD+ Deficiency Inflicts Lifelong Muscle Damage
The research team used gene editing to knockout the NADS gene throughout the body of mice. NADS is a key enzyme in the deamidation pathway for NAD+ synthesis and is highly expressed in organs such as the liver and kidneys. This genetic modification effectively cuts off a major NAD+ production pathway, creating a cohort of mice with early-life NAD+ deficiency.
There are two NAD+ synthesis pathways in mammals: the deamidation pathway, where the key enzyme NADS plays a dominant role in systemic NAD+ supply during early childhood; and the amidation pathway (salvage pathway), the main way cells recycle NAD+ using precursors such as nicotinamide (NAM), which may play a more important role in adulthood.
As expected, these gene-edited mice appeared normal at birth, but after weaning (20 days to 2 months after birth), their skeletal muscle NAD+ levels plummeted to well below those of normal mice. Meanwhile, the mice exhibited stunted growth, reduced muscle mass, and decreased exercise endurance.
Interestingly, when these mice reached middle age (12 months old), their skeletal muscle NAD+ levels spontaneously recovered to normal levels. Yet, the muscle dysfunction that began in early childhood did not improve with the rebound of NAD+ levels and persisted into old age (24 months old).
To determine whether this irreversible damage stems from systemic dysfunction or an intrinsic muscle defect, the research team specifically knocked out the NADS gene only in skeletal muscle. Surprisingly, the skeletal muscle NAD+ levels of these mice did not decrease, and their muscle function was identical to that of normal mice. This indicates that NADS in muscle itself is not essential for maintaining muscle NAD+ levels.
So where lies the real crux? Re-examining the mice with systemic NADS knockout, researchers found that the sharp drop in muscle NAD+ during early childhood was accompanied by a significant reduction in blood NAM levels—a core precursor for NAD+ synthesis.
This leads to the following inference: Muscle NAD+ in early childhood is mainly synthesized by organs with high NADS expression (e.g., the liver) and transported to muscle in the form of NAM. Systemic NADS knockout destroys this supply chain. In adulthood, however, muscle can synthesize NAD+ independently through other pathways (the amidation pathway), restoring its NAD+ levels. Nevertheless, the muscle developmental damage caused by the lack of raw materials in early childhood is irreversible.
NAD+ Deficiency "Locks" the Has2 Gene
How does early-life NAD+ deficiency leave a lifelong mark on muscle health? The answer lies in epigenetic modification—a process in which chemical markers (e.g., methyl groups) act like a "lock" on specific genes. The gene itself does not mutate, but its genetic information can no longer be read and executed by cells.
Early-life NAD+ deficiency places such a lock on the Has2 gene, which is responsible for producing hyaluronic acid (HA). Beyond its well-known role as a skin moisturizer, HA also activates the proliferation and repair of muscle stem cells, keeping muscles firm and strong. This lock permanently suppresses the Has2 gene through a three-step cascade reaction:
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NAD+ deficiency causes a sharp drop in α-ketoglutarate (AKG)NAD+ is a core coenzyme for cellular energy metabolism. Its deficiency impairs the tricarboxylic acid cycle, leading to a significant reduction in the metabolic intermediate AKG.
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The "unlocking enzyme" JMJD3 loses its activityAKG depletion inactivates the JMJD3 enzyme, which is dependent on AKG for its function. JMJD3’s primary role is to remove repressive epigenetic marks (histone H3K27me3) from genes, enabling their normal expression.
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The Has2 gene is "locked" and permanently silencedWith JMJD3 inactivated, repressive H3K27me3 marks accumulate at the promoter region of the Has2 gene, halting HA synthesis. Even when NAD+ and AKG levels recover later in life, this epigenetic lock cannot be removed.
This also explains why adult mice with 85% NAD+ depletion maintain normal muscle function: their Has2 genes were never locked in early childhood. Short-term NAD+ deficiency in adulthood is merely a temporary lack of raw materials, which the body can compensate for through other metabolic pathways.
Two Remedial Pathways to Bypass NAD+ Deficiency
While supplementing NAD+ in adulthood cannot reverse muscle damage caused by early-life NAD+ deficiency, there are effective alternative approaches—targeting the downstream consequences of NAD+ depletion rather than NAD+ itself.
Since the root cause is AKG depletion → JMJD3 inactivation → Has2 silencing, the research team first tested supplementing AKG directly to reverse this epigenetic lock. Mice with early-life NAD+ deficiency that received AKG supplementation showed a significant reduction in repressive H3K27me3 marks on the Has2 gene, restoring Has2 expression and HA production. These mice exhibited thicker muscle fibers and improved exercise capacity, nearly matching normal mice—effectively reversing muscle aging.
A second, more direct approach is to bypass the locked Has2 gene by directly delivering its end product, HA. Cell experiments confirmed that adding HA directly to NAD+-depleted cells yielded effects equivalent to AKG supplementation. This finding provides a basis for the future development of muscle repair therapies that directly deliver HA.
Can AKG/HA Supplementation Replicate the Remedial Effects in Humans?
AKG and HA show promising remedial potential for muscle damage, but their effects in humans cannot be directly replicated from mouse studies. Existing research confirms both substances have positive effects on human muscle health, yet they have clear application limitations and considerations:
- AKG: Long-term supplementation is essential for efficacy
AKG effectively inhibits muscle protein degradation and helps preserve muscle mass—especially important for the elderly, as it alleviates age-related muscle loss and has great potential as a nutritional supplement for individuals with sarcopenia.
However, efficacy depends on long-term adherence and appropriate dosages; occasional supplementation will yield no meaningful benefits. Studies have shown that a single 3000mg dose of an AKG complex (e.g., AAKG) does not improve muscle endurance or metabolic levels. Additionally, there are currently no dedicated studies on AKG supplementation for individuals with muscle damage caused by early-life nutritional deficiency, though it holds significant potential as a remedial strategy.
- HA: The right delivery method is critical
HA is a key signaling molecule for human muscle repair, as it activates muscle stem cells and promotes muscle regeneration. Currently, the only effective supplementation method is local injection (e.g., intra-articular injection), which delivers HA directly to damaged sites to reduce inflammation, lubricate tissues, and promote repair.
Oral HA, the more familiar form, has obvious limitations: it is degraded in the human body and cannot reach skeletal muscle at effective concentrations. Most existing research on oral HA focuses on its skin benefits. Like AKG, more targeted clinical trials are needed to validate its efficacy for muscle repair.
In summary, AKG and HA may be considered for maintaining muscle health or alleviating age-related muscle loss, but current scientific evidence cannot guarantee that they can reverse early developmental disadvantages caused by childhood NAD+ deficiency.
Early childhood is a golden critical period for life development. Any nutritional deficiency or environmental disturbance during this stage may cause irreversible damage. Protecting the body’s health potential from an early age is therefore essential for long-term muscle health and overall vitality.
References
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