NAD+

Every living cell contains NAD+, a coenzyme sitting at the middle of energy metabolism. It ferries electrons through the mitochondrial reactions that turn food into ATP, and it feeds the enzymes that mend DNA, control which genes are expressed, and handle cellular stress. Run short of it and none of that happens efficiently.

Levels drop considerably as people age — by middle age, tissue concentrations may sit around half of youthful ones. That decline is why the molecule attracts research attention as a possible driver of age-related metabolic trouble. Supplementing NAD+ itself orally does not work well, so most protocols use precursors — nicotinamide riboside or nicotinamide mononucleotide — or put NAD+ straight in by IV or injection.

People come to it for energy, focus, recovery and longevity generally, and the expectations deserve setting honestly. This is not a weight-loss drug. The human trial evidence is early. And the striking anti-aging results from animal models have not reproduced in large human trials. The biology is genuinely interesting; the clinical proof in healthy adults is not there yet, and those are different claims.

Two enzyme families consume it. Sirtuins — deacylases involved in silencing genes, repairing DNA and regulating metabolism — cannot work without it, so their activity falls as it falls. PARPs also consume it while repairing single-strand DNA breaks, which sets up an unpleasant loop: heavy DNA damage drains NAD+ faster, and the depletion then degrades the repair capacity that was drawing on it [1][2].

The oral precursors enter cells through their own transporter routes and get converted inside. Human studies confirm both lift levels in blood and tissue. Delivering NAD+ intravenously puts it into circulation directly and skips the gut entirely — whether that translates into higher tissue levels, or better outcomes, than swallowing a precursor is a question no published human trial has answered.

The bioavailability groundwork came in 2016, when a single dose of nicotinamide riboside was shown to raise whole-blood NAD+ metabolites in healthy men [3]. That confirmed an oral precursor can genuinely lift NAD+ in people. It was a pharmacokinetic study rather than an outcomes trial, and it validated the approach without saying anything about benefit.

Then the clinical endpoints arrived, and they were sobering. A randomized placebo-controlled crossover trial in healthy middle-aged and older adults ran six weeks: NAD+ metabolites rose reliably, and blood pressure, arterial stiffness and resting metabolic rate did not budge against placebo [4]. A 12-week randomized trial in men with obesity found the same shape — NAD+ up, insulin sensitivity and skeletal muscle mitochondrial function unchanged [7]. Two honest null results, in the populations most people asking about this belong to.

The more encouraging signals come from narrower groups. A 2019 study found supplementation enriching the NAD+ metabolome in aged human skeletal muscle alongside anti-inflammatory changes in gene expression, though functional performance did not improve significantly [5]. In postmenopausal women carrying prediabetes and extra weight, a 2021 trial found ten weeks of NMN at 250 mg daily improved insulin signaling in muscle — the most clinically specific human finding in this literature so far. Nobody's weight shifted, and the study was small [6].

Reviews synthesizing the animal and early human work land in the same place: boosting NAD+ extends healthspan across multiple animal models and blunts metabolic disease phenotypes there, and whether that carries to humans remains unresolved [1][2]. Grade B — the mechanism is plausible, the NAD+-raising effect in humans is consistent, and the downstream clinical benefit in healthy adults is not. Note also that IV NAD+ specifically has no published controlled human trial at all; its use rests on mechanism and clinical anecdote.

Trials of the oral precursors have used 250 to 1,000 mg a day of nicotinamide riboside and 250 to 500 mg a day of NMN, taken all at once or split, with food. Prescribers generally work within those ranges. For compounded injectable or IV NAD+, protocols scatter — commonly 0.5 to 4 mg/kg a session infused slowly to limit reactions — and none of it has been validated in a large published trial.

Nothing establishes how long to continue, either: published human trials ran four to twelve weeks. Ongoing maintenance dosing is clinical judgment rather than evidence, and it should be described that way rather than presented as a protocol. Work it out with a licensed provider who can weigh your own health situation.

The oral precursors have been well tolerated in trials at up to 1,000 to 2,000 mg a day over weeks to months. Mild stomach effects — nausea, loose stools, some flushing — are the usual complaints and generally settle with a dose adjustment. No serious adverse event has been attributed to an oral precursor in any published human trial so far.

The injectable and IV forms are a different story on comfort: flushing, chest tightness, palpitations and nausea are frequently reported, especially when the infusion runs fast, and generally ease when it is slowed. Whether anything worse happens is unknown, because serious adverse events from IV NAD+ have never been systematically characterized in a large trial — unknown here means unstudied, not established as safe. It is not FDA-approved as a drug. Avoid it in pregnancy and breastfeeding, and in anyone with active cancer, since upregulating this pathway is theorized to support tumor metabolism in some cancers, a question still open. Cardiac conditions or significant metabolic disease warrant a physician's assessment first.