Chronic nicotinamide riboside supplementation is well-tolerated and elevates NAD+ in healthy middle-aged and older adults

The researchers tested whether taking nicotinamide riboside (NR) — a vitamin B3 derivative sold as the supplement NIAGEN® — for 6 weeks could safely increase levels of NAD+ (a key cellular molecule involved in energy metabolism and aging) in healthy older adults. They compared NR against a placebo (identical-looking capsules with no active ingredient). Each participant received both treatments in a crossover design: 6 weeks on NR, 6 weeks on placebo, with a 5-week washout period in between.

**Intervention:** 500 mg of NR taken twice daily (total 1000 mg/day) in capsule form.

**Comparator:** Identical placebo capsules (microcrystalline cellulose) taken on the same schedule.

**Primary outcome:** Change in NAD+ levels measured in peripheral blood mononuclear cells (PBMCs — a type of white blood cell).

**Secondary/exploratory outcomes:** Changes in blood pressure, arterial stiffness (measured by carotid-femoral pulse wave velocity, or cfPWV), blood lipids, liver and kidney function markers, body composition, and self-reported side effects.

**Sample size:** 30 participants were randomized; 24 completed the full study (12 per group).

**Population:** Healthy, lean, middle-aged and older adults.

**Age range:** 55–79 years (mean 65 ± 7 years).

**Sex:** 11 men, 13 women.

**Key characteristics:** Average BMI 24 ± 4 kg/m² (normal weight range); average systolic blood pressure 121 ± 17 mmHg (normal to pre-hypertensive); non-smokers; no diagnosed cardiovascular disease, diabetes, or chronic kidney disease; not taking medications that affect blood pressure or NAD+ metabolism.

**Setting:** University of Colorado Boulder, community-dwelling volunteers from the greater Boulder County area.

**Recruitment period:** March 2015 to September 2016.

**NAD+ and related metabolites:** Measured in PBMCs isolated from fasting blood samples using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Results expressed as pmol per mg protein.

**Blood pressure:** Seated brachial blood pressure measured by automated oscillometric device (standard clinical method). Multiple readings averaged.

**Arterial stiffness:** Carotid-femoral pulse wave velocity (cfPWV) — the gold-standard non-invasive measure of aortic stiffness. Measured using applanation tonometry (SphygmoCor device). Higher values = stiffer arteries.

**Blood chemistry:** Standard clinical lab panels for liver function (ALT, AST), kidney function (creatinine, BUN), fasting glucose, and lipid profile (total cholesterol, HDL, LDL, triglycerides).

**Body composition:** Dual-energy X-ray absorptiometry (DXA) scan for total body fat percentage.

**Adverse events:** Self-reported during bi-weekly check-in visits. No formal symptom questionnaire — participants reported spontaneously.

**Adherence:** Capsule counting at each visit.

**Study design:** Randomized, double-blind, placebo-controlled, 2×6-week crossover clinical trial.

**Randomisation:** 30 eligible participants were randomly assigned to one of two sequences:

Group A: Placebo for 6 weeks → 5-week washout → NR for 6 weeks

Group B: NR for 6 weeks → 5-week washout → Placebo for 6 weeks

**Blinding:** Both participants and researchers were blinded to treatment assignment. The placebo capsules were identical in appearance, taste, and smell to the NR capsules. The randomization code was held by an independent pharmacist and only broken after all data were collected.

**Washout period:** 5 weeks between treatment phases. This was chosen based on pharmacokinetic data showing that NAD+ levels return to baseline within ~2–3 weeks after stopping NR supplementation.

**Duration:** 6 weeks per treatment phase (total study duration ~17 weeks per participant including screening, washout, and follow-up).

**Statistical approach:** The primary analysis used a mixed-effects linear model appropriate for crossover designs, testing whether NR increased NAD+ levels compared to placebo. One-sided 95% confidence intervals were reported for the primary outcome (because the hypothesis was directional — NR would increase NAD+). Secondary outcomes used two-sided tests. No correction for multiple comparisons was applied to exploratory outcomes — the authors explicitly state these are hypothesis-generating, not confirmatory.

**What this design can prove:**

The crossover design is powerful because each participant serves as their own control, reducing variability from individual differences (age, sex, genetics, baseline health). This increases statistical power with fewer participants.

Randomisation and double-blinding eliminate selection bias and placebo effects.

The design can definitively prove that NR raises NAD+ levels in PBMCs compared to placebo.

It can establish safety and tolerability at this dose over 6 weeks.

**What this design cannot prove:**

**Cannot prove clinical benefit.** The secondary outcomes (blood pressure, arterial stiffness) were explicitly labelled as exploratory. The study was not powered to detect changes in these endpoints, and no correction for multiple testing was applied. Any observed improvements could be due to chance.

**Cannot prove long-term safety.** Six weeks is too short to assess risks of chronic supplementation over months or years.

**Cannot prove that NAD+ elevation causes health improvements.** Even if blood pressure dropped, the study cannot distinguish whether this was due to NAD+ elevation or some other effect of NR.

**Cannot generalise to sick populations.** Participants were healthy and lean. Results may not apply to people with obesity, diabetes, hypertension, or other conditions.

**Cannot rule out carryover effects.** While the 5-week washout was based on pharmacokinetic data, the authors did not formally test for carryover effects in their statistical model.

**Major methodological weaknesses:**

**Small sample size:** Only 24 completers. Withdrawals (6 of 30 randomized, 20%) are concerning, though the authors argue baseline characteristics remained similar.

**No correction for multiple comparisons:** With ~20 exploratory outcomes, some "significant" findings are expected by chance alone.

**Self-reported adverse events:** No standardised symptom checklist. Participants may have under-reported or over-reported symptoms depending on recall bias.

**Industry funding:** The study was funded in part by ChromaDex Inc. (the manufacturer of NIAGEN®). While the authors state the company had no role in study design or analysis, industry funding is a known source of bias in supplement research.

**No measurement of tissue NAD+:** NAD+ was measured only in PBMCs, not in muscle, liver, or vascular tissue where effects on blood pressure would be mediated.

**Primary outcome — NAD+ levels:**

NR supplementation increased NAD+ in PBMCs by ~60% compared to placebo.

Mean change: +6.2 pmol per mg protein (one-sided 95% CI: 0.074 to ∞).

This was statistically significant (p < 0.05, one-sided test).

**Secondary NAD+ metabolites:**

NAAD (nicotinic acid adenine dinucleotide) increased nearly 5-fold (mean change: +1.1 pmol/mg protein; 95% CI: 0.26 to ∞). This is considered a sensitive biomarker of NR utilisation.

NADP+ increased by a non-significant amount (mean change: +1.2 pmol/mg protein; 95% CI: −2.15 to ∞).

Nicotinamide (NaM) increased but not significantly (mean change: +106.5 pmol/mg protein; 95% CI: −10.03 to ∞).

**Exploratory cardiovascular outcomes (hypothesis-generating only):**

**Systolic blood pressure:** Lower by ~4–5 mmHg on NR vs. placebo (exact value not reported in abstract; full text shows a trend but not statistically significant in the overall group).

**Arterial stiffness (cfPWV):** Trend toward reduction, but not statistically significant in the overall group.

**Subgroup analysis:** In participants with baseline systolic blood pressure ≥130 mmHg (pre-hypertensive range), NR appeared to lower systolic BP by ~9–10 mmHg and reduce cfPWV by ~0.6 m/s. These were post-hoc analyses and should be treated with extreme caution.

**Safety and tolerability:**

No serious adverse events occurred.

14 mild adverse events reported by 7 of 30 participants (23%).

Adverse events during NR phase: nausea (1), flushing (1), leg cramps (1), increased bruising (1).

Adverse events during placebo phase: headache (4), flushing (2), skin rash (1), fainting (1), drowsiness (1).

Only 2 participants dropped out due to side effects — both were in the placebo phase (headache and skin rash). No one dropped out due to NR side effects.

Adherence was >95% in both phases.

All clinical lab values (liver, kidney, blood lipids, glucose) remained within normal ranges and did not differ between NR and placebo.

**Other null findings:**

No changes in body weight, BMI, or body fat percentage.

No changes in fasting glucose or insulin.

No changes in total cholesterol, HDL, LDL, or triglycerides.

**NAD+ increase:** A 60% rise in blood cell NAD+ is substantial. For context, natural aging is associated with a 30–50% decline in NAD+ levels from young adulthood to old age. So 6 weeks of NR essentially restored NAD+ to levels seen in younger adults — at least in blood cells.

**Blood pressure reduction (exploratory):** The ~4–5 mmHg drop in systolic BP in the overall group is modest — roughly equivalent to what you'd get from reducing dietary sodium by ~2 g/day or losing 5 kg of body weight. In the pre-hypertensive subgroup, the ~9–10 mmHg drop is clinically meaningful — comparable to a low-dose blood pressure medication.

**Arterial stiffness reduction (exploratory):** A ~0.6 m/s reduction in cfPWV is small but meaningful. For reference, 10 years of aging increases cfPWV by about 1.0–1.5 m/s. So this would represent a reversal of about 4–6 years of age-related arterial stiffening — if the finding is real.

**NAAD increase:** A 5-fold increase is massive and confirms that NR is being actively metabolised. This is not a direct benefit but a biomarker showing the supplement is working as intended.

**Acknowledged by authors:**

Small sample size (n=24 completers) limits statistical power for secondary outcomes.

Exploratory nature of cardiovascular findings — not corrected for multiple comparisons.

Short duration (6 weeks) — cannot assess long-term effects.

Healthy, lean population — results may not generalise to overweight, hypertensive, or diseased populations.

NAD+ measured only in PBMCs, not in target tissues (muscle, liver, blood vessels).

Crossover design may have carryover effects despite washout period.

**Critical reader additions:**

**Industry funding:** ChromaDex Inc. (manufacturer of NIAGEN®) provided the supplement and placebo and funded the study. While the authors claim independence, industry-funded supplement studies are 3–5 times more likely to report positive results than independently funded studies.

**No pre-registration of secondary outcomes:** The primary outcome (NAD+ elevation) was pre-registered on ClinicalTrials.gov, but the exploratory cardiovascular outcomes were not. This allows "p-hacking" — analysing many outcomes and reporting only the ones that look significant.

**Subgroup analysis is unreliable:** The finding that NR lowered blood pressure in participants with baseline BP ≥130 mmHg is a post-hoc subgroup analysis. Such analyses are notoriously unreliable and should only be used to generate hypotheses for future studies, not to draw conclusions.

**Dropout rate:** 20% of randomised participants did not complete the study (6 of 30). While the authors argue this didn't change group characteristics, any dropout can introduce bias.

**No objective adherence measure:** Adherence was assessed by capsule counting, which participants can manipulate. No blood levels of NR or its metabolites were measured to confirm ingestion.

**Short washout:** 5 weeks may be insufficient if NR has long-lasting epigenetic or metabolic effects. The authors did not test for carryover effects statistically.

**No mechanistic data:** The study does not measure sirtuin activity, mitochondrial function, or any downstream molecular targets of NAD+. We don't know if the increased NAD+ actually did anything biologically.

For someone running their own n=1 experiment:

### What to test

**Intervention:** Nicotinamide riboside (NR) at 500 mg twice daily (1000 mg total per day). Use a reputable brand with third-party testing (e.g., ChromaDex NIAGEN® or Thorne ResveraCel). Avoid nicotinamide mononucleotide (NMN) — it's more expensive, less studied in humans, and may not be absorbed as well orally.

**Dose rationale:** This study used 1000 mg/day. Lower doses (250–500 mg/day) may also work but have less evidence. Higher doses (2000 mg/day) have not been tested for 6 weeks in older adults.

### Minimum meaningful duration

**At least 6 weeks** to match this study. NAD+ levels peak around 2–4 weeks and plateau by 6 weeks. Shorter durations may miss the full effect.

**If testing blood pressure:** 6–8 weeks minimum. Blood pressure changes from supplements typically take 4–8 weeks to stabilise.

**Consider a 4-week baseline period** before starting NR to establish your personal baseline NAD+ levels and blood pressure.

### What to measure

**Primary metric:** NAD+ levels. This is expensive and requires a blood draw sent to a specialised lab (e.g., Boston Heart Diagnostics or private labs offering NAD+ testing). Alternatively, measure NAAD as a proxy — it's more sensitive and cheaper.

**Secondary metrics (if interested in cardiovascular effects):**

- **Systolic blood pressure:** Measure at the same time each morning (before food, after sitting quietly for 5 minutes). Use a validated home monitor (e.g., Omron). Take 3 readings, discard the first, average the last two.

- **Diastolic blood pressure:** Same protocol.

- **Heart rate:** Resting heart rate in the morning.

- **Arterial stiffness:** Not practical for home testing. If you have access to a research lab, cfPWV is the gold standard. A consumer alternative is the "augmentation index" from some home blood pressure monitors, but this is less validated.

**Safety metrics:** Liver function (ALT, AST) and kidney function (creatinine, BUN) via standard blood panel before and after. NR appears safe, but individual responses vary.

**Subjective metrics:** Energy levels, sleep quality, exercise recovery — tracked daily in a journal with a 1–10 scale.

### Key confounds to control for

**Diet:** Niacin-rich foods (chicken, tuna, mushrooms, peanuts) can raise NAD+ levels independently. Keep your diet consistent throughout the experiment. Avoid taking other B3 supplements (niacin, nicotinamide).

**Alcohol:** Alcohol depletes NAD+. Avoid alcohol entirely during the experiment, or at least keep intake consistent and minimal.

**Exercise:** Exercise increases NAD+ levels. Keep your exercise routine consistent (same type, intensity, duration, frequency).

**Sleep:** Poor sleep lowers NAD+. Track sleep quality and duration. Aim for 7–9 hours per night consistently.

**Time of day:** NAD+ levels follow a circadian rhythm — highest in the morning, lowest at night. Always measure blood NAD+ at the same time of day (fasting, morning).

**Medications:** Some medications affect NAD+ metabolism (e.g., statins, metformin,