Supplements for Men's Energy: What the Evidence Actually Supports

Part 2 of 3 in The Energy Audit. Part 1 was the audit you do before buying anything. Part 3 is how to run the test on yourself.

There is one rule that sorts almost every energy supplement into a useful pile or a useless one.

A supplement can refill an empty tank. It cannot enlarge a full one. Correct a genuine gap and you get a real, sometimes dramatic improvement. Take the same compound when you are not short of it and you get, on average, nothing. This is not a caveat at the bottom of the article. It is the whole thing.

The reason most "energy" products can promise so much is that they blur those two situations together. The trial that made the ingredient look good was run in people who were deficient. The bottle is sold to everyone.

The clearest example of the rule: vitamin D, twice

In 2016, a double-blind randomised trial published in Medicine enrolled 120 people who had both fatigue and vitamin D deficiency, defined as blood 25(OH)D under 20 µg/L. A single 100,000 unit dose of vitamin D significantly improved fatigue against placebo. Real effect, real trial.

In 2019, a study in Scientific Reports asked a different question of 327,478 people in UK Biobank, using genetic variants that lower vitamin D as a natural experiment. If low vitamin D caused tiredness, the people genetically predisposed to lower levels should report more of it. They did not. The odds ratio was 1.05, with a confidence interval spanning no effect.

Neither study is wrong. They asked different questions. Correcting a deficiency helped. Being at the lower end of normal did not cause tiredness in the first place. Read those two results as one claim about "vitamin D and energy" and you will reach a conclusion neither study supports.

That is the pattern to hold on to for everything below.

Tier 1: worth taking, once a test says you are short

These are the ones with the largest possible upside, and the upside is conditional. Test, then take, then retest.

Vitamin D

Worth checking for most men in the UK, on numbers alone. National Diet and Nutrition Survey data for 2019 to 2023 put 18% of adults aged 19 to 64 below the deficiency threshold across the year, rising to 31% between January and March. Separately from any fatigue question, the NHS advises everyone to consider 10 micrograms a day in autumn and winter, which is a bone and muscle recommendation rather than an energy one.

• If you are tired: ask for a 25(OH)D test rather than assuming. The Mendelian randomisation result above is the reason.
• If low: correction doses are a conversation with your GP, since they depend on how low. The 10 microgram maintenance dose is not a correction dose.
• Timeline: weeks, not days. Retest rather than guessing.

Vitamin B12

B12 is where the correction rule is at its starkest. A 2021 systematic review and meta-analysis in Nutrients looked at B12 supplementation for cognition, depressive symptoms and fatigue, and found no evidence of benefit in people without overt deficiency. In people who are deficient, correcting it can be transformative.

• Higher risk: vegan or near-vegan diets, long-term metformin, long-term proton pump inhibitors for reflux, previous gastric surgery, and age over 60.
• Otherwise: a B12 supplement is not doing anything for your energy, and neither is the high-dose B complex sold beside it.

Iron, and the reason this section is different for men

Read this one carefully, because it is the place where an honest article and a careless one diverge most.

The two trials that made iron famous for unexplained fatigue were run in women. Verdon and colleagues randomised 144 non-anaemic women with unexplained fatigue to oral iron or placebo in the BMJ in 2003, and found fatigue fell 29% against 13% on placebo, with the benefit confined to women whose ferritin was at or below 50 µg/L. Vaucher and colleagues replicated the effect in menstruating women with low ferritin in CMAJ in 2012.

Those are good trials. They are also trials in menstruating women, who have an obvious and benign reason to be losing iron. That reason does not exist in men.

In an adult man, unexplained iron deficiency is a finding to investigate rather than a gap to fill. British Society of Gastroenterology guidance is that men and postmenopausal women with newly diagnosed iron deficiency anaemia should generally have gastroscopy and colonoscopy as first-line investigations, because roughly a third of them turn out to have an underlying problem, most often in the gut, and some of those are cancers.

• So: if you are a man, do not self-prescribe iron. Take the ferritin result to a GP and let them decide what it means. Taking iron off your own bat can normalise the number and hide the reason it was low.
• And: iron you do not need is not neutral. There is no efficient route for the body to excrete a surplus.

Tier 2: does something without a gap, in a specific situation

These are not correcting a shortage. They are buying a defined effect, usually a modest one, in a defined situation. Take them for the situation, not as a daily habit that never gets reviewed.

Creatine monohydrate, 3 to 5 g a day

Creatine is not a stimulant and it will not make a well-rested man feel more energetic. What it appears to do is reduce how much you degrade under load. A 2024 study in Scientific Reports gave a single high dose during overnight sleep deprivation and found it blunted the decline in processing speed and reaction time, alongside measurable changes in brain energy compounds. A 2025 crossover trial in Nutrients loaded 14 physically active men with 20 g a day for a week and found better performance on tasks demanding sustained cognitive effort, plus lower perceived mental fatigue.

• Honest framing: less degradation under load, not more energy at baseline. Both trials were small, and the sleep-deprivation one is a laboratory situation rather than your Tuesday.
• Dose: 3 to 5 g a day of plain monohydrate. Loading is optional and mostly about reaching saturation faster.
• More on it: creatine beyond the gym.

Caffeine with L-theanine

The combination has better evidence than caffeine alone for attention without the edge. A 2025 crossover trial in the British Journal of Nutrition gave 37 overnight sleep-deprived adults 160 mg of caffeine with 200 mg of L-theanine, an amino acid found in tea, and measured better hit rates and faster reaction times on an attention task than placebo, backed by changes in brain electrical activity.

The counterweight matters more than the trial. Peter Rogers argued in the Journal of Caffeine Research that for habitual coffee drinkers, most of the lift is reversal of withdrawal rather than a net gain. Regular consumers who take caffeine feel sharper than they did without it, but do not end up sharper than people who never drink it.

• What that means practically: caffeine is a scheduling tool, not a supply of energy. It moves alertness to where you want it and borrows from later.
• Watch the total: stay under about 400 mg a day, and keep the last dose well clear of bedtime, or you are funding tomorrow's tiredness. Part 1 covered why that matters.
• More on it: L-theanine and caffeine together.

Rhodiola rosea, standardised extract

An adaptogen with a small but real set of trials specifically in stress-related fatigue rather than in general tiredness.

Olsson and colleagues randomised 60 people diagnosed with fatigue syndrome to 576 mg a day of the standardised SHR-5 extract or placebo in 2009, and reported improvements in fatigue, attention and the cortisol response to waking. An earlier crossover study in Phytomedicine tested a low repeated dose in physicians working night duty and found better mental performance.

• Read it as: moderate evidence from small trials, mostly for the fatigue that comes with sustained stress. Not a substitute for sleep.
• More on it: adaptogens without the hype.

Coenzyme Q10, and the one case where it is close to essential

A 2022 systematic review and meta-analysis in Nutrients pooled 13 randomised trials and 1,126 participants and found CoQ10 reduced fatigue scores against placebo, with a small to moderate effect size, and with bigger effects at higher doses and longer durations. Side effects were close to non-existent.

The counterweight: a 2025 meta-analysis looking at depressive symptoms and fatigue found no significant benefit for fatigue, though it had only two trials reporting that outcome. Two meta-analyses, different inclusion criteria, different answers. Treat CoQ10 as promising rather than proven.

• The specific case: statins lower the body's own CoQ10 production through the same pathway they lower cholesterol. If you are on a statin and tired, this is worth raising with your GP rather than guessing.
• More on it: ubiquinol against ubiquinone.

Ashwagandha, for the stress version of tired

Better evidence for stress and sleep than for energy directly. A 12-week randomised trial gave 200 mg of a standardised root extract twice daily to overweight or mildly obese adults aged 40 to 75 who reported high stress and fatigue. Fatigue fell significantly against placebo, and in the men taking it, free testosterone and luteinising hormone were higher than placebo (Journal of Psychopharmacology, 2023). A separate 2019 randomised trial found greater reductions in morning cortisol than placebo.

• Caution: do not read the testosterone finding as a testosterone treatment. It was one secondary result in one trial. See the next section.
• Also: ashwagandha has rare reports of liver injury and it interacts with thyroid medication. Not one to start casually if you take other things.
• More on it: KSM-66 and Sensoril explained.

Tier 3: the ones the search results are full of, that the trials do not support

Testosterone boosters

A systematic review in the International Journal of Impotence Research went through the marketed products and found the evidence for raising serum testosterone thin to absent for most ingredients.

• Tribulus terrestris: eight of ten studies found no change in the androgen profile. The two that found an increase reported a small one, in men who were hypogonadal to start with.
• D-aspartic acid: increases were seen in untrained men in early work, then a randomised trial in resistance-trained men found no change, and in one arm a reduction.

If low testosterone is genuinely the problem, it is diagnosed on repeated morning blood tests and managed by a doctor. If it is not the problem, these do nothing except cost money. Neither outcome is improved by the bottle.

High-dose B complex when you are not short of B vitamins

B vitamins are cofactors in the pathways that release energy from food, which is why the label can say "supports energy metabolism" without lying. That claim is about biochemistry, not about how you feel. In people who are not deficient, the fatigue evidence is not there.

The "energy" multivitamin

A multivitamin is a reasonable insurance policy against a patchy diet. It is not an energy product, and buying the version with "energy" on the front usually means paying more for caffeine, a B-vitamin megadose, or both. Worth reading: when a multivitamin is not enough, and when it is.

The order of operations

If you take nothing else from this article, take the sequence.

• Handle the drains from Part 1. Sleep, alcohol, mood, movement.
• Get the bloods. Full blood count, ferritin, thyroid, HbA1c, coeliac, and vitamin D or B12 if you have a risk factor.
• Correct what is actually low, with a GP involved if the low thing is iron.
• Only then, and only if a specific situation calls for it, add one Tier 2 item.
• Give it a fixed window, measure it, and drop it if it does nothing. That is Part 3.

Most men who arrive at step four have already fixed the problem at step one or three. That is a good outcome, not a wasted article.

Where Stack Almanac fits

Two jobs here. The first is dose truth: the Almanac Advisor reads the actual amount in what you take, not the number on the front of the bottle, so it can tell you when something is below a dose that has ever been trialled or stacked past a sensible ceiling across two products.

The second is the gate. Log a statin and CoQ10 stops being an optional extra. Log a thyroid medication and ashwagandha gets flagged. Add a blood test result and the advice you get is aimed at your number rather than at men in general.

None of that replaces the GP appointment. It stops you buying the thing the appointment would have told you not to.

Where this comes from

Sources for every claim above, in the order they appear.

• Nowak A et al. Effect of vitamin D3 on self-perceived fatigue: a double-blind randomized placebo-controlled trial. Medicine 2016;95(52)
• Investigating causality in the association between vitamin D status and self-reported tiredness. Sci Rep 2019;9:2880
• NHS. Vitamin D (vitamins and minerals)
• Markun S et al. Effects of vitamin B12 supplementation on cognitive function, depressive symptoms, and fatigue: a systematic review, meta-analysis and meta-regression. Nutrients 2021;13(3):923
• Verdon F et al. Iron supplementation for unexplained fatigue in non-anaemic women: double blind randomised placebo controlled trial. BMJ 2003;326:1124
• Vaucher P et al. Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin: a randomized controlled trial. CMAJ 2012;184(11):1247-54
• British Society of Gastroenterology. Guidelines for the management of iron deficiency anaemia in adults
• Gordji-Nejad A et al. Single dose creatine improves cognitive performance and induces changes in cerebral high energy phosphates during sleep deprivation. Sci Rep 2024;14:4937
• Effects of creatine monohydrate loading on sleep metrics, physical performance, cognitive function and recovery in physically active men. Nutrients 2025;17(24):3831
• High-dose L-theanine-caffeine combination improves neurobehavioural and neurophysiological measures of selective attention in acutely sleep-deprived young adults. Br J Nutr 2025
• Rogers PJ. Caffeine and alertness: in defense of withdrawal reversal. J Caffeine Res 2014;4(1):3-8
• Darbinyan V et al. Rhodiola rosea in stress induced fatigue: a double blind cross-over study of a standardized extract SHR-5 with a repeated low-dose regimen on the mental performance of healthy physicians during night duty. Phytomedicine 2000;7(5):365-71
• Effectiveness of coenzyme Q10 supplementation for reducing fatigue: a systematic review and meta-analysis of randomized controlled trials. Nutrients 2022
• Effects of coenzyme Q10 supplementation on depressive symptoms and fatigue: a systematic review and meta-analysis of randomized controlled trials. 2025
• Smith SJ, Lopresti AL, Fairchild TJ. Exploring the efficacy and safety of a novel standardized ashwagandha (Withania somnifera) root extract (Witholytin) in adults experiencing high stress and fatigue in a randomized, double-blind, placebo-controlled trial. J Psychopharmacol 2023
• Lopresti AL et al. An investigation into the stress-relieving and pharmacological actions of an ashwagandha extract: a randomized, double-blind, placebo-controlled study. Medicine 2019
• Do 'testosterone boosters' really increase serum total testosterone? A systematic review. Int J Impot Res 2023
• The effects of d-aspartic acid supplementation in resistance-trained men over a three month training period: a randomised controlled trial. PLoS One 2017

Nothing here is medical advice, and none of it replaces a conversation with your GP. Supplements are not medicines, they interact with medicines, and the iron section above is the clearest example of why the order matters.

Next in the series: The Self-Experiment: Testing a Supplement Properly on Yourself.