Persistent fatigue: check before you buy
The “energy” aisle sells stimulants to people who are short of iron, vitamin D or sleep. This comparison inverts the logic: first the causes you can measure, and only then the products — of which there are few.
Contents
“I am tired” is one of the most common reasons for seeing a doctor, and one of the worst served by online retail. The reason is simple: the most frequent causes are measured or corrected for free, and none of them comes in a capsule.
The rule: check, then correct, then only supplement
Taking an “energy” supplement without knowing why you are tired amounts to adding noise. Worse: some products — iron in particular — are harmful in the absence of a deficiency. Here is the order we apply.
The five causes to rule out first
1. Sleep debt, and above all its irregularity
This is the first cause, by a long way. Not just duration: regularity. Going to bed at 11pm on weekdays and 2am at weekends produces a shift comparable to a weekly transatlantic flight. Before any blood test, ask yourself about your wake-up time — is it the same seven days a week? Our piece on the three systems of sleep explains the mechanism.
2. Iron deficiency
The leading correctable nutritional cause of fatigue, particularly common in menstruating women, regular blood donors, endurance athletes and people on plant-based diets. It often precedes anaemia: you can have low ferritin with normal haemoglobin and already feel the fatigue.
Ferritin + CRP (inflammation falsely raises ferritin) before any supplementation. If deficiency is confirmed: iron bisglycinate with vitamin C, away from tea and coffee, with every-other-day dosing improving absorption. Never blind.
3. Vitamin D insufficiency
From October to April, above the 40th parallel, skin synthesis is virtually nil. Insufficiency is therefore very widespread in France in winter. Supplementing corrects a shortfall; it produces no notable effect in people whose status is already adequate. Ideally, measure 25(OH)D before and after three months.
4. Undiagnosed sleep disorders
Obstructive sleep apnoea first of all. Warning signs: snoring with pauses noticed by those around you, daytime sleepiness despite eight hours in bed, waking with headaches, dry mouth, resistant high blood pressure. It is common, widely underdiagnosed, and no supplement changes anything about it.
5. The thyroid, and the other medical causes
Hypothyroidism is screened with a TSH test. Diabetes, depression, inflammatory syndromes, medication side effects, coeliac disease: the list of medical causes of fatigue is long. That is precisely why the “see a doctor” step comes before the “buy something” step.
Full blood count, ferritin + CRP, TSH, fasting glucose, and depending on context 25(OH)D. Prescribed and interpreted by a doctor. It is the best health investment on this page — and it earns us nothing.
What has a place, once the causes are checked
| Product | Evidence | Who for | Condition |
|---|---|---|---|
| Vitamin D3 + K2 | Solid (if deficient) | Almost everyone, October to April | Testing recommended |
| Iron bisglycinate + C | Solid (if deficient) | Confirmed deficiency only | Ferritin + CRP mandatory |
| Magnesium bisglycinate | Emerging | Cramps, tension, restless sleep | Low dietary intake |
| 10,000 lux lamp | Solid (circadian) | Getting up before daylight, winter low mood | Mornings only |
| Caffeine + L-theanine | Solid (alertness) | Work blocks, not chronic fatigue | Before 1 pm |
| Electrolytes | Emerging | Heavy sweating, heat | Not on a low-salt diet |
The particular case of caffeine
Caffeine does not create energy: it blocks the perception of sleep pressure. It is an advance on wages, repaid with interest by the end of the day. Used properly — a moderate dose, early in the day, with L-theanine to smooth the peak — it remains an effective alertness tool. Used to make up for chronic sleep debt, it sustains the very problem it masks.
Practical rule: last intake eight to ten hours before your target bedtime. And count tea, cola drinks and dark chocolate in the total.
What we ruled out
- “Vitality” multivitamin ampoules. Inconsistent doses, no targeting, and no effect in the absence of a deficiency. A multivitamin does not replace a blood test.
- Ginseng and guarana. Weak data, or amounting to the caffeine effect for the second.
- Vitamin B12 in omnivores with no deficiency. Essential on a vegan diet, with no demonstrated value elsewhere.
- Energy drinks. Caffeine + sugar, often late in the day: the worst possible pattern for a fatigue problem.
- “Detox” cures. No physiological basis: the liver and kidneys do not wait for an ampoule to work.
The fourteen-day protocol
- Days 1–3: fix an identical wake-up time, seven days a week. Ten minutes of light outside on waking.
- Day 4: book a blood test. Supplement nothing before it.
- Days 5–7: last coffee before 1 pm. Watch the effect on falling asleep.
- Days 8–10: aim for 1.6 g/kg of protein and three structured meals. Reactive hypoglycaemia late in the morning explains a lot of “slumps”.
- Days 11–14: add thirty minutes of brisk walking a day. Counter-intuitive, and one of the best documented interventions on fatigue.
- When the results come back: correct what needs correcting — and only that.
Fatigue of sudden onset, involuntary weight loss, prolonged fever, unusual breathlessness, chest pain, swollen glands, night sweats, bleeding, or fatigue that worsens despite decent sleep. In those situations no comparison page replaces a clinical examination.
What has evidence, once the causes are checked
None of these products “provides energy”. They correct a shortfall or act on a specific mechanism. The order follows the level of evidence for the intended use.
Amazon affiliate links. We earn a commission if you buy, at no extra cost to you: it changes neither the ranking nor the level of evidence shown.
* Budget bands are indicative and change often. Only the price shown on Amazon at the moment you buy counts: check it before you order.
What we get asked most
Short answers, sourced where they can be, honest where the data is missing.
This piece, filtered through your profile
Eleven questions, no sign-up, everything stays in your browser. The site then reorders its recommendations according to your chronotype, your budget and your contraindications.
Start my assessment →The 5 sources behind this piece
Every reference is clickable and checkable. When a claim has no solid source, we write that down rather than citing an off-topic study.
The site’s full bibliography →Iron supplementation reduces fatigue in women with low ferritin, even without anaemia. Hence the mandatory test before taking any.
The benefit is concentrated in people who are genuinely deficient: vitamin D corrects a shortfall, it does not boost a normal status.
400 mg of caffeine six hours before bed still significantly reduces sleep time — without participants always noticing.
A small trial on insomnia in older people: limited numbers, hence an “emerging” rather than “solid” level of evidence.
The basis of 10,000 lux light therapy in the morning for seasonal affective disorder.
Read next
All the long readsMorning light: the most powerful lever nobody sells
How light sets the circadian clock: melanopsin cells, real lux values, light therapy. And what to do when it is dark at 8am.
Bloating and transit: where to actually start
Digestion comparison: psyllium, probiotics, fibre, hydration. What the trials say, and what to check before buying.
What to measure before buying any supplement at all
Ferritin, vitamin D, B12, lipid panel: the four tests that decide whether a supplement helps you or not. What to ask for, when, and how to read the result.