Health Reference LibraryWhy do I have no energy? Working through the causes
This entry is part of the Nutri Tailor Health Reference Library — cited research on supplements, nutrients and adjacent areas of health.
Summary
A lack of physical energy, as distinct from sleepiness, tends to point toward physical causes. Iron deficiency is the first candidate, particularly in menstruating women, followed by B12 deficiency, low vitamin D, thyroid problems, under-eating and deconditioning. The blood-readable causes are worth reading properly, with ferritin, B12 and vitamin D checked against their thresholds, because studies show that correcting a genuine deficiency restores energy over weeks. Eating enough, moving a little each day, and the 7 to 9 hours of sleep NHS guidance recommends cover the remaining ground. Depletion that is unexplained, or that comes with warning signs, needs a GP.
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How it works
Physical energy depends on oxygen delivery and on the conversion of fuel inside cells, and the leading deficiencies affect exactly these processes. Iron shortage reduces the oxygen-carrying capacity of haemoglobin, so muscles run short of oxygen even before formal anaemia develops. B12 is required by the enzymes of energy metabolism. Thyroid hormone sets the resting metabolic rate. Chronic under-eating leaves the system underfuelled. Deconditioning adds to this, because reduced activity leads to reduced capacity, which is experienced as having no energy.
Safety profile
Depletion with breathlessness at rest, chest discomfort or fainting needs urgent care rather than a self-check. Unexplained weight loss or night sweats alongside low energy mean a prompt GP appointment. The standing rule on iron also applies here. Iron should not be supplemented without a ferritin reading, because studies show benefit only in people who are genuinely deficient, and iron can harm someone whose level is already high.
Special populations
In menstruating women, iron comes first, and the [15 band](/apps/learn/ferritin-15-is-that-low-and-what-should-you-do) describes the commonest finding. Endurance trainers can be both depleted and deconditioned at once after a break for illness. Adults over 60 should weight B12 higher, because absorption falls with age. Anyone eating restrictively, whatever the reason, should check intake before looking to biochemistry, because insufficient food is the commonest unmeasured cause of low energy.
Guideline positions
Ferritin below 30 µg/L confirms iron deficiency in UK practice, and below 15 by the WHO standard, with the full scale at the [band hub](/apps/learn/ferritin-blood-test-results-explained-uk-what-your-number-means). Thyroid function and blood sugar are GP-ordered tests when the picture warrants them. The warning signs are unchanged. Unexplained weight loss, breathlessness at rest, night sweats or chest symptoms move this from a self-check to an appointment.
Practical framework
The evidence supports a physical work-through. Blood results come first where any are held, with ferritin, B12 and vitamin D read against their bands and corrected properly where low, and the [iron timeline](/apps/learn/iron-supplementation-timeline-symptom-resolution-ferritin-normalisation) setting expectations of pace. Food comes second, meaning enough of it, eaten regularly, with protein present. Movement comes third, and small daily amounts are better supported than occasional intense efforts, because capacity rebuilds through regular use. Sleep holds at the 7 to 9 hours NHS guidance recommends. Three months of genuine effort without improvement is a GP conversation, taken with the record of what has been tried. Nutri365 tracks which change moves energy, reads results and reported symptoms together over time, checks for interactions and contraindications, and refers questions outside scope to a health specialist.
Sources
- 2024. Tiredness and fatigue. NHS.
- 2024. Iron deficiency guidelines. NHS Scotland.
- 2020. WHO ferritin guideline. WHO.