Persistent tiredness is one of the most common complaints people bring to their GP — and one of the most common reasons people start taking supplements. The challenge is that fatigue has many causes, not all of them nutritional. But where the cause is a deficiency in a specific vitamin or mineral, correcting that deficiency can produce a meaningful and relatively rapid improvement in energy levels.
Here’s a breakdown of the nutrients most strongly linked to tiredness, what the evidence says, and what to look for in a supplement.
Magnesium
Magnesium is arguably the nutrient most frequently overlooked in discussions of fatigue. It’s required for over 300 enzymatic reactions in the body, including the production of ATP — the molecule cells use as an energy currency. Without adequate magnesium, ATP synthesis is impaired even when calorie intake is sufficient, which is why magnesium deficiency can cause profound fatigue that isn’t explained by diet or sleep alone.
Magnesium deficiency is also strongly associated with disrupted sleep quality. Magnesium activates the parasympathetic nervous system and regulates GABA receptors in the brain — both critical for the ability to fall asleep and stay in deep sleep. Poor sleep is itself a major driver of daytime fatigue, so the magnesium-tiredness connection operates through both direct energy metabolism and sleep architecture.
The UK reference intake for magnesium is 300 mg daily for men and 270 mg for women, but surveys suggest a significant proportion of UK adults fall below this. Dietary sources include leafy green vegetables, nuts, seeds, legumes, and dark chocolate — foods that are often underrepresented in processed-food-heavy diets.
For supplementation, the form matters. Our Magnesium Glycinate is chelated to glycine, an amino acid that enhances absorption and is gentle on the stomach — making it one of the better-tolerated forms for daily use. Standard magnesium oxide, the cheapest and most common form, has low bioavailability and is primarily useful as a laxative rather than for raising magnesium status.
Vitamin D3
Fatigue is one of the most consistently reported symptoms of vitamin D deficiency — and in the UK, deficiency is widespread. Up to one in three UK adults has insufficient vitamin D levels by the end of winter, and in populations with darker skin or limited sun exposure, this proportion is higher year-round.
Vitamin D receptors are present in muscle tissue and play a role in mitochondrial function — the cellular machinery responsible for energy production. Deficiency impairs this function and is associated with fatigue, muscle weakness, and impaired exercise tolerance. Several clinical trials have shown that correcting vitamin D deficiency significantly improves self-reported fatigue and physical performance, particularly in people confirmed deficient at baseline.
Signs vitamin D deficiency may be driving your fatigue
Fatigue on its own is non-specific, but the pattern of vitamin D deficiency often includes a cluster of signs rather than tiredness alone. Worth paying attention to:
- Muscle weakness or aching, particularly in the thighs, shoulders or lower back — vitamin D deficiency is a well-documented cause of diffuse musculoskeletal pain and proximal muscle weakness.
- Tiredness that is worse through the winter months and improves noticeably in spring or after a sunny holiday.
- Frequent colds or infections — vitamin D modulates immune function and deficiency is associated with increased susceptibility to respiratory infections.
- Low mood that tracks the seasons (see our guide on seasonal affective disorder).
- Bone aches or tenderness, particularly in the shins or ribs in severe deficiency.
A simple serum 25-hydroxyvitamin D blood test through your GP or a private lab is the only way to confirm deficiency. Levels below 25 nmol/L are considered deficient by UK standards; 25–50 nmol/L is insufficient; above 50 nmol/L is sufficient.
Vitamin D3 (cholecalciferol) is the preferred form — it raises and maintains serum levels more effectively than D2. Our Vitamin D3 K2 supplement combines D3 with K2 MK-7, zinc, boron, and MCT oil in a single capsule. The MCT oil base supports fat-soluble absorption, and the zinc supports vitamin D receptor function at the cellular level. For a full overview of what vitamin D does, see our article on the health benefits of vitamin D. If you’re thinking about combining D3, zinc and magnesium for fatigue support, our guide on vitamin D with zinc and magnesium covers how they interact and when to take each.
B vitamins
B vitamins — particularly B12, B6, and B9 (folate) — are essential for converting food into the glucose your cells use for energy. They are also required for red blood cell production: without adequate B12 and folate, the body produces fewer, larger, and less functional red blood cells (megaloblastic anaemia), reducing oxygen delivery to tissues and causing fatigue, breathlessness, and cognitive difficulties.
Vitamin B12 deserves particular attention. It’s found almost exclusively in animal products, making deficiency significantly more common in people following vegan or vegetarian diets. The NHS estimates that up to 10% of older adults are B12 deficient, and deficiency can take years to become apparent because the body stores B12 in the liver for months to years. Symptoms of B12 deficiency — fatigue, memory problems, tingling extremities — can be subtle and are often attributed to other causes.
If you’re vegan, vegetarian, or over 50, checking your B12 level through a blood test and supplementing if needed is one of the most directly actionable steps for energy support. Methylcobalamin and adenosylcobalamin are the active, better-absorbed forms compared to cyanocobalamin.
Iron
Iron is essential for haemoglobin — the protein in red blood cells that carries oxygen from the lungs to tissues. Iron deficiency anaemia reduces the oxygen available to every cell in the body, causing fatigue, pallor, breathlessness on exertion, and poor concentration. It is the most common nutritional deficiency worldwide and disproportionately affects women of reproductive age, vegetarians, and people with gut conditions that impair absorption.
Iron supplementation should only be started after a blood test confirms deficiency. Iron is not a “good if in doubt” supplement — excess iron can be harmful and supplementing in someone without deficiency provides no benefit. If you suspect iron deficiency, a GP can order a full blood count and serum ferritin test to confirm. Good dietary sources include red meat, liver, lentils, chickpeas, and dark leafy greens (with vitamin C to improve non-haem iron absorption from plant sources).
Omega-3 fatty acids (DHA)
Omega-3 fatty acids — particularly DHA — support brain and neurological function and have anti-inflammatory properties relevant to chronic fatigue. Some evidence suggests omega-3 supplementation improves fatigue, particularly in people with chronic fatigue syndrome and in those with high inflammatory burden. The mechanism is likely related to reducing neuroinflammation, which is associated with both cognitive fog and physical fatigue.
Beyond fatigue, adequate DHA supports the integrity of cell membranes throughout the body, which affects how efficiently cells produce and use energy. Good dietary sources include oily fish and algae-derived DHA supplements — the latter provides the same active EPA and DHA without the contamination concerns of fish oil and is suitable for plant-based diets.
N-acetyl cysteine (NAC)
NAC is the supplement form of cysteine, an amino acid the body uses to synthesise glutathione — the most important intracellular antioxidant. Oxidative stress and mitochondrial dysfunction are increasingly recognised as contributors to chronic fatigue, particularly in people whose fatigue is persistent despite adequate nutrition and sleep.
NAC has been studied in the context of chronic fatigue syndrome and post-viral fatigue, where oxidative stress appears to play a role. By replenishing glutathione, NAC may help reduce this oxidative burden and support cellular energy production. It also has well-documented effects on respiratory health — NAC thins mucus and is used clinically for chronic bronchitis and respiratory conditions that reduce oxygenation. Our NAC+ supplement provides 600 mg per capsule alongside selenium and molybdenum for full glutathione pathway support.
A note on sleep and tiredness
No supplement can replace consistent, good-quality sleep. Even with optimal nutrition, chronic sleep deprivation produces fatigue that supplements cannot address. Magnesium glycinate is one of the few supplements with reasonable evidence for improving sleep quality rather than just addressing nutritional deficiency — it reduces the time to sleep onset and supports deeper sleep stages. But this works best as part of good sleep hygiene, not as a replacement for it. See our article on how vitamin D3 and K2 work together for more context on nutrient synergies.
If you are experiencing persistent fatigue that doesn’t improve with rest, good nutrition, and basic supplementation, a blood panel covering vitamin D, B12, iron/ferritin, folate, and thyroid function is the most practical starting point — it covers the most common correctable causes and gives you evidence to act on rather than guessing.
Frequently asked questions
What is the best vitamin for tiredness and fatigue?
There is no single “best” vitamin because the right answer depends on what’s causing the fatigue. The most common correctable nutritional causes are vitamin D deficiency (widespread in the UK), vitamin B12 deficiency (particularly common in older adults and vegans), iron deficiency anaemia (particularly in women of reproductive age), and magnesium deficiency (impairs both energy metabolism and sleep quality). A blood test covering these four is the most useful starting point before supplementing.
Can vitamin D deficiency cause tiredness?
Yes. Vitamin D deficiency is one of the most commonly identified nutritional causes of fatigue in the UK. Vitamin D receptors are present in muscle tissue and influence mitochondrial function — the cellular energy-production process. Multiple clinical trials have found that correcting vitamin D deficiency significantly improves self-reported fatigue scores, particularly in people confirmed deficient at baseline. If your energy is low through winter in particular, vitamin D is one of the first things worth checking.
What are the signs that vitamin D deficiency is making you tired?
Vitamin D deficiency fatigue rarely shows up as tiredness alone — it tends to cluster with other signs. Common ones include muscle weakness or aches (particularly in the thighs, shoulders and lower back), tiredness that is clearly worse in winter and improves in summer, more frequent colds or infections, and seasonally-patterned low mood. Bone aches in the shins or ribs can appear in severe deficiency. A serum 25-hydroxyvitamin D blood test is the only way to confirm — levels below 25 nmol/L are deficient by UK standards, and 25–50 nmol/L is insufficient.
How long does it take for supplements to improve energy levels?
It depends on the deficiency being corrected. Magnesium tends to produce noticeable effects on sleep quality within 1–2 weeks. Vitamin D levels rise within 2–4 weeks of supplementation, with symptom improvement typically noticeable within 4–8 weeks. Iron deficiency anaemia correction takes longer — serum ferritin levels may take 3–6 months to normalise even with daily supplementation. B12 deficiency can take several months to fully correct when levels are very low.
Can a vegan diet lead to tiredness?
It can, if key nutrients are not planned for carefully. The main risks for vegans are vitamin B12 (found almost exclusively in animal products and requires supplementation on a vegan diet), iron (plant iron is less bioavailable than haem iron from meat), omega-3 (ALA from plant sources converts poorly to DHA and EPA), and zinc (phytates in plant foods reduce zinc absorption). A vegan diet can be nutritionally complete with careful planning and targeted supplementation, but fatigue in vegans should prompt a check of B12, iron, and vitamin D specifically.
Is persistent fatigue always caused by a vitamin deficiency?
No. While nutritional deficiencies are among the most common and correctable causes of fatigue, persistent tiredness can also result from thyroid dysfunction, anaemia from causes other than iron deficiency, sleep disorders such as sleep apnoea, depression and anxiety, chronic infection, and many other conditions. If fatigue persists despite good sleep, a reasonable diet, and corrected nutritional deficiencies, a GP appointment and full blood panel is the appropriate next step — not more supplements.
What is the difference between tiredness and chronic fatigue syndrome?
Tiredness is a normal physiological response to physical or mental exertion and resolves with rest. Chronic fatigue syndrome (CFS, also known as ME) is a complex medical condition characterised by severe, persistent fatigue that does not improve with rest, is often worsened by physical or mental activity (post-exertional malaise), and is accompanied by other symptoms including cognitive difficulties, unrefreshing sleep, and pain. CFS is a distinct diagnosis that requires medical evaluation — it is not simply tiredness that supplements can address, though some people with CFS do benefit from nutritional support as part of a broader management plan.
References
- de Baaij JH, et al. Magnesium in Man: Implications for Health and Disease. Physiological Reviews. 2015;95(1):1–46. doi:10.1152/physrev.00012.2014
- Scientific Advisory Committee on Nutrition (SACN). Vitamin D and Health Report. Public Health England, 2016.
- Nowak G, Szewczyk B. Mechanisms contributing to antidepressant zinc actions. Polish Journal of Pharmacology. 2002;54(6):587–592. (PubMed)
- World Health Organization. Worldwide prevalence of anaemia 1993–2005. WHO, 2008.
- Abbasi B, et al. The effect of magnesium supplementation on primary insomnia in elderly: a double-blind placebo-controlled clinical trial. Journal of Research in Medical Sciences. 2012;17(12):1161–1169. (PubMed)
- Bjørklund G, et al. Vitamin B12 in Health and Disease. Current Medicinal Chemistry. 2020;27(33):5489–5497. doi:10.2174/0929867326666190902154908
This article is for informational purposes only and does not constitute medical advice. If you are experiencing persistent fatigue, speak with your GP to rule out underlying medical causes before starting a supplement regimen.
