Always Tired? The Two Deficiencies Your Doctor Should Test For
The two most common nutritional causes of persistent fatigue:
1. Vitamin D deficiency — affects 42% of US adults. Required for mitochondrial function and energy metabolism. Get a blood test (25(OH)D) — it's accurate and widely available. If deficient: vitamin D3 4,000-5,000 IU/day.
2. Magnesium deficiency — affects ~50% of US adults. Required for ATP production (your cells' energy currency). Blood tests are unreliable. Safe to try empirically: magnesium glycinate 400mg/day ($0.23/day).
Both together cost less than $0.50/day and address the two most widespread nutritional gaps in the American diet.
Why You're Tired: The Cellular Energy Connection
If you're getting 7-8 hours of sleep and STILL exhausted, your body might be missing the raw materials it needs to produce energy at the cellular level.
Every cell in your body runs on ATP (adenosine triphosphate). Making ATP requires:
- Magnesium — directly required by over 300 enzymes in the ATP production process. Without it, your mitochondria can't efficiently convert food to energy.
- Vitamin D — plays a role in mitochondrial function and muscle energy metabolism. Deficiency is associated with fatigue, muscle weakness, and reduced exercise capacity.
- Iron — carries oxygen to cells. Without oxygen, ATP production drops dramatically. (More relevant for women, especially with heavy periods.)
- B vitamins — cofactors in the citric acid cycle and electron transport chain. B12 deficiency is common in vegans/vegetarians and older adults.
The uncomfortable truth: about half of Americans are low in at least one of the first two. If you're not supplementing and you eat a typical modern diet, the odds are good that a simple deficiency is dragging your energy down.
Step-by-Step: What to Do
Step 1: Get Tested (Vitamin D)
Unlike magnesium, vitamin D has an accurate, cheap blood test. Ask your doctor for a 25(OH)D test. It tells you exactly where you stand:
- Below 20 ng/mL → Deficient → Take D3 4,000-5,000 IU/day for 8-12 weeks
- 20-29 ng/mL → Insufficient → Take D3 2,000-4,000 IU/day
- 30-80 ng/mL → Adequate → Maintenance dose 1,000-2,000 IU/day
Full vitamin D deficiency guide · Dosage guide · Always use D3, not D2
Step 2: Start Magnesium (Today)
Magnesium blood tests are unreliable (only 1% is in blood), but supplementation is safe and cheap. Start magnesium glycinate 300-400mg/day empirically if you have any of these risk factors: stress, processed diet, alcohol, PPIs/diuretics, age 65+.
| Product | Cost/Day |
|---|---|
| Vitamin Shoppe Mg Glycinate 400mg | $0.23 |
| Nature Made Mg Glycinate (USP) | $0.47 |
Step 3: Give It 4-6 Weeks
Replenishing vitamin D takes 8-12 weeks for full effect. Magnesium effects are often noticed within 1-2 weeks (better sleep, less muscle tension) but full energy improvement takes longer.
Step 4: If Still Tired — See Your Doctor For
- Thyroid function (TSH) — Hypothyroidism is a common cause of fatigue
- Iron / ferritin — Especially for women. Ferritin below 30 ng/mL can cause fatigue even without full-blown anemia
- B12 — If vegan/vegetarian, over 60, or on metformin
- Sleep study — Sleep apnea is dramatically underdiagnosed. You can sleep 8 hours and still be exhausted if you stop breathing repeatedly.
- Complete blood count (CBC) — Screens for anemia and other blood disorders
The Daily Stack for Fatigue
| Supplement | Dose | Cost/Day | Why |
|---|---|---|---|
| Vitamin D3 | 2,000-5,000 IU (based on blood test) | ~$0.05-0.10 | 42% of adults deficient. Mitochondrial function. |
| Magnesium glycinate | 400mg elemental | $0.23 | ~50% of adults deficient. Required for ATP. |
Total: ~$0.28-0.33/day ($8-10/month) to address the two most common nutritional causes of fatigue. Less than a single coffee.
One click adds all supplements to your Amazon cart · $0.28/day total
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The evidence · Magnesium Glycinate
What the published reviews of Magnesium Glycinate concluded
- Magnesium supplementation as an adjunct to fluoxetine therapy for depression. · East Asian archives of psychiatry : official journal of the Hong Kong College of Psychiatrists = Dong Ya jing shen ke xue zhi : Xianggang jing shen ke yi xue yuan qi kan, 2026
“Magnesium supplementation as an adjunct to fluoxetine therapy improves HDRS total score in patients with depression.”
- Preloading magnesium attenuates cisplatin-associated nephrotoxicity: pilot randomized controlled trial (PRAGMATIC study). · ESMO open, 2022
“The magnesium-preloading regimen was safe and significantly showed a decreased incidence of cis-AKD. The encouraging results of our pilot study need to be confirmed in a large-scale randomized controlled trial.”
- The effect of combined magnesium and vitamin D supplementation on vitamin D status, systemic inflammation, and blood pressure: A randomized double-blinded controlled trial. · Nutrition (Burbank, Los Angeles County, Calif.), 2022
“A combined MagD treatment may be more effective in increasing serum 25OHD concentrations compared with VitD supplementation alone in Owt/Ob individuals.”
Method: PubMed searched in humans for randomised trials with Magnesium Glycinate in the title; these 3 of the 31 that qualified are shown with the authors’ own conclusion quoted rather than summarised. What it cannot tell you: whether Magnesium Glycinate will work for you — each of these reports an average across trials published 2022–2026, and null findings are shown here as readily as positive ones.
Frequently Asked Questions
Why am I always tired even when I sleep enough?
The two most common nutritional causes are vitamin D deficiency (42% of adults) and magnesium deficiency (~50% of adults). Both are required for cellular energy production. Get a vitamin D blood test and try magnesium glycinate 400mg/day. If still tired after 4-6 weeks, see your doctor for thyroid and iron testing.
What supplements help with fatigue?
Start with vitamin D3 (2,000-5,000 IU based on blood level) and magnesium glycinate 400mg/day ($0.23/day). Together ~$0.28-0.33/day. These address the most common deficiencies. If fatigue persists, test thyroid, iron/ferritin, and B12.
Related
- Vitamin D Deficiency Signs & Testing
- Magnesium Deficiency Signs
- Vitamin D Dosage Guide
- Magnesium Dosage Guide
- Can't Sleep? — Fatigue and poor sleep often co-occur
- Brain Fog — Cognitive fatigue shares many of the same nutritional causes
- Feeling Depressed? — Fatigue and low mood frequently overlap with vitamin D deficiency
- Afternoon Crash — When fatigue hits specifically in the afternoon
- Iron: Ferrous Sulfate vs Bisglycinate — If your doctor flags low iron as a fatigue cause
- B12: Methylcobalamin vs Cyanocobalamin — Another common deficiency causing fatigue
Sources
- Rosanoff A, et al. "Suboptimal magnesium status in the United States." Nutr Rev. 2012;70(3):153-164. PMID: 22364157
- NIH Office of Dietary Supplements. "Vitamin D: Fact Sheet for Health Professionals." ods.od.nih.gov
- NIH Office of Dietary Supplements. "Magnesium: Fact Sheet for Health Professionals." ods.od.nih.gov