Several nutrient deficiencies can cause low energy, but iron deficiency is the most common worldwide, affecting an estimated 5 billion people who don’t consume enough. Other frequent culprits include vitamin B12, vitamin D, magnesium, B vitamins involved in metabolism, and iodine. Each of these nutrients plays a distinct role in how your body produces and uses energy, and being low in even one of them can leave you persistently tired.
Iron Deficiency
Iron is needed to make hemoglobin, the protein inside red blood cells that carries oxygen to every tissue in your body. When iron is low, your red blood cells can’t deliver enough oxygen, and your heart has to pump harder to compensate. The result is a type of fatigue that rest alone won’t fix: you feel winded climbing stairs, mentally foggy, and drained by activities that used to feel easy.
Iron deficiency is the single most widespread nutritional gap on the planet. According to a 2024 modeling analysis in The Lancet Global Health, more than 5 billion people globally don’t consume adequate iron. Women of reproductive age are especially vulnerable because of monthly blood loss, but vegetarians, frequent blood donors, and people with digestive conditions that reduce absorption are also at higher risk.
What you eat alongside iron-rich foods matters. Calcium, tannins in tea and wine, and phytates in whole grains can all interfere with absorption of the plant-based (non-heme) iron found in beans, spinach, and fortified cereals. Spacing tea or dairy away from iron-rich meals and pairing those meals with a source of vitamin C can make a meaningful difference in how much iron your body actually takes up.
Vitamin B12 Deficiency
Vitamin B12 does double duty: it helps form red blood cells, and it supports the nervous system. Low B12 can cause a type of anemia similar to iron deficiency, but it also impairs nerve function in ways that produce their own kind of exhaustion. B12 is essential for producing the protective coating around nerve fibers (myelin) and for making signaling chemicals like adrenaline. When levels drop, the nervous system slows down, which can show up as fatigue, muscle weakness, numbness, or brain fog.
What makes B12 tricky is that neurological symptoms often appear months before anemia does. You can feel profoundly tired with a normal-looking blood count if your B12 is low enough to affect nerve function but not yet low enough to shrink your red blood cell supply. Adults need about 2.4 micrograms per day. B12 is found almost exclusively in animal products (meat, fish, eggs, dairy), so vegans and strict vegetarians need a supplement or fortified foods. Older adults also absorb B12 less efficiently from food due to declining stomach acid production.
Vitamin D Deficiency
Vitamin D affects energy at the cellular level. Inside your muscle cells, it helps mitochondria (the structures that generate energy) consume oxygen efficiently and maintain the calcium balance they need to function. When vitamin D is deficient, mitochondria can’t keep up. Research published in the European Journal of Applied Physiology found that vitamin D deficiency decreases the oxygen consumption rate of muscle cells, disrupts mitochondrial function, and over time contributes to muscle weakness and atrophy.
The practical effect is that low vitamin D doesn’t just make you tired in a vague way. It makes physical effort feel harder. Your muscles fatigue faster, recovery takes longer, and you may notice generalized weakness or soreness. In one study, correcting vitamin D deficiency with supplementation improved the rate at which muscles could regenerate their energy stores, and every participant reported improvement in fatigue and muscle symptoms. The recommended daily intake for adults is 600 IU, though many health professionals suggest higher amounts for people who are already deficient or who get minimal sun exposure.
Magnesium Deficiency
Your cells run on a molecule called ATP, but ATP can’t actually function without magnesium. Magnesium binds directly to ATP to create the compound (MgATP) that enzymes throughout your body recognize as usable fuel. Without enough magnesium, ATP is present but essentially inactive, like having a full gas tank with no ignition. Research in the Proceedings of the National Academy of Sciences confirmed that this magnesium-ATP complex is the principal energy source of the cell and the required form for hundreds of energy-transferring reactions.
Magnesium also serves as a cofactor for converting thiamine (vitamin B1) into its active form, creating a chain reaction: low magnesium can worsen the effects of other B vitamin deficiencies. Common signs of low magnesium include fatigue, muscle cramps, poor sleep, and irritability. Nuts, seeds, leafy greens, and whole grains are the richest food sources.
B Vitamins Beyond B12
The entire B-vitamin family is woven into your body’s energy metabolism. Thiamine (B1), riboflavin (B2), niacin (B3), pantothenic acid (B5), and pyridoxine (B6) all act as helpers in the chemical reactions that break down food into usable energy. Any single B vitamin deficiency can impair the mitochondrial metabolism of sugars, fats, and amino acids.
Thiamine deserves special mention because it’s a cofactor at critical steps in the energy cycle where your cells convert glucose into fuel. A thiamine deficiency doesn’t just slow energy production; it can cause a dramatic fatigue along with neurological symptoms like confusion and poor coordination. Alcohol use, bariatric surgery, and highly processed diets low in whole grains are common risk factors. The daily requirement is small (1.1 to 1.2 mg for adults), but chronic shortfalls add up.
Iodine Deficiency
Iodine is the raw material your thyroid gland needs to produce hormones that regulate your metabolic rate. The two main thyroid hormones, T3 and T4, control how fast every cell in your body burns fuel. When iodine is too low, the thyroid can’t make enough of these hormones, and your metabolism slows. The fatigue from iodine deficiency feels like your whole system is running at half speed: you gain weight more easily, feel cold, think sluggishly, and lack the drive to do much of anything.
Iodine deficiency is less common in countries that use iodized salt, but it remains a significant issue in parts of the world without salt fortification programs, and it’s becoming more common among people who use sea salt or specialty salts that aren’t iodized. Seafood, dairy, and eggs are reliable dietary sources.
How These Deficiencies Overlap
In practice, nutrient deficiencies rarely show up alone. Someone eating a limited diet may be low in iron, B12, and vitamin D simultaneously. The fatigue from multiple mild deficiencies can be just as debilitating as one severe deficiency, and the overlapping symptoms (tiredness, weakness, difficulty concentrating) make it hard to pinpoint the cause by feel alone.
A standard blood panel can check for most of these. Ferritin measures your iron stores, serum B12 captures your circulating vitamin B12 level, and 25-hydroxyvitamin D reflects your vitamin D status. Thyroid function tests (TSH, free T4) can reveal whether iodine or other thyroid-related issues are contributing. Magnesium is trickier because most of the body’s magnesium lives inside cells and bones rather than in the blood, so a normal blood level doesn’t always rule out a deficiency.
If you’ve been persistently tired despite getting enough sleep and managing stress, a nutrient deficiency is one of the most correctable explanations. Identifying which nutrients are low, rather than guessing with a multivitamin, lets you target the actual gap and notice real improvement within weeks to months depending on the severity.