Why You’re Always Tired: Nutritional Causes of Low Energy (and What to Do About It)
If you’re constantly tired despite getting a full night’s sleep, you’re not imagining it, and it’s not necessarily “just stress.” Healthcare professionals often miss several well-documented nutritional causes of low energy, partly because standard blood tests only flag the most extreme cases. This guide walks through the most common ones seen in clinic, what the evidence actually says about each, and what’s worth doing about it.
It’s Not Always “Just Stress”
Fatigue is one of the most common complaints in general practice, and it’s also one of the easiest to dismiss. However, it’s worth ruling out the nutritional factors that quietly drain energy day after day, before blaming stress, poor sleep, or “just getting older. Several of these are surprisingly common, and correcting them can make a noticeable difference within weeks.
Common Nutritional Causes of Low Energy:
Iron Deficiency (Even Without Anaemia)
Most people assume iron only matters once anaemia shows up on a blood test. In fact, a meta-analysis of randomised controlled trials found that correcting iron deficiency significantly reduced fatigue. This held true even in people whose iron levels hadn’t dropped low enough to count as anaemic. Women are particularly at risk due to menstrual blood loss, so if your ferritin (iron stores) sits at the low end of “normal,” it may still be worth addressing.
Vitamin B12 Deficiency
B12 is essential for energy production and nervous system function. Deficiency is more common than many people realise, particularly among those following a vegetarian or vegan diet, taking certain medications, or producing less stomach acid. Because animal products are the only natural source of B12, this is one of the more straightforward deficiencies to test for and correct.
Blood Sugar Swings
Frequent blood sugar spikes and crashes are one of the most common, and most fixable, causes of daytime fatigue. If your energy dips hard an hour or two after eating, or you rely on caffeine or sugar to get through the afternoon, this is worth investigating further I’ve written a full guide to blood sugar balance that covers exactly why this happens and how to stabilise it.
Magnesium Deficiency
Magnesium drives the cellular processes that actually produce energy, so low levels can leave you feeling depleted even when everything else looks fine on paper. Not all forms of magnesium are equally useful here, though I’ve broken down which forms of magnesium are best for which symptoms, including the ones I typically recommend for fatigue specifically.
Under-Eating or Erratic Meal Patterns
Skipping meals, chronic under-eating, or restrictive dieting can all suppress energy availability over time, even if weight loss is the goal. Rather than pushing through on willpower, the more sustainable approach is building meals that actually meet your needs. I cover this in my recent blog post how I support my hormones daily, since consistent, well-structured eating is foundational to stable energy.
Poor Gut Health & Nutrient Absorption
Even a nutrient-rich diet won’t help much if your gut isn’t absorbing what you eat. Conditions affecting the gut lining, chronic bloating, or an imbalanced microbiome can all interfere with nutrient uptake. That’s part of why gut health is often one of the first things I look at with clients who feel constantly drained despite eating well.
Thyroid Function
Your thyroid regulates how quickly your body uses energy, so an underactive thyroid can produce fatigue that doesn’t budge no matter how well you eat or sleep. I’ve written more about how thyroid hormones fit into the bigger picture in my guide to appetite and metabolism hormones, including which nutrients support healthy thyroid function.
What About Vitamin D?
People constantly blame vitamin D deficiency for fatigue, so it’s worth being honest about what the evidence actually shows. A large genetic study of over 300,000 people found no causal link between vitamin D levels and self-reported tiredness. This suggests other factors, rather than vitamin D itself, likely explain the association seen in smaller studies. That doesn’t mean vitamin D isn’t important for other aspects of health, particularly in the UK where deficiency is common over winter, but it’s probably not your main fatigue culprit.
What to Do About It
Rather than guessing and supplementing everything at once, the more effective approach is testing first. A full blood panel covering ferritin, B12, folate, thyroid function, and vitamin D gives a genuinely useful starting point, and it’s worth asking your GP specifically for ferritin rather than just a standard full blood count, since a standard count can otherwise miss early iron deficiency. From there, building consistent, balanced meals, prioritising sleep, and addressing any gut symptoms tend to make the biggest difference alongside targeted supplementation where it’s genuinely needed.
When to See a Professional
If fatigue has lasted more than a few weeks, is worsening, or comes with other symptoms like unexplained weight changes, hair loss, or heavy periods, it’s worth seeing your GP first. This rules out underlying medical causes. Once your GP has ruled these out, or if you’d like a more personalised, nutrition-led approach, that’s exactly the kind of thing we’d work through together and we can look at your specific picture.
Final Thoughts
Persistent tiredness is common, but it isn’t something you simply have to live with. Whether it’s iron, B12, blood sugar, magnesium, or something else entirely, there’s usually a nutritional thread worth pulling on. Identifying the right one, rather than guessing, is what actually moves the needle.
References
Yokoi, K. and Konomi, A. Iron deficiency without anaemia is a potential cause of fatigue: meta-analyses of randomised controlled trials and cross-sectional studies. Br J Nutr. 2017;117(10):1422-1431. https://www.cambridge.org/core/journals/british-journal-of-nutrition/article/iron-deficiency-without-anaemia-is-a-potential-cause-of-fatigue-metaanalyses-of-randomised-controlled-trials-and-crosssectional-studies/F7E59D4BFC154E9687E42CDCC4968EAF
Havdahl A, Mitchell R, Paternoster L, Davey Smith G. Investigating causality in the association between vitamin D status and self-reported tiredness. Sci Rep. 2019;9:2880. https://www.nature.com/articles/s41598-019-39359-z
Photo credit: Tired woman – Photo by Ephraim Mayrena via Unsplash
