You sleep, you drink your coffee, you get through the day. But by 3pm you are running on empty, and weekends are spent recovering rather than living. If that sounds familiar, you are not alone. Tiredness is one of the most common reasons people visit their doctor, and one of the most common reasons people come to see me.
The good news: fatigue often has causes you can actually find and work on. Some show up clearly on a blood test. Others are hidden in your daily habits. Here are the eight nutritional reasons I look at first, and the blood tests worth asking for.
1. Low iron (even without anaemia)
Iron carries oxygen around your body. When stores run low, energy, concentration and exercise tolerance often drop first.
The key point many people miss: you can have low iron stores without being anaemic. Your haemoglobin may look fine while your ferritin (your iron “savings account”) is low. In randomised trials, iron supplementation reduced fatigue in women with unexplained tiredness and ferritin below 50 µg/L, even though they were not anaemic.
Who is most at risk? Women with heavy periods, vegetarians and vegans, endurance athletes, frequent blood donors, and people with coeliac disease or other gut conditions affecting absorption.
What helps: Pair iron-rich foods (lentils, chickpeas, red meat, mussels, dark leafy greens) with vitamin C, and keep tea and coffee away from iron-rich meals.
2. Low vitamin B12 or folate
B12 and folate are essential for making healthy red blood cells and for your nervous system. Low levels can cause tiredness, low mood, brain fog, and tingling in hands or feet.
Who is most at risk? Vegans and vegetarians, people over 50, and anyone taking metformin or long-term acid-reducing medication such as omeprazole (very commonly prescribed in Spain).
What helps: Eggs, fish, meat and dairy for B12; leafy greens, legumes and citrus for folate. If you eat a fully plant-based diet, a B12 supplement is not optional.
3. Low vitamin D (yes, even in Spain)
It surprises many of my clients, but low vitamin D is common in Spain despite all the sunshine. A large Spanish population study found that around one in three adults may be at risk of deficiency. Indoor office life, sunscreen, and darker winter months all play a part.
Low vitamin D is linked with fatigue, muscle aches and low mood, and correcting a deficiency has been shown to improve fatigue scores in some studies.
What helps: Short, regular sun exposure on arms and legs (without burning), oily fish such as sardines and mackerel, and eggs. Many people benefit from a supplement in winter, ideally guided by a blood test.
4. Blood sugar swings
A pastry and a café con leche for breakfast, nothing until a late lunch, then a sugary snack to survive the afternoon. This pattern creates energy peaks and crashes.
Research suggests sugary foods do not give the mood and energy boost we hope for. A meta-analysis found that within an hour of eating sugar, people actually felt more tired and less alert.
What helps: Build meals around protein, fibre and healthy fats. Think eggs with wholegrain toast, Greek yoghurt with nuts and fruit, or a lentil salad with olive oil. Steady fuel means steady energy.
5. Not eating enough
This one is common among busy professionals, people trying to lose weight, and active people who train hard. When energy intake does not match what your body uses, it starts saving energy. You feel tired, cold, irritable, and recovery from exercise suffers. In sport, this is known as Relative Energy Deficiency in Sport (REDs), and it can affect hormones, bones and immunity.
Low protein intake, especially at breakfast and lunch, is a frequent pattern I see.
What helps: Regular meals, a source of protein at every meal, and eating enough around training. If you are very active, your needs are probably higher than you think.
6. Poor sleep (and how you use caffeine)
No diet can make up for consistently short or poor-quality sleep. Most adults need 7 to 9 hours.
What you eat and drink affects sleep too. Caffeine can stay in your system for many hours, so an afternoon coffee may still be disrupting your sleep at midnight. Alcohol helps you fall asleep but reduces sleep quality later in the night. Late, heavy dinners (very normal in Spain) can also make it harder to rest well.
If you want to go deeper, read my post on Why We Sleep: practical lessons from Matthew Walker, and my guide on caffeine timing and dosage.
What helps: A caffeine cut-off around early afternoon, a lighter evening meal where possible, and a consistent sleep and wake time.
7. Mild dehydration
Even mild dehydration can lower mood, increase feelings of fatigue and make concentration harder. In one study, losing just over 1% of body weight in fluid was enough to make young women feel more tired.
Hot summers, exercise, coffee-heavy mornings and simply forgetting to drink all add up.
What helps: Keep water visible on your desk, drink with every meal, and pay extra attention in summer or when training. My post on electrolytes and hydration explains when water alone is enough and when it is not.
8. An underactive thyroid
Your thyroid sets your body’s energy pace. When it is underactive, common signs include tiredness, feeling cold, constipation, dry skin and weight gain.
This is a medical condition that needs diagnosis and treatment by a doctor, but nutrition plays a supporting role. The thyroid relies on iodine, selenium and iron to work well.
What helps: Get tested if symptoms fit, and include iodised salt, fish, eggs and a couple of Brazil nuts a week as part of a balanced diet. Avoid high-dose iodine or selenium supplements unless advised.
Which blood tests should you ask for?
If you have felt tired for more than a few weeks, ask your GP (médico de cabecera) about these tests:
Full blood count (FBC): checks for anaemia.
Ferritin and iron studies: shows your iron stores, not just haemoglobin. Ideally alongside CRP, as inflammation can falsely raise ferritin.
Vitamin B12 and folate: active B12 (holotranscobalamin) can be more informative if available.
Vitamin D (25-hydroxyvitamin D): especially at the end of winter.
TSH (and ideally free T4): to check thyroid function.
HbA1c or fasting glucose: for blood sugar regulation.
Coeliac screen (tTG-IgA): particularly if you also have low iron or digestive symptoms.
A tip: Always ask for a copy of your results with the reference ranges. A result at the very low end of “normal” can still be worth discussing.
It is also worth being honest: in many people with fatigue, blood tests come back normal. That does not mean nothing is wrong. It often points back to the everyday factors above (meals, sleep, stress, hydration), which is where nutritional therapy can make the biggest difference.
When fatigue needs a doctor first
Please see your GP promptly if tiredness comes with unexplained weight loss, fever or night sweats, breathlessness, chest pain, very heavy periods, blood in your stool, or persistent low mood or loss of interest in things you used to enjoy.
The bottom line
Feeling tired all the time is common, but it should not be your normal. Start with the basics: regular balanced meals, enough protein, water, sleep and smarter caffeine. Then test rather than guess. If a supplement is needed, quality matters, so read my guide to choosing high-quality supplements in Spain before you buy.
If you would like help making sense of your blood results and building a plan that fits your life, book a free call and let’s find out what is behind your low energy.
FAQ
Why am I always tired even though I sleep enough?
Common nutritional reasons include low iron stores, low vitamin B12 or vitamin D, blood sugar swings from irregular or sugary meals, not eating enough, and mild dehydration. Sleep quality also matters, not just hours in bed. Caffeine late in the day and alcohol can reduce deep sleep without you noticing.
What blood tests should I ask for if I am always tired?
Useful tests include a full blood count, ferritin and iron studies, vitamin B12 and folate, vitamin D (25-hydroxyvitamin D), thyroid function (TSH and free T4), HbA1c or fasting glucose, and a coeliac screen if you also have low iron or digestive symptoms. Your GP will decide what is appropriate for you.
Can low iron cause fatigue without anaemia?
Yes. You can have low iron stores (low ferritin) while your haemoglobin is still normal. Randomised trials in women with unexplained fatigue and ferritin below 50 µg/L found that iron supplementation reduced fatigue compared with placebo. Never take iron without testing first, as excess iron can build up in the body and cause iron overload.
Is vitamin D deficiency really common in Spain?
Surprisingly, yes. A large Spanish population study found that about one in three adults may be at risk of vitamin D deficiency. Spending most of the day indoors, using sunscreen, and the darker winter months all contribute. A simple blood test tells you where you stand.
Do sugary snacks give you energy?
Not really. Research suggests that within an hour of eating sugar, people tend to feel more tired and less alert rather than more energised. Meals that combine protein, fibre and healthy fats give steadier energy throughout the day.
When should I see a doctor about tiredness?
See your GP if tiredness lasts more than a few weeks, or straight away if it comes with unexplained weight loss, fever or night sweats, breathlessness, chest pain, very heavy periods, blood in your stool, or persistent low mood.
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