Why am I always so tired?

1. Iron deficiency

When iron is low, you can’t make enough healthy red blood cells, so your organs and muscles get less oxygen. The outcome? You feel tired, weak, and short of breath even doing things that used to feel “easy.” 

Iron plays a starring role in making haemoglobin, the protein in red blood cells that carries oxygen from your lungs to every tissue in your body. (NHS: Iron deficiency anaemia)

What’s tricky is that iron deficiency doesn’t always show up as full-blown anaemia right away. You might have “latent” or non-anaemic iron deficiency, where your iron stores are depleted but your haemoglobin is still in the normal range. Even that early stage can cause symptoms of fatigue, reduced concentration, or struggling to exercise. (Patient Info: Non-anaemic Iron Deficiency)

Typical causes include:

• Blood loss from periods, or hidden bleeding from the gut

• Eating less iron than you need, especially common in vegetarians and vegans

• A reduced ability to absorb the iron you eat, usually caused by intestinal diseases.

Some hints that iron deficiency might be an issue include persistent fatigue, pale skin or inner eyelids, shortness of breath on exertion, heart palpitations, dizziness, brittle nails, and, in some people, cravings to eat things that are not food (medically called pica) - in iron deficiency, the commonest one is a desire to chew and eat ice cubes. In older people, constant itching skin can also occur. 

If you have several of these symptoms, consider asking your GP for an investigation. Remember that iron supplements you can buy yourself are for preventing iron deficiency. If you are anaemic, you will be prescribed iron at a significantly higher dose. 

2. Vitamin B12 deficiency

Vitamin B12 (cobalamin) is vital for making healthy red blood cells and for maintaining nerve health. A deficiency leads to megaloblastic anaemia, where newly formed red blood cells are abnormally large but nonfunctional. It can also cause neurological symptoms. Both of these sap energy and vitality.

One complicating factor: the body stores B12 in the liver, so it can take many years for deficiency to emerge. Often, symptoms develop slowly and may be dismissed as “just aging” or stress.

Risk groups include:

• Vegans or strict vegetarians (since B12 is mostly found in animal products)

• People with absorption disorders (e.g. pernicious anaemia, atrophic gastritis)

• Anyone using long-term acid-lowering medications, or who've had gastrointestinal surgery

Symptoms of B12 deficiency, according to the NHS, can include fatigue, breathlessness, pale skin, but also tingling or numbness in hands/feet, balance issues, memory problems, and mood changes. Importantly, B12 deficiency can happen without obvious anaemia.

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3. Folate (Vitamin B9) deficiency

When folate is low, red blood cell production becomes inefficient, contributing to megaloblastic anaemia and symptoms of fatigue, weakness, and sometimes poor concentration.

Folate (or folic acid in its supplement form) works hand-in-glove with B12 in DNA synthesis, red blood cell formation, and cellular repair. 

Risk factors include:

• Not eating enough vegetables, legumes, leafy green foods, according to the NHS

• Needing more folate than usual (e.g. because of pregnancy, growth spurts)

• Inability to absorb folate properly, usually caused by conditions affecting gut health

Symptoms generally overlap with B12 deficiency: tiredness, pale skin, glossitis (a swollen, red tongue) and irritability. 

4. Vitamin D deficiency

We often hear about vitamin D in relation to bones, but low vitamin D is also associated with muscle weakness, aches, slowed recovery, and fatigue. It may also be linked to depression and reduced ability to exercise. 

(Patient: Vitamin D deficiency)

Why is this so common, especially in northern countries?

• In autumn and winter, sunlight is too weak

• Many people spend little time outdoors or cover up their skin

• People with darker skin need more sunlight to make the same amount of vitamin D

• We don’t eat enough oily fish, eggs and other sources

In the UK, most people are advised to consider a vitamin D supplement from October to March. (NHS: Vitamin D)

If your level is very low, symptoms might include persistent fatigue, muscle aches, bone pain, and slower recovery from exercise. 

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5. Low magnesium status

Magnesium is essential for multiple functions around the body that maintain normal energy levels. It supports the metabolism in turning food into energy. It continues supporting cellular energy creation throughout a series of chemical reactions. And it’s also the master electrolyte that makes sure our muscles and other organs can receive and respond to signals to keep going. 

It is a cofactor in hundreds of enzyme systems, including those that produce ATP (the cell’s energy currency) and it helps regulate nerve and muscle function, electrolyte balance, and nerve signalling. (Gloucestershire Hospitals: Hypomagnesaemia guideline)

In severe magnesium deficiency, The British Medical Journal reports that symptoms can include confusion, tremors, muscle spasms, and arrhythmias. But even milder chronic deficits (especially inside cells) can manifest as fatigue, muscle cramps, poor sleep, and irritability. (Cleveland Clinic: Signs of magnesium deficiency)

Magnesium deficiency may arise from:

• A diet low in magnesium - this is very common in people who don’t often eat nuts, seeds, legumes and whole grains

• Gastrointestinal losses (e.g. diarrhea, malabsorption)

• Increased urinary loss (e.g. with diuretics, certain medications)

• Alcohol use, chronic stress

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6. Insufficient B-vitamins

The B-vitamin family is critical for turning the fuel you eat - carbs, fats and proteins - into usable energy called ATP. They also help in nerve function, skin health, and hormone pathways.

The most important energy-supporting vitamins in a B-complex supplement are B1, B2, B3, B5, B6. 

If your diet is highly processed or limited, or you're under chronic stress, your B-vitamin stores may run low. The result: sluggishness, irritability, lack of resilience, poor stamina, and slower recovery, symptoms easily dismissed as “being busy” or “just life.”

Because their deficiencies tend to produce vague, overlapping symptoms, they often fly under the radar. A well-balanced diet rich in whole grains, legumes, nuts, seeds, meats and dairy tends to supply sufficient B vitamins in most people, reports the NHS. However, B vitamins cannot be stored in the body, and if your intake is low of you are in need of higher than average servings, a supplement may help. 

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7. Low protein intake

Protein is much more than “muscle food.” Amino acids—the building blocks of proteins—are needed to make key components of the energy machinery in your body cells (called mitochondria).

If protein intake is too low over a prolonged period (e.g. restrictive diets, illness, appetite loss), your body may struggle to repair tissues, maintain muscle mass, and sustain metabolic processes. The result can be a baseline feeling of fatigue or inability to “bounce back” after exertion.

You might notice weak muscles, slower athletic recovery, frequent infections (immune compromise), or feeling flat even after adequate sleep. In more extreme cases, muscle wasting or weakness becomes obvious.

8. Blood sugar fluctuations 

Your brain and muscles run largely on glucose. If you skip meals, your calorie intake is too low, or your diet is high in refined carbohydrates like sugar and white bread but low in fibre or protein, you risk spikes and crashes in blood sugar. This puts you on a rollercoaster that taxes your energy reserves and gives you constant crashes. 

A crash in glucose can lead to symptoms like dizziness, irritability, “brain fog,” fatigue, and cravings. Repeatedly under-eating also causes hormonal adaptations (lower thyroid, suppressed metabolic rate) and forces the body into energy conservation mode, resulting in chronic low output.

A more balanced meal pattern (smaller frequent meals, combined carbs + protein + fibre + fats) can help smooth blood sugar and maintain steady energy levels across the day.

9. Dehydration & electrolyte imbalance

It’s easy to underestimate how much water and electrolytes affect your energy. Even mild dehydration (1-2 % of body weight) slows down cognition, impairs concentration, weakens physical performance, and makes everything feel harder.

Electrolytes—especially sodium, potassium, magnesium, and calcium—regulate nerve impulses, muscle contraction, and fluid balance. Losses through sweat, illness, diuretics or simply drinking “water only” without salts can lead to subtle imbalances that worsen fatigue.

Symptoms you may notice: dry mouth, headache, lightheadedness, muscle cramps, reduced stamina. The remedy? Hydrate regularly (don’t wait until you’re thirsty), include natural electrolyte sources (a pinch of salt, potassium-rich foods, nuts) especially when exercising or in hot weather.

10. Under-eating and restrictive dieting

When your body is starved of calories and nutrients over time, it enters a survival mode. Basal metabolic rate drops, hormonal systems (thyroid, reproductive hormones, cortisol) shift, muscle mass may decline, and repair processes slow. The result is a pervasive sense of tiredness, low resilience, and inability to sustain normal energy demands.

Malnutrition can result from eating disorders, reports the NHS, or food insecurity, chronic illness, poor appetite, or gastrointestinal conditions. Its signs include weight loss (sometimes hidden), repeated infections, muscle wasting, poor wound healing, and persistent lethargy.

Bringing it together: What to do next

Feeling constantly low in energy is rarely “just in your head.” Nutrition plays a central role in how your body powers itself. 

Fatigue and low energy are among the most common complaints in GP appointments. While factors like poor sleep, stress, thyroid problems, mental health issues and chronic illness can be at play, nutritional shortfalls frequently underlie or accompany the problem in subtle ways.

While the list above may seem long, in practice you’ll often find that one or two causes are especially relevant to you.

For a more useful appointment with your GP, consider the following: 

1. Track your symptoms. Note when you feel most tired, and how severely this affects you. Try to be as specific as possible about the symptoms you feel. Give examples of what the fatigue stops you doing in daily life. 

2. Track your diet and sleep for a few weeks. Note what and how much you eat, and  whether you skip meals. Note when you go to bed, when you get up, and whether you are awake in the night. 

3. Make targeted nutritional adjustments based on this. You may realise you need to add iron-rich foods, eat more protein, or hydrate more regularly. 

4. Talk to your GP if the fatigue persists despite your efforts. Your notes should help move the conversation forward with better information for your doctor. 

If any of your symptoms are severe, including breathlessness, chest pain, fainting, weight loss, or neurological signs, seek medical advice immediately. 

Energy is a delicate balance, like a symphony of biochemical pathways. A deficiency in one key player (iron, B12, D, magnesium, etc.) can throw the whole performance off course.

By understanding these 10 nutritional causes of low energy, you can begin to see where things might be going off-track in your own body. Use this as a compass: look for patterns, have thoughtful conversations with your doctor, and take small but consistent steps toward rebalancing your nutrition.