Sources of vitamin A, B, C, D, E and K are essential to understand for maintaining optimal health through diet. Vitamins are organic compounds required in small amounts for normal physiological function, growth and disease prevention. The body cannot produce most vitamins in sufficient quantities, making dietary intake crucial. Fat-soluble vitamins (A, D, E, K) are stored in the liver and adipose tissue, whilst water-soluble vitamins (B complex, C) require regular replenishment. This article explores the primary food sources of these essential vitamins, daily requirements, absorption mechanisms and how to recognise deficiency states requiring medical attention.

Understanding Essential Vitamins: A, B, C, D, E and K

Vitamins are organic compounds essential for normal physiological function, growth, and maintenance of health. Unlike macronutrients, vitamins are required in small quantities but play critical roles in numerous biochemical processes. The human body cannot synthesise most vitamins in adequate amounts, making dietary intake or supplementation necessary to prevent deficiency states.

Vitamins are classified into two main categories based on their solubility properties. Fat-soluble vitamins (A, D, E, and K) are absorbed alongside dietary fats in the small intestine and can be stored in the liver and adipose tissue for extended periods. This storage capacity means deficiencies develop slowly but also increases the risk of toxicity with excessive supplementation. Water-soluble vitamins (B complex and C) are generally not stored in significant amounts (with vitamin B12 being a notable exception, as it can be stored in the liver for several years) and excess quantities are typically excreted in urine, necessitating regular dietary intake.

Each vitamin has distinct biochemical functions. Vitamin A is crucial for vision, immune function, and cellular differentiation. The B complex vitamins (B1, B2, B3, B5, B6, B7, B9, B12) serve as coenzymes in energy metabolism and DNA synthesis. Vitamin C acts as an antioxidant and is essential for collagen synthesis. Vitamin D regulates calcium homeostasis and bone metabolism. Vitamin E provides antioxidant protection to cell membranes, whilst vitamin K is indispensable for blood coagulation and may also play a role in bone health.

Understanding the sources and functions of these vitamins enables individuals to make informed dietary choices and healthcare professionals to identify at-risk populations. The NHS recommends obtaining vitamins primarily through a balanced, varied diet rather than relying on supplements, except in specific circumstances such as pregnancy or diagnosed deficiencies.

Dietary Sources of Fat-Soluble Vitamins (A, D, E, K)

Vitamin A exists in two primary forms: preformed vitamin A (retinol) found in animal products, and provitamin A carotenoids (particularly beta-carotene) present in plant foods. Excellent animal sources include liver (particularly lamb, beef, and chicken liver), oily fish, eggs, and dairy products such as cheese, butter, and fortified milk. The bioavailability of retinol from animal sources is generally higher than that of carotenoids. Plant-based sources rich in beta-carotene include orange and yellow vegetables (carrots, sweet potatoes, butternut squash, pumpkin), dark green leafy vegetables (spinach, kale, spring greens), and orange fruits such as mangoes, apricots, and cantaloupe melon. The body converts beta-carotene to active vitamin A as needed, though conversion efficiency varies between individuals. People who are pregnant or trying to conceive should avoid vitamin A supplements and liver/liver products due to potential risks to the developing baby, and adults should not exceed 1.5 mg per day of retinol from diet and supplements combined.

Vitamin D is unique as it can be synthesised in the skin upon exposure to ultraviolet B (UVB) radiation from sunlight. However, in the UK, sunlight of adequate intensity for vitamin D synthesis is only available from approximately April to September. Dietary sources are limited but include oily fish (salmon, mackerel, sardines, pilchards), red meat, liver, egg yolks, and fortified foods such as breakfast cereals, fat spreads, and some plant-based milk alternatives. The NHS and UK government advise that all UK residents consider taking a daily 10 microgram (400 IU) vitamin D supplement during autumn and winter months. Year-round supplementation is recommended for people with limited sun exposure (those who are housebound or cover their skin) and those with darker skin tones. Adults should not exceed 100 micrograms (4000 IU) of vitamin D per day from all sources.

Vitamin E (tocopherols and tocotrienols) is found predominantly in plant oils (sunflower, rapeseed, olive oil), nuts (almonds, hazelnuts, peanuts), seeds (sunflower seeds), wheat germ, and green leafy vegetables. Vitamin K exists as phylloquinone (K1) in green vegetables (kale, spinach, broccoli, Brussels sprouts, cabbage) and menaquinones (K2) produced by gut bacteria and found in fermented foods, meat, dairy products, and natto (fermented soybeans). People taking warfarin should maintain a consistent intake of vitamin K-rich foods and consult their healthcare provider before taking supplements containing vitamin K. Cooking and processing generally have minimal impact on vitamin K content.

The level of fortification varies between products, so it's advisable to check food labels for specific vitamin content.

Food Sources Rich in Vitamin C and B Complex

Vitamin C (ascorbic acid) is abundant in fresh fruits and vegetables but is heat-sensitive and water-soluble, meaning cooking methods can significantly reduce content. Excellent sources include citrus fruits (oranges, lemons, grapefruit), berries (strawberries, blackcurrants, raspberries), kiwi fruit, peppers (particularly red peppers), tomatoes, and cruciferous vegetables (broccoli, Brussels sprouts, cauliflower). Potatoes, despite moderate vitamin C concentration, contribute significantly to UK dietary intake due to high consumption levels. To maximise vitamin C retention, consume fruits and vegetables raw when possible, or use cooking methods such as steaming or microwaving rather than prolonged boiling.

The B complex vitamins comprise eight distinct compounds with varied food sources. Vitamin B1 (thiamine) is found in wholegrain cereals, fortified breakfast cereals, pork, nuts, and pulses. Vitamin B2 (riboflavin) sources include milk and dairy products, eggs, fortified cereals, and mushrooms. Vitamin B3 (niacin) is present in meat, fish, wheat flour, eggs, and fortified cereals. Vitamin B5 (pantothenic acid) is widely distributed in foods including meat, vegetables, eggs, and wholegrains.

Vitamin B6 (pyridoxine) is found in pork, poultry, fish, bread, eggs, vegetables, peanuts, and milk. Chronic high-dose vitamin B6 supplementation can cause peripheral neuropathy, so supplements should not exceed recommended amounts. Vitamin B7 (biotin) sources include eggs (particularly yolk), nuts, seeds, salmon, and sweet potatoes. Vitamin B9 (folate/folic acid) is abundant in green leafy vegetables, peas, chickpeas, fortified breakfast cereals, and liver. Vitamin B12 (cobalamin) is exclusively found in animal products: meat, fish, dairy products, and eggs, with fortified foods providing the only reliable plant-based sources. Individuals following vegan diets require B12 supplementation or regular consumption of fortified foods to prevent deficiency.

Processing and cooking can reduce B vitamin content, particularly thiamine and folate. In the UK, non-wholemeal wheat flour is fortified with thiamine and niacin, contributing to overall intake. Consuming a variety of wholegrain products, fresh produce, and animal or fortified foods ensures adequate B complex intake for most individuals.

Daily Requirements and Absorption of Key Vitamins

The UK Department of Health establishes Reference Nutrient Intakes (RNIs) representing the amount of a nutrient sufficient for approximately 97% of the population. For adults, RNIs include: vitamin A (700 µg for men, 600 µg for women), vitamin D (10 µg for all adults, as recommended by the Scientific Advisory Committee on Nutrition), vitamin E (no UK RNI, but adults need approximately 4 mg for men, 3 mg for women), vitamin K (no UK RNI, but a safe/adequate intake of approximately 1 µg per kg body weight is suggested), vitamin C (40 mg), and varying amounts for B vitamins depending on the specific compound. Requirements increase during pregnancy, lactation, periods of rapid growth, and in certain medical conditions.

Absorption of fat-soluble vitamins (A, D, E, K) requires adequate dietary fat and normal pancreatic and biliary function. These vitamins are incorporated into micelles in the small intestine and absorbed via passive diffusion or carrier-mediated transport. Conditions affecting fat absorption—such as coeliac disease, Crohn's disease, chronic pancreatitis, or cholestatic liver disease—can impair fat-soluble vitamin absorption, necessitating monitoring and potential supplementation. Certain medications, including orlistat (a lipase inhibitor used for weight management) and bile acid sequestrants, may also reduce absorption. If taking orlistat, it's advisable to take multivitamin supplements at a different time of day to reduce malabsorption.

Water-soluble vitamins (B complex and C) are absorbed primarily in the small intestine through various mechanisms. Vitamin B12 absorption is particularly complex, requiring intrinsic factor (a glycoprotein secreted by gastric parietal cells) for absorption in the terminal ileum. Conditions affecting the stomach (pernicious anaemia, gastrectomy) or terminal ileum (Crohn's disease, surgical resection) can therefore cause B12 deficiency. Metformin, commonly prescribed for type 2 diabetes, may reduce B12 absorption with long-term use. The Medicines and Healthcare products Regulatory Agency (MHRA) advises testing for B12 deficiency if symptoms develop and considering periodic monitoring in those at risk of deficiency.

Alcohol consumption can interfere with absorption and metabolism of several vitamins, particularly thiamine, folate, and vitamin B6. Individuals with alcohol dependence are at substantially increased risk of deficiency and may require supplementation.

Recognising and Addressing Vitamin Deficiencies

Vitamin deficiencies develop when dietary intake, absorption, or utilisation is inadequate to meet physiological requirements. Vitamin A deficiency is rare in the UK but presents with night blindness, dry eyes (xerophthalmia), and increased infection susceptibility. Vitamin D deficiency is relatively common, particularly in at-risk groups (individuals with darker skin, limited sun exposure, or malabsorption disorders), causing rickets in children and osteomalacia in adults, characterised by bone pain, muscle weakness, and increased fracture risk. Vitamin D status is assessed by measuring serum 25-hydroxyvitamin D; deficiency is typically defined as levels below 25 nmol/L, though laboratory reference ranges may vary. The National Institute for Health and Care Excellence (NICE) and NHS recommend vitamin D supplementation for at-risk populations.

Vitamin E deficiency is uncommon but may occur in fat malabsorption syndromes, presenting with peripheral neuropathy, ataxia, and muscle weakness. Vitamin K deficiency causes prolonged bleeding and easy bruising due to impaired coagulation factor synthesis. Newborns receive prophylactic vitamin K to prevent haemorrhagic disease of the newborn, as recommended in NICE guidance (NG194).

Vitamin C deficiency (scurvy) is rare but presents with fatigue, gingivitis, poor wound healing, petechiae, and perifollicular haemorrhages. B vitamin deficiencies produce varied clinical pictures: thiamine deficiency causes beriberi (peripheral neuropathy, heart failure) or Wernicke-Korsakoff syndrome in alcohol dependence; riboflavin deficiency causes angular stomatitis and glossitis; niacin deficiency causes pellagra (dermatitis, diarrhoea, dementia); B6 deficiency causes peripheral neuropathy and anaemia; folate deficiency causes megaloblastic anaemia and is associated with neural tube defects in pregnancy; B12 deficiency causes megaloblastic anaemia and neurological complications including peripheral neuropathy and subacute combined degeneration of the spinal cord. Suspected B12 deficiency with neurological symptoms warrants urgent intramuscular hydroxocobalamin treatment and prompt assessment.

Investigation of suspected deficiency involves clinical assessment and serum vitamin level measurement. Management depends on severity and underlying cause. Mild deficiencies may respond to dietary modification, whilst moderate to severe deficiencies require supplementation. Patients should contact their GP if experiencing persistent fatigue, unusual bleeding or bruising, neurological symptoms (numbness, tingling, balance problems), visual disturbances, or poor wound healing. Pregnant women should take 400 µg folic acid daily from before conception until 12 weeks' gestation to reduce neural tube defect risk. Some women at higher risk (e.g., those with previous neural tube defects, taking antiepileptic medications, with diabetes or malabsorption) require a higher dose of 5 mg daily, available on prescription.

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