VITAMIN A AND ITS DERIVATIVES AND THEIR SOURCES
Vitamin A is a fat-soluble vitamin that is available in different forms known as retinoids. These forms include retinol (alcohol), retinal (aldehyde), retinoic acid, and retinyl esters. Retinoic acid is unstable and can be converted into retinol or retinal through reversible reactions.
PRE VITAMIN
Pre-vitamins are those forms of vitamin A that are directly absorbed from the small intestine and do not require conversion to act as vitamin A. Retinol and retinyl esters are pre-vitamins. Dairy products like milk, butter, cheese, fish, organ meats such as liver, and eggs are pre-vitamin sources in the diet.
PRO VITAMIN
Pro-vitamins are plant pigments that are inactive precursors and later converted into vitamin A in the small intestine. They are mostly carotenoids, including alpha-carotene, beta-carotene, and beta-cryptoxanthin.
NON PRO VITAMIN
Non provitamins include lycopene, lutein, and zeaxanthin. Non-provitamins are carotenoids that are not involved in the formation of vitamin A. Carotenoids present in the cell wall of fruits are easily absorbed, unlike those in dark green leafy vegetables and carrots, which are not readily released into the body.
VITAMIN A SUPPLEMENTS
Vitamin A supplements mostly contain a combination of pre-vitamin and pro-vitamin forms.
VITAMIN A DEFICIENCY AND MALIGNANCY
Vitamin A is a powerful antioxidant and plays a critical role in cell differentiation, so it was once considered a very effective preventive micronutrient against cancer. The following roles of vitamin A in the body support the statement that its deficiency may contribute to cancer development.
VITAMIN A AND CELL DIFFERENTIATION
Vitamin A has a vital role in controlling cell differentiation processes. Loss of this control results in abnormal cell growth, which is malignancy in other words, the formation of cancerous cells.
VITAMIN A AND OXIDATION
Vitamin A is a strong antioxidant. Oxidation, peroxidation, or auto-oxidation causes gradual changes in cells, initiating the process of carcinogenesis (cancer formation). Vitamin A inhibits this oxidation process because of its high carbon content and multiple double bonds.
VITAMIN A AND KERATIN
As discussed earlier, vitamin A controls the process of cell differentiation. Keratin is a protein that helps form tissues such as hair, nails, and the outer layer of the skin. It is also present in epithelial cells lining organs and glands such as the lungs, breast, colon, bladder, and head and neck. Vitamin A deficiency induces specific keratin species and causes morphological changes in epithelial cells, leading to abnormal cell growth. Keratin can act as a tumor marker, as certain keratin types may be found in higher-than-normal amounts in various cancers.
GLYCOSYLATED DERIVATIVES OF VITAMIN A
Due to pollution, genetic disorders, toxins, and an imbalanced diet, continuous cellular changes can occur, especially in individuals with vitamin A deficiency. Vitamin A is required for the formation of mucus (mucus polysaccharides) and glycoproteins, which keep epithelial surfaces smooth and healthy.
Retinol, after receiving mannose, becomes phosphorylated to form mannosyl retinyl phosphate, a glycosylated derivative of vitamin A that inhibits certain cellular changes and prevents the formation of cancerous cells.
VITAMIN A AND NATURAL KILLER CELLS
Natural killer (NK) cells are an important part of the immune system. They are effective against infections and cancer. If there is any abnormal cell, NK cells target and destroy it. Vitamin A is required for maintaining the number and activity of natural killer cells.
HOW VITAMIN A DEFICIENCY INDUCES CANCER IN SMOKERS
Cigarette smoke decreases vitamin A levels in the body. Due to this deficiency, neoplastic changes can occur in the epithelial lining of the trachea and bronchi. Vitamin A deficiency reduces the expression of retinoic acid beta receptors, which promote transcription of tumor suppressor genes, and increases the expression of retinoic acid alpha receptors. This imbalance increases the risk of lung cancer in smokers.
BETA-CAROTENE SUPPLEMENTS AND CANCER
Increased intake of vitamin supplements can raise the risk of lung cancer. Many people use these supplements to improve immunity and eyesight, but experts warn otherwise.
PEKING UNIVERSITY OF CHINA
Experts from Peking University of China warn that vitamin supplements may be a causative factor for lung cancer. Recent research found that excessive use of vitamin A supplements can increase the risk of uterine cancer, and for the first time, a link was established between vitamin A supplements and lung cancer.
FRONTIERS IN NUTRITION
According to research published in Frontiers in Nutrition, an increased intake of vitamin A supplements can raise the risk of squamous cell and adenocarcinoma lung cancers. Lung cancer is a major cause of death in Britain. Every year, approximately 47,000 people in Britain are diagnosed with lung cancer, and around 34,700 die because of it. Squamous cell carcinoma and adenocarcinoma are the most common types.
INTERNATIONAL AGENCY FOR RESEARCH ON CANCER (IARC)
In 1998, IARC concluded that supplements of beta-carotene at high doses (≥15 mg/day) lose their cancer-preventive activity. High doses of beta-carotene increased the risk of lung cancer among smokers and asbestos workers.
WORLD CANCER RESEARCH FUND (WCRF)
In 2007, the WCRF concluded that there is a valid relationship between beta-carotene supplements and an increased risk of lung cancer, based on studies showing a noticeable interaction between beta-carotene, smoking, and genotype.
Smokers lacking the carcinogen-detoxifying enzymes glutathione-S-transferase 1 and 2 (GST1 and GST2) due to genetic variations have a greater risk of lung cancer. High-dose supplements of vitamin A (greater than 20 mg/day of beta-carotene) in smokers were associated with higher lung cancer risk than in smokers taking lower doses.
According to WCRF, beta-carotene supplements also showed unexpected effects on other cancers such as prostate and non-melanoma skin cancers.
A META-ANALYSIS (2010)
A 2010 meta-analysis indicated a convincing relationship between high doses of vitamin A supplements and increased lung cancer mortality. Other studies have shown strong associations between beta-carotene supplements and cancers such as bladder cancer, stomach cancer, and bowel adenoma. These effects are amplified in smokers and asbestos workers.
WORLD HEALTH ORGANIZATION (WHO)
According to a 2003 WHO expert report, there is insufficient evidence that carotenoids reduce the risk of cancer. WHO did not specify whether this insufficient evidence refers to vitamin A supplements or dietary carotenoids.
DIETARY BETA-CAROTENE AND RISK OF CANCER
Dietary beta carotene reduces the risk of lung and other cancers. Several studies support the idea that dietary beta-carotene decreases cancer risk.
UNITED STATES OF AMERICA
In the USA, consumption of raw fruits and vegetables (dietary beta-carotene) reduces the risk of lung cancer in non-smoking men and women.
WORLD CANCER RESEARCH FUND
According to WCRF, dietary carotenoids are probably protective against various cancers such as lung, mouth, pharynx, larynx, and esophageal cancers.
RECOMMENDATIONS OF THE CANCER COUNCIL
The Cancer Council recommends meeting nutritional requirements from food rather than supplements.
Avoid taking high doses (greater than 18 mg/day) of beta-carotene, especially for smokers.
Consume a wide variety of fruits and vegetables to obtain maximum benefits.
CONCLUSION
In conclusion, vitamin A deficiency can contribute to cancer because of its crucial role in cell differentiation and regulation. However, vitamin A supplements especially high-dose beta-carotene can increase the risk of lung cancer, particularly among smokers.
In contrast, dietary carotenoids reduce cancer risk, especially lung cancer. Vitamin A shows dose-dependent effects in smokers. Extensive research is still required to establish the safe limit of beta-carotene supplementation and to clarify the differences between natural (dietary) and synthetic (supplemental) beta-carotene. Many questions remain about the precise role of vitamin A in cancer prevention and development.
FAQs
1. Is vitamin A deficiency linked to cancer?
Yes. Vitamin A plays a key role in cell differentiation and immune regulation. Its deficiency can lead to uncontrolled cell growth and increase the risk of certain cancers, especially in epithelial tissues such as the lungs and respiratory tract.
2. Are beta-carotene supplements carcinogenic?
High-dose beta-carotene supplements (≥15–20 mg/day) have been shown to increase the risk of lung cancer, particularly in smokers and asbestos workers. This effect has been confirmed by studies from IARC, WCRF, and multiple meta-analyses.
3. Is dietary beta-carotene safe?
Yes. Beta-carotene from natural food sources such as carrots, pumpkins, and leafy vegetables is safe and may help reduce the risk of several cancers, including lung, mouth, and esophageal cancers.
4. Should smokers take vitamin A or beta-carotene supplements?
No. Smokers should avoid high-dose vitamin A or beta-carotene supplements because they increase the risk of lung cancer. Instead, obtaining beta-carotene through fruits and vegetables is recommended.
5. What is the difference between pre-vitamin and pro-vitamin A?
Pre-vitamin A (retinol, retinyl esters) is the active form found in animal sources, while pro-vitamin A (carotenoids such as beta-carotene) is a plant pigment that converts into vitamin A inside the body.
DISCLAIMER
This article is for educational purposes only and is not a substitute for professional medical advice. Always consult a qualified healthcare provider or dietitian before taking any vitamin or dietary supplement, especially if you are a smoker, have chronic illness, or are at risk for cancer. Misuse or high-dose supplementation of vitamin A or beta-carotene can be harmful.
CALL TO ACTION
To protect your health:
• Focus on a balanced diet rich in colorful fruits and vegetables rather than high-dose supplements.
• Avoid smoking and limit exposure to environmental toxins that can deplete vitamin A.
• If you are considering supplementation, consult your healthcare professional for personalized advice based on your health status and diet.
Stay informed, eat naturally, and let food be your strongest line of defense against disease.
REFERENCES
1. Peking University of China Research findings linking vitamin A supplements with increased risk of lung and uterine cancer.
2. Frontiers in Nutrition Study reporting higher incidence of squamous cell carcinoma and adenocarcinoma of the lung among high-dose supplement users.
3. International Agency for Research on Cancer (IARC, 1998) Concluded that high-dose beta-carotene (≥15 mg/day) increases lung cancer risk in smokers and asbestos workers.
4. World Cancer Research Fund (WCRF, 2007) Reported an association between beta-carotene supplementation, smoking, and genetic susceptibility to cancer.
5. Meta-Analysis (2010) Demonstrated a convincing relationship between high-dose vitamin A supplements and increased lung cancer mortality.
6. World Health Organization (WHO, 2003) Found insufficient evidence that carotenoids reduce cancer risk.
7. Cancer Council Recommendations Advocated obtaining nutrients from food sources rather than supplements, and avoiding beta-carotene doses above 18 mg/day, especially for smokers. 8. U.S. Observational Studies Reported lower lung cancer rates in non-smokers consuming raw fruits and vegetables rich in beta carotene.