Sunlight, composed of radiation in the form of electromagnetic energy at mixed wavelength is essential to the continued survival of living creatures on Earth, whether it be autotrophs using energy from sunlight to synthesize food via photosynthesis or heterotrophs using light to obtain food indirectly. The main source of Vitamin D that is so beneficial for humans is through sunlight and it helps keep the body especially the bone and neurological functions in good condition. While there are widely accepted benefits, it is equally imperative to be wary of the risk that comes with persistent sun exposure. Carcinomas of the skin contribute to 1.6% of all world-wide cancers and ultraviolet light that forms part of the spectrum of sunlight has been implicated as a carcinogen.
The essential necessity that leads to the successful survival of mankind has long been disputed. Air, food, water, shelter and even love have been considered fundamental to our continued existence. However, to state the Sun is what maintains life on Earth is no mere exaggeration as life has evolved and flourished under its watchful glare.
In strictly scientific terms, sunlight is composed of radiation from the sun in the form of electromagnetic energy at mixed wavelengths, ranging from infrared, visible light to ultraviolet light, all traveling at a speed of around 3.0 x 108 m/s in vacuum. Albert Einstein referred to this form of solar energy as photons, where its characteristics such as colour and energy level are dependent on its wavelength.
This expanse of electromagnetic energy differs in its intensity depending on the latitude, the time of year, and time of day. It peaks at noon when the Earth’s surface is inclined perpendicular to the sunlight so it collects the maximum amount of sunlight.
Each day, enough sunlight falls on the earth’s surface to meet the world’s energy demand. Many creatures, plants, animals and humans have established physiological reactions in response to the sun’s spectral idiosyncrasies, taking into consideration any daily or seasonal variations.
Autotrophs, such as plants, use energy from sunlight to synthesize their own food from inorganic substances via a process called photosynthesis.
Heterotrophs, such as animals, use light to obtain food in an indirect manner, either by consuming autotrophs, by consuming their products, or by consuming other heterotrophs. Through a process of cellular respiration, the heterotrophs convert the by-products of their feed to give them energy needed for survival.
There are crucial health implications to human exposure of solar radiation.
The main source of vitamin D is through sunlight. Adequate vitamin D status is fundamental for all round good health. Vitamin D is necessary for bone, joint, muscle and neurological function. Low levels are associated with a number of complications and an increased risk of chronic diseases.
However, there is growing evidence of harm that comes from over-exposure to sunlight. Skin cancers including malignant melanoma are among the more severe health effects.
Mark Twain famously quoted, ‘Too much of anything is bad’. It has become imperative to find a harmony between achieving enough sun exposure and avoiding an increase in risk of skin cancer.
Vitamin D, first discovered in 1920, is part of a group of fat soluble chemicals. Several forms of Vitamin D exists but the two major forms vital for humans are Vitamin D3 (cholecalciferol) and Vitamin D2 (ergocalciferol). The epidermis of the skin has the capability of synthesizing Vitamin D3. The production takes place through the photochemical process of UV radiation of type B (UVB) with a wavelength between 290-315 nm, penetrating skin and leading to the formation of cholecalciferol.
7-dehydrocholesterol, is the functioning provitamin in the skin, found in its highest concentration in the basal and spinous layer of the epidermis. In normal circumstances, about 25–50 mg/cm2 of 7-dehydrocholesterol is formed, which is enough to meet body’s requirements.
In the presence of UVB, 7-dehydrocholesterol is converted to the intermediate isomer pre-vitamin D3, which undergoes spontaneous isomerisation to cholecalciferol (Vitamin D3)
There are many causes to consider when determining important factors that drive the generation of Vitamin D. These include season, the quantity of melanin in an individual’s skin acting as a barrier to effective UV light absorption, sunscreen, latitude, time of day, environmental factors such as cloud cover and smog.
The Liver and Kidney are responsible for the conversion of Cholecalciferol to its active metabolite – Calcitriol by action of enzymes in the Liver (Vitamin D 25-hydroxylase) and Kidney (25-Hydroxyvitamin D3 1-alpha-hydroxylase)
The actions of calcitriol are considerable. In the intestine, calcitriol causes an increase in calcium and phosphorous absorption and a decrease in magnesium absorption. Indirectly, through its influence in calcium homeostasis and regulation, calcitriol increases the mineralization of bone and preserves its density. Hence, Vitamin D maintains a normal serum balance of calcium and phosphate through the response of the active metabolites on target organs: the intestine, the bone and parathyroid gland.
Without an adequate level and intake of Vitamin D, the body can only absorb 10-15% of dietary calcium leading to a state of low calcium stores called hypocalcemia. Therefore, deteriorating levels of vitamin D predisposes a child to Rickets and the adult to its milder form – Osteomalacia.
Osteomalacia (adults) and rickets (children) share the same underlying process of failure of the mineralization process of the normal bone tissue.
Presenting symptoms of Osteomalacia include diffuse pain of the skeleton, bone tenderness, weakness of the muscles and difficulty in walking.
Rickets is among the most frequent childhood diseases in developing countries. The dominant cause is vitamin D deficiency. Symptoms of Rickets include tender bone, dental defects, deformity of the cranium and spine, problems with growth and weakness of the muscle.
Not surprisingly, the treatment often focuses on increasing dietary intake as well as increasing exposure to Ultraviolet B light
Osteoporosis is defined as a decrease in bone density of bone, which is normally mineralized causing thinning and increased porosity of the bone. The remaining bone is fragile with a high risk of fractures and pain.
The production of Vitamin D decreases during the winter due to lower intensity, and it has been estimated that around 80% of the UK population do not get sufficient exposure to UVB for Vitamin D synthesis. Thus, people have to rely on dietary sources.
It has been theorized that levels of at least 50 – 75 nmol/L of Vitamin D is optimal to sustain bone form.
Strong links have been found between poor Vitamin D condition and bowel cancer, metabolic syndrome, obesity, ischemic heart disease and Type 2 Diabetes.
One study conducted by Dr. Thomas Wang revealed that individuals with low vitamin D levels had a 60% increase in incidence of heart attacks and strokes. The theory behind this could be postulated to Vitamin D reducing arterial calcification by diverting calcium to bones and teeth instead of soft tissues such as the arteries. Arterial calcifications reduce the diameter of the arteries, for example in the coronary arteries, causing a diminished flow of blood to areas of the heart muscle supplied by the coronary arteries. This indicates a strong disposition to future heart attacks.
Seasonal Affective Disorder (SAD) is a type of mood disorder caused by decreasing light levels. It is a winter depressive episode common in areas with low light levels such as Scandinavia.
The suprachiasmatic nucleus in the hypothalamus oversees the electromagnetic energy that penetrates the eye, which is then used to coordinate the output of hormones such as melatonin and serotonin, within the endocrine system. One of the many theories to the formation of SAD in certain individuals is an imbalance in this delicate system of hormones and regulation by light.
People with SAD suffer from lethargy, lack of interest in activities, weight gain, slowing of psychomotor, anxiety and low motivation.
Although the importance of sunlight and Vitamin D cannot be taken lightly, it is advisable to limit the skin being over-exposed to UV rays from the sun.
Melanoma of the skin contributes to 1.6% of all cancers world-wide. The Department of Health attributes that 1.5 million skin cancers are diagnosed annually in USA. In the UK, 2000 people die from the disease each year.
Carcinogenesis is the course of formation of cancer is a multistage process, where a series of genetic and external influences leads to the emergence of mutated cancer cells.
Ultraviolet light of both A and B type have been incriminated as carcinogens. The two wavelengths of radiation (UVA 320–400 nm) infiltrate differing depths of epidermis and dermis of the skin. UVA has the capability of deeper penetration to reach lower levels and causes oxidative damage to components of the skin cells.
UVB, on the other hand, interacts and causes direct damage to the DNA molecule. This damages genetic information carried by the DNA causing molecular lesions and photoproducts leading to the development of skin cancer. The two photoproducts are cyclobutane yrimidine dimer (CPD) and 6-4 pyrimidine –pyrimidone and they execute their destructive behavior by interfering with DNA replication and causing specific mutations.
Actinic (solar) keratosis is common to those exposed to the sun in a frequent manner. It is an area of crusty, scaly skin on sun exposed areas such as the face, neck and scalp. While it causes no damage of sorts, it is considered to be a ‘premalignant lesion’. If it is not addressed and treated, it can transform into cancer.
Melanomas are cancers of the skin arising from pigmented cells called melanocytes found between the epidermis and dermis. Melanoma is usually caused by damage to the DNA through production of photoproducts. Melanomas can be aggressive and spread to distant areas and organs, making the condition incurable and increasing the mortality rate.
Basal cell carcinoma is the most common type of skin cancer and arises from the basal layer of the epidermis. Intense, intermittent exposure to the sunlight has been implicated as a strong predisposition for developing Basal cell carcinoma. The prognosis for patients with this type of cancer is highly favourable and has a very strong curative rate.
In conclusion, cultural attitudes and behavioural changes in the 20th century have seen an increase in popularity of tanning and sun bathing and lax reactions to the growing threat of skin cancer. Public education is vital not only to reduce the risks posed by sunlight but also to understand that it would be equally harmful to avoid it. Balance and moderation is the key to attain the maximum benefits that the sun gives us.