vitamin D as hormone?
Sunlight as the ultimate energy source
- Powering the food chain: At the base of every terrestrial and marine food chain are “primary producers” such as green plants and phytoplankton. These organisms capture energy from the sun and use it to turn water and carbon dioxide into sugars in a process called photosynthesis . National Geographic notes that the sun is the primary source of energy for almost every ecosystem; plants use solar energy to make glucose, and when herbivores eat plants and carnivores eat herbivores, energy flows from one trophic level to the next . In practical terms, the peanut butter sandwich or bowl of rice you eat ultimately originated as solar energy stored in the carbon–hydrogen bonds of carbohydrates, fats and proteins.
- Extracting energy in our cells: Once plants (or animals that ate plants) are consumed, human digestive enzymes break down proteins, fats and carbohydrates into amino acids, fatty acids and simple sugars . In our cells, glucose is then oxidized in controlled steps. During glycolysis, each glucose molecule is converted to two molecules of pyruvate, producing a small amount of ATP and reduced carrier molecules (NADH) . Pyruvate enters mitochondria where it is converted to acetyl‑CoA; the acetyl group is oxidized to CO₂ in the citric‑acid cycle, and electrons are passed through the electron‑transport chain in the inner mitochondrial membrane . The energy released from these electrons drives oxidative phosphorylation – the phosphorylation of ADP to ATP – which captures roughly half of the energy stored in the original food molecules . In essence, the energy that plants packaged using sunlight is unpacked by our mitochondria to power muscle contraction, nerve impulses, biosynthesis and heat production.
Vitamin D – the direct photochemical link
Humans do have a direct photochemical interaction with sunlight: the production of vitamin D. When ultraviolet‑B (UV‑B) radiation (wavelengths 290–315 nm) hits the skin, it is absorbed by 7‑dehydrocholesterol (a cholesterol precursor) in the epidermis. This triggers a photolysis reaction converting 7‑dehydrocholesterol to previtamin D₃, which spontaneously isomerizes to vitamin D₃ . Vitamin D₃ enters the bloodstream and travels to the liver, where it is hydroxylated to 25‑hydroxyvitamin D₃ (calcidiol). In the kidneys, a second hydroxylation produces 1,25‑dihydroxyvitamin D₃ (calcitriol), the active hormone . Calcitriol binds to vitamin D receptors in many tissues and regulates genes involved in calcium and phosphate absorption, bone mineralization, immune function and cell differentiation .
Why evolution tied us to the sun
- Nutritional necessity: Because nearly all of the chemical energy available to heterotrophs (animals, fungi and many bacteria) originates from photosynthesis, life on Earth has evolved around the sun. Our metabolic pathways are geared to oxidize photosynthetically derived carbohydrates and fats to produce ATP, and a constant supply of solar‑powered food keeps us alive and active.
- Hormonal and skeletal health: Vitamin D is not just a vitamin but a hormone that humans cannot obtain in sufficient quantities from food alone; sensible sun exposure is therefore essential. Without adequate calcitriol, intestinal calcium absorption falls, parathyroid hormone rises, and bone mineral is mobilized, leading to rickets in children and osteomalacia in adults . Evolutionary selection favoured skin photochemistry that links sunlight to bone health; this is why melanin pigment adjusts UV‑B penetration and why some sun exposure is beneficial .
Take‑home message
Humans owe their vitality to the sun even though we can’t photosynthesize. Through plants, sunlight becomes the glucose and fats we metabolize into ATP; through vitamin D synthesis, UV‑B light becomes a hormone that keeps our bones strong and influences many physiological processes. In a very real sense, every breath, heartbeat and smile is powered by photons that left the sun eight minutes ago – a beautiful reminder of our connection to the cosmos .