Light, as the source of life on earth, not only gives life and color to all things, but also is an important source of information for organisms to perceive the outside world and regulate their own physiological rhythms, metabolism, emotions and cognition and other complex life processes. However, with the rapid development of society and the acceleration of modernization, the widespread application of artificial light sources has changed our lighting environment, including spectrum, light intensity and illumination duration, thus posing new challenges to human health. In recent years, epidemiological surveys have shown that nighttime light pollution is closely related to the increase in the incidence of metabolic diseases such as obesity and diabetes. This suggests that inappropriate light patterns may interfere with the body's normal metabolic function, which in turn can lead to disease. However, the biology behind this has been unknown. In order to solve this puzzle, the mainland scientific research team has carried out innovative research on the mechanism of "photoreceptor regulation of blood glucose metabolism", and has made breakthroughs.
Unraveling the process of light regulating blood sugar metabolism
Through experiments on mice and humans, it was found that after several hours of light stimulation, both during the day and at night, the blood glucose metabolism of mice and humans was significantly reduced. This means that light exposure directly reduces the body's ability to process blood sugar. Mammals sense light through a variety of photoreceptors on the retina, including cones and rods, which are responsible for image perception, and self-sensing retinal ganglion cells (ipRGCs), which are capable of sensing short wavelengths of blue light. Through genetic engineering, the photosensing function of these three cells was turned off one by one, and the study revealed that the effect of light on blood glucose metabolism was mainly mediated by ipRGC.
In the brain, the hypothalamus is the center that regulates the body's metabolism, and the areas with dense connections to ipRGC are the suprachiasmatic nucleus and the supraoptic nucleus. The researchers manipulated these two hypothalamic nuclei separately and found that the supraoptic nucleus is a key node in the photoregulation of blood glucose metabolism. Subsequently, after a series of experiments, a new neural pathway was revealed: from the ipRGC photoreceptor cells in the eye to the oxytocinergic neurons and antidiuretic hormone neurons in the supraoptic nucleus of the hypothalamus, and then to the paraventricular nucleus, the solitary tract nucleus of the brainstem, and the pallilose nucleus of the middle suture, which are responsible for converting the light signal into neural activity that regulates blood glucose metabolism.
After the brain perceives the ambient light signal, the regulation of blood glucose metabolism ultimately needs to be carried out by peripheral tissues and organs. The study found that these nerve signals eventually act on peripheral brown adipose tissue, an important tissue that metabolizes glucose to produce heat to maintain body temperature, through sympathetic nerves emitted by the medulla oblongata. Further experiments confirmed that the reason for the reduction of blood glucose metabolism by light is to inhibit the thermogenesis function of adipose tissue by blocking the connection between sympathetic nerves and brown adipose tissue, and by using thermoneutral ambient temperature to suppress the thermogenic activity of brown adipose tissue.
Are the above-mentioned discoveries and biological mechanisms of photoregulation of blood glucose metabolism also present in humans? To solve this problem, the researchers used the characteristics of ipRGC that are sensitive to short-wave blue light but not long-wave red light, by testing the ability of people to metabolize blood sugar under different wavelengths of light, and using a thermoneutral temperature environment to inhibit the activity of brown fat in the human body. The results showed that consistent with the results of the mouse study, the ability of light to reduce human blood glucose metabolism was also mediated by ipRGC sensing light, which in turn affected the activity of brown adipose tissue.
Taken together, this study uncovers the key role of ipRGC, revealing a complete regulatory neural network from photoreceptors to metabolic effectors. This new "eye-brain-peripheral brown adipose axis" includes the ipRGC in the eye, the supraoptic nucleus of the hypothalamus, the paraventricular nucleus, the nucleus of the solitary tract of the brainstem, and the pallilose nucleus of the middle suture, and acts on the peripheral brown adipose tissue through sympathetic nerves. This study elaborated on the biological mechanism of photoregulation of blood glucose metabolism and expanded the new function of photoreceptor regulation of life processes.
The evolutionary origin and practical significance of photoregulation of blood glucose
Why did living organisms evolve the neurophysiological function of light influencing blood glucose metabolism? One possible guess is that it helps animals quickly adapt to different solar radiation environments to maintain a balanced body temperature. The body's total heat comes from its own heat production and external radiation (sunlight). Outdoors, when the sun is abundant, the heat provided by the sun can replace part of its own heat production to maintain body temperature. Therefore, light energy can rapidly reduce the utilization rate of glucose in fat cells and reduce its own heat production through the "eye-brain-peripheral brown fat axis" pathway discovered in this work. When animals enter places with less solar radiation, such as burrows or tree shades, the animal's body will mobilize brown adipose tissue to quickly consume blood sugar to produce heat through this pathway, compensating for the reduced heat from solar radiation, and keeping the body temperature unchanged.
The study also revealed the physiological basis behind the perception of warm and cold light. In life, short-wave light (blue light) often causes a feeling of coolness, while long-wave light (red light) brings warmth, which may be caused by light regulating the production of heat in adipose tissue. The high sensitivity of ipRGC cells to blue light allows them to effectively inhibit thermogenesis, thus imparting a "cool" experience in a blue light environment, adding a new perspective to the interaction between light and human senses.
More importantly, this finding suggests that a neural mechanism that would otherwise help the body maintain a stable body temperature is likely to contribute to the increase in metabolic diseases in the excessive artificial light pollution of modern industrialized society. It is known that the body's ability to metabolize glucose at night is weaker than that during the day, and light pollution will further reduce the body's ability to metabolize glucose at night through this newly discovered pathway. Repeated and long-term exposure to artificial light sources at night, combined with the habit of eating late-night snacks, will bring a heavy burden on the body's metabolism, which may lead to obesity, diabetes and other metabolic diseases in the long run.
At the same time, the study deepened the understanding of the effects of light on glucose metabolism and revealed the potential association between light pollution and metabolic diseases. In modern society, the widespread use of artificial light sources prolongs the duration of light, which may increase the risk of diabetes and obesity by affecting adipose tissue activity. This discovery is of great significance for basic science and public health, which not only provides a new theoretical support for the prevention and treatment of glucose metabolism disorders caused by light pollution, but also predicts potential treatment directions. Therefore, understanding the metabolic effects of artificial light sources suggests that we need to adjust our indoor lighting design and screen usage habits to reduce health risks.
The future of light and health
In deepening scientific exploration, the scientific community still needs to study more deeply the role of light in regulating the physiological and pathological processes of organisms. This includes understanding how light regulates lipid and protein metabolism through neural mechanisms, and how long-term nocturnal light exposure affects the regulation of plasticity in the brain and peripheral organs. At the same time, the impact of light on human body functions in extreme light environments such as deep space, deep sea, polar regions and tunnels is evaluated, so as to provide a scientific basis for life support and protection under extreme conditions.
In terms of application prospects, with the deepening of research on the relationship between light and health, personalized light management systems are about to emerge. The system will accurately meet the physiological and health needs of individuals, and intelligently regulate the lighting conditions. With the help of IoT and AI technology, the lighting system will realize the deep integration of environmental perception and demand response, dynamically optimize light intensity and spectrum, and create the most suitable lighting environment around the clock to protect human health. In addition, new light sources with spectra close to the characteristics of natural light can be developed to simulate the natural spectrum or flexibly adjust the spectral composition to reduce adverse effects on the human body.
In summary, this study not only deepens the basic scientific understanding of the mechanism of photosensory regulation of life, but also has a profound impact on the promotion of human health, the guiding development of future lighting technology, and the formulation of public health policies. Light, as an indispensable lighting tool in our daily life, has a deeper meaning of having a profound impact on human life and health. Only under the guidance of scientific research, the rational and intelligent use and utilization of light resources can make light truly a companion for our healthy life.
(The author is a chair professor at the Department of Life Science and Medicine, University of Science and Technology of China)