Vitamins are natural organic (carbon-containing) substances that can be found in food (and in the very popular “supplements” that many people take in probably much higher amounts that they may need to) and that our bodies require for a variety of processes that occur inside us. They are also known as micronutrients, they are needed in small amounts and they are also small in structure compared to proteins, fats and carbohydrates that need to be broken down in order to be used, whereas vitamins do not. They are also “essential” nutrients, meaning they are essential for us to get from food as our bodies can not make them on their own except for vitamin D, which the body can make when exposed to sunlight- not very frequent event for most people in the winter. Calcium, which is absorbed in the small intestines in a process that needs vitamin D, is a common supplement that is taken with vitamin D especially for women in and after menopause to prevent osteoporosis. Vitamin K2 (a type of vitamin K) is made by some of our good gut bacteria, but we don't get much from this source.

Both vitamin deficiencies (much lower amounts than our bodies need) and excess (higher than the maximum our bodies need/tolerate) may lead to adverse symptoms for some vitamins. For example, vitamin A deficiency may result in night blindness first leading to blindness later on, and tissues like eyes and skin may become dry and damaged while infections are more likely due to the immune system not working properly especially in infants and children, whose growth and development may be slowed. However this deficiency can be reversed by taking high vitamin A doses for several days. On the other hand, excess vitamin A may result in hair loss, cracked lips, dry skin, weakened bones, headaches, and increased pressure in the brain, although these effects requires intake of really high doses and are reversed by stopping vitamin A intake.

From: http://www.stylecraze.com/articles/3-vital-vitamins-for-hair-growth/#gref

There are a total of 13 vitamins, which can be divided in two groups: those soluble in water, and those soluble in fat. All B vitamins and vitamin C are water-soluble and our bodies can’t store them. They leave us quickly via our kidneys and then in our urine, so we need to provide our bodies with them often, ideally every day. In contrast, vitamins A, D, E, and K are fat-soluble, absorbed in our intestines in fatty forms and easily stored in our bodies in the liver and fatty tissues for long periods of time (except vitamin K). This is why toxicity may occur if too much is taken of a fat-soluble vitamin, especially vitamin A or D.

All vitamins are absorbed in the small intestine, and then transported to specific tissues in the bloodstream; fat-soluble vitamins need to be transported first by the lymphatic system after intestinal absorption to the blood. At the right destination, vitamins help "reactions" occur, several carried out by enzymes that need a little help from "coenzymes" which some of these vitamins act as. Vitamins C and E are well known as "antioxidants", meaning they protect our cells from excessive accumulation of "free radicals" that may cause "oxidative stress" and result in an increased risk for cardiovascular disease, cancer and other diseases.

For a fun and informative 5min TED Ed video on how we absorb vitamins, how they are transported and what they help us with, watch: https://www.youtube.com/watch?v=ISZLTJH5lYg

                                     Water Soluble Vitamins

Information for tables above came mostly from Harvard Medical School at: https://www.health.harvard.edu/staying-healthy/listing_of_vitamins

If you eat fortified cereals for breakfast (cereals with added vitamins and minerals) you may be getting more than the daily amount of vitamins recommended, especially children, and even more so if you are taking vitamin supplements, eating fortified snack bars, etc. The daily values shown in cereal boxes (that come from the US FDA) are usually the ones for adults- children need less; some cereals show both amounts.

If we have access to good food rich in vitamins, we don’t really need to supplement our regular intake with extra vitamins, but there may be situations/places in which taking specific vitamin supplements is recommended (for all or some of them) if the food is not rich in these, or you have a diet that restricts food with certain vitamins. Keep in mind that man-made vitamin supplements usually contain the full recommended daily amount of the different vitamins, which we are taking in addition to what we are ingesting with our food, and don't forget to count the fortified food you eat that contains added vitamins (milk, cereals, flour). As I am a petite person, I feel that for things like supplements or pain killers I am probably taking way more than I need (compared to someone twice my size/weight for example) so I may take less than the recommended adult dose sometimes, and/or less frequently- disclaimer here: this is just my approach; I am not the MD kind of doctor so my advice is not that of a clinician.

In honor of the Nobel Prize in Physiology or Medicine 2017 given to Michael Young, Michael Rosbash and Jeffrey Hall for revealing molecular mechanisms controlling the circadian rhythm, this post is dedicated to this topic.
 
Circadian rhythm or clock refers to a complex mechanism underlying the capacity of living organisms (plants and animals including us) to adjust several of our body processes to the sunlight during the day and darkness at night in 24h (solar) oscillating cycles. This figure taken from the press release of the 2017 Nobel prize awarded this past October 2nd shows how the circadian clock helps us adapt our physiology to the different phases of the day and regulates sleep patterns, feeding behavior, hormone release, blood pressure, and body temperature.

“Molecularly” speaking, this clock is regulated by specific genes in different cells, that get activated and transcribed into RNA, which leads to protein production and accumulation within cells. Once a certain level is achieved, this acts as a “feedback loop” turning off expression of the genes that were activated before. The cycle takes 24h total to begin again. Several genes have been identified thanks to many years of research, mostly in Drosophila (fruit fly) and mice as the animal model for mammals. Although the central “clock” is located in the suprachiasmatic nucleus (SCN) in the hypothalamus in mammals, formed by neurons that act based on sensing light (“photic”) input from the retina, circadian rhythm clocks are also present in most peripheral tissues and cells. Food schedule also has an important impact on this clock. Organs such as liver, pancreas, adrenal gland, spleen, thymus, and heart each have their own rythm, as well as fatty cells. The hormones insulin, cortisol and melatonin are secreted (produced and released to act on target organs/tissues/cells) in a circadian-dependent manner.

From:
https://www.intechopen.com/books/molecular-mechanisms-of-the-aging-process-and-rejuvenation/circadian-clock-gene-regulation-in-aging-and-drug-discovery

Several genes have been identified that participate in the main feedback loop responsible for circadian cycles. A transcription factor formed by a combo of two proteins (BMAL1 and CLOCK) controls expression of the repressor genes PER and CRY (and also transcription of other clock-controlled genes), which together form a complex with other proteins to repress BMAL1-CLOCK function on chromatin. This is known as negative feedback (=repression or silencing of genes) that inhibits synthesis of the PER and CRY genes. Once PER and CRY protein concentrations are reduced significantly, negative feedback ends and a new molecular cycle starts. This loop in almost all cells/tissues results in a self-sustaining molecular 24 h rhythm, with specific genes and resulting activated/repressed processes involved in different tissues. The complex genetic regulation that underlies this circadian clock working smoothly in different types of cells involves a great deal of epigenetics (another topic awaiting a Nobel Prize- for details on this see my homepage) occurring at the level of histones, DNA and RNA modifications.

Experimentally, to determine whether a cell is cycling (or whether its circadian rhythm is intact) expression of a clock-related gene is measured, for example BMAL1 expression in a young normal mouse heart shown in the figure below.

From: http://physiologyonline.physiology.org/content/29/1/72
 
As explained in my home page, thanks to molecular biology and applied fields, once important genes in a biological phenomenon under study are identified, awesome tools are generated (in this case in mice) by mutating or deleting these genes and observing the effect in physiology, behavior, gene expression, cell biology, etc- whatever is now disturbed in these mutants are processes in which the mutated genes (when not mutated) have essential roles.  Some “explant” experiments have been used to show that cells taken away from the whole organism can maintain the circadian rhythm autonomously, and this gets affected when clock genes are mutated- these “arrhythmic” effects upon mutations in clock genes have been reported in lung, liver, cornea, kidney, fibroblasts and the SCN.
 
It is not difficult to infer then how important it is to help our bodies keep this internal clock that provides the rhythm for us to function daily with as little disturbance as possible. But modern lifestyle results in a number of disruptions: jetlag/traveling, night shift jobs, irregular eating and sleeping patterns, light pollution (from the street or our tablets/phones/screens) that affect and confuse our internal cellular clocks as well as the central master SCN regulator. Even before clock genes were considered mechanistically, daily patterns for some diseases symptoms severity were observed: rheumatoid arthritis patients usually feel worse in the morning (thus, nighttime rather than daytime administration of slow release medication is an effective treatment) while osteoarthritis patients feel worse throughout the day. Many studies have demonstrated that shift workers and people with chronic sleep disruption have an increased risk to develop certain conditions such as obesity, type 2 diabetes, hyperlipidemia, high blood pressure, cancer and cardiovascular disease, and they can experience higher levels of inflammation.
 
Knowing what the rhythm is in special cells/tissues in our bodies can inform medicine approaches, one example of this is the so called chronotherapy, consisting of synchronizing drug administration with circadian rhythms to achieve maximum therapeutic effect and minimum side effects. We already know that many genes that encode important metabolic enzymes including some that process drugs used as medicine show circadian rythms in cells. Medication could then be administered when it is expected to have maximum effect on its target (assuming the drug gets access soon after administration) and/or when enzymes that degrade them in our bodies are at the lowest level.

Another interesting research angle is the circadian rhythm disruption associated with eating schedules (eating/fasting) leading to obesity. The hormone insulin, produced in the pancreas, enhances transfer of glucose from the blood (after eating) into liver, muscle and fat cells, and blocks fat burning. Studies in mice have shown that tissues are somewhat “resistant” to insulin during the inactive/fasting phase of the circadian cycle (during which glucose is converted primarily into fat) and sensitive to insulin during the high activity/eating phase. Mice mutant in clock genes get “locked” into an insulin-resistant mode characteristic of the inactive/fasting phase, and when fed a high-fat diet they gain more weight and fat than normal mice. Interestingly, the same thing happens to normal mice placed under constant light, which may explain the high rate of obesity and diabetes among night-shift workers.

From: https://hackyourgut.com/2017/06/29/circadian-rhythms-weight-loss-and-leaky-gut/