question: Why do our bodies store excess fat, sugar, cholesterol etc but not water or vitamins?
comment: Because water is actually pretty hard to store. We need a *lot* of it, far more than we need fat or sugar. Water is also pretty easy to obtain, so we don't *need* to store it. Things that do only do so because they can't get it regularly: cacti, camels... Water is also not only something we discard, it's how we discard *other* things, things that we have to discard due to their toxicity like urea. A high water intake and output is important for our overall health because it keeps us efficiently removing toxic waste products from our systems. We actually do store certain vitamins - fat soluble vitamins. Some vitamins are water soluble though, so we have no way of storing them. To store any chemical our body has to sequester it from the water inside our cells. Fat-soluble molecules naturally remove themselves from water anyway, but water soluble ones have to be made water *insoluble* in order to store them, which means modifying them so that they're not vitamins anymore. The problem with storage of fats and sugars is that we evolved to be able to survive droughts and other unpredictable conditions. We store excess energy (in the form of fats and sugars) so that we can't be caught off-guard if we ever enter a period of starvation. Fat storage is also a useful insulating tool remaining from when we hadn't invented central heating. So, we have all these mechanisms for storing fats and sugars, but we now live in an environment where starvation pretty much never happens unpredictably and we don't need to build large energy stores. The body always assumes the worst because it's evolved to do so, and if you're over-eating regularly its assuming that a starvation period is coming. So if you're constantly over-eating your body is constantly trying to stockpile energy in fear of an oncoming winter or something, but that never comes and you just keep on eating. The body is actually *incredibly* efficient about what it does and doesn't store. It stores everything it needs to be able to store (and can store at all). That's how evolution works after all - it tends towards making the most efficient versions of things possible, as long as more efficiency means a higher survival rate. If it doesn't need to store more energy, then it sends us messages telling us we don't need to eat. If you're storing more fat and sugar than you need, it's because you're *eating* too much fat and sugar, ignoring those messages, or because of some genetic defect that is meaning that messaging system isn't working properly. Or because of gut bacteria. That's the new focus of dietary science. Also, the reason we can't store water soluble things is because it's actually toxic to do so. A lot of our body works on the relative concentration of water soluble molecules, and if we have too much of a water soluble molecule, we actually can't absorb any more of it without converting the water soluble molecule to an insoluble form. We actually have to use a very advanced mechanism to absorb glucose at all, that relies on the fact there's more sodium in our intestine than inside our cells.
passages: Section::::Protein.
  
  Protein attracts water and plays an important role in water balance. In cases of severe protein deficiency, the blood may not contain enough protein to attract water from the tissue spaces back into the capillaries. This is why starvation often shows an enlarged abdomen. The abdomen is swollen with edema or water retention caused by the lack of protein in their diet.
   Water makes up about 60% of the body weight in men and 55% of weight in women. A baby is composed of about 70% to 80% water while the elderly are composed of around 45%.
   In terms of mineral nutrients intake, it is unclear what the drinking water contribution is. However, inorganic minerals generally enter surface water and ground water via storm water runoff or through the Earth's crust. Treatment processes also lead to the presence of some minerals. Examples include calcium, zinc, manganese, phosphate, fluoride and sodium compounds. Water generated from the biochemical metabolism of nutrients provides a significant proportion of the daily water requirements for some arthropods and desert animals, but provides only a small fraction of a human's necessary intake.
   Wetlands span the surface/sub-surface interface, storing water at various times as groundwater, soil moisture and surface water. They are vital ecosystems that support wildlife and perform valuable ecosystem services, such as flood protection and water cleansing. They also provide livelihoods for millions of people who live within and around them. For example, the Inner Niger River Delta in the Western Sahel zone supports more than a million people who make their living as fishermen, cattle breeders or farmers, using the annual rise and fall of the river waters and its floodplains.
   Water is excreted from the body in multiple forms; including urine and feces, sweating, and by water vapour in the exhaled breath. Therefore, it is necessary to adequately rehydrate to replace lost fluids.
   Section::::On Earth.:Fresh water storage.
   A constant supply is needed to replenish the fluids lost through normal physiological activities, such as respiration, sweating and urination. Water generated from the biochemical metabolism of nutrients provides a significant proportion of the daily water requirements for some arthropods and desert animals, but provides only a small fraction of a human's necessary intake.
  
  In the normal resting state, input of water through ingested fluids is approximately 1200 ml/day, from ingested foods 1000 ml/day and from aerobic respiration 300 ml/day, totaling 2500 ml/day.
  
  Section::::Routes of fluid loss and gain.:Input.:Regulation of input.
   In 2009, the European Food Safety Authority (EFSA) included water as a macronutrient in its dietary reference values for the first time. Recommended intake volumes in the elderly are the same as for younger adults (2.0 L/day for females and 2.5 L/day for males) as despite lower energy consumption, the water requirement of this group is increased due to a reduction in renal concentrating capacity.
  
  Section::::Thirst quenching.
   There are a variety of trace elements present in virtually all potable water, some of which play a role in metabolism. For example, sodium, potassium and chloride are common chemicals found in small amounts in most waters, and these elements play a role in body metabolism. Other elements such as fluoride, while arguably beneficial in low concentrations, can cause dental problems and other issues when present at high levels. Water is essential for the growth and maintenance of our bodies, as it is involved in a number of biological processes.
  
  Section::::Applications.:Medical use.
  
  Section::::Applications.:Medical use.:Effects of illness.
   Plants share with animals the problems of obtaining water but, unlike in animals, the loss of water in plants is crucial to create a driving force to move nutrients from the soil to tissues. Certain plants have evolved methods of water conservation.
   Metabolic water
  
  Metabolic water refers to water created inside a living organism through their metabolism, by oxidizing energy-containing substances in their food. Animal metabolism produces about 110 grams of water per 100 grams of fat, 42 grams of water per 100 g of protein and 60 grams of water per 100 g of carbohydrate.
  
  Some organisms, especially xerocoles, animals living in the desert, rely exclusively on metabolic water. Migratory birds must rely exclusively on metabolic water production while making non-stop flights. Humans, by contrast, obtain only about 8-10% of their water needs through metabolic water production.
   Section::::Active absorption.:Active non-osmotic water absorption.
  
  This theory was given by Thimann (1951) and Kramer (1959). According to the theory, sometimes water is absorbed against a concentration gradient. This requires expenditure of metabolic energy released from respiration of root cells. There is no direct evidence, but some scientists suggest involvement of energy from respiration. In conclusion, it is said that, the evidences supporting active absorption of water are themselves poor.
  
  Section::::Passive absorption.
   The usual way of adding water to a body is by drinking. Water also enters the body with foods, especially those rich in water, such as plants, raw meat, and fish.
  
  The amount of this water that is retained in animals is affected by several factors. Water amounts vary with the age of the animal. The older the vertebrate animal, the higher its relative bone mass and the lower its body water content.
   Water storage is a broad term referring to storage of both potable water for consumption, and non potable water for use in agriculture. In both developing countries and some developed countries found in tropical climates, there is a need to store potable drinking water during the dry season. In agriculture water storage, water is stored for later use in natural water sources, such as groundwater aquifers, soil water, natural wetlands, and small artificial ponds, tanks and reservoirs behind major dams. Storing water invites a host of potential issues regardless of that waters intended purpose, including contamination through organic and inorganic means. 
   Fats are also sources of essential fatty acids, an important dietary requirement. They provide energy as noted above. Vitamins A, D, E, and K are fat-soluble, meaning they can only be digested, absorbed, and transported in conjunction with fats. Fats play a vital role in maintaining healthy skin and hair, insulating body organs against shock, maintaining body temperature, and promoting healthy cell function. Fat also serves as a useful buffer against a host of diseases. When a particular substance, whether chemical or biotic, reaches unsafe levels in the bloodstream, the body can effectively dilute—or at least maintain equilibrium of—the offending substances by storing it in new fat tissue. This helps to protect vital organs, until such time as the offending substances can be metabolized or removed from the body by such means as excretion, urination, accidental or intentional bloodletting, sebum excretion, and hair growth.
   Adipocytes, the cells that normally function as lipid store of the body, are well equipped to handle the excess lipids. Yet, too great of an excess will overburden these cells and cause a spillover into non-adipose cells, which do not have the necessary storage space. When the storage capacity of non-adipose cells is exceeded, cellular dysfunction and/or death result. The mechanism by which lipotoxicity causes death and dysfunction is not well understood. The cause of apoptosis and extent of cellular dysfunction is related to the type of cell affected, as well as the type and quantity of excess lipids.
   Vitamins, minerals, fiber, and water do not provide energy, but are required for other reasons. A third class of dietary material, fiber (i.e., nondigestible material such as cellulose), seems also to be required, for both mechanical and biochemical reasons, though the exact reasons remain unclear. For all age groups, males need to consume higher amounts of macronutrients than females. In general, intakes increase with age until the second or third decade of life.
   Section::::Physiology.:Genetics.
  
  The thrifty gene hypothesis (also called the famine hypothesis) states that in some populations the body would be more efficient at retaining fat in times of plenty, thereby endowing greater resistance to starvation in times of food scarcity. This hypothesis, originally advanced in the context of glucose metabolism and insulin resistance, has been discredited by physical anthropologists, physiologists, and the original proponent of the idea himself with respect to that context, although according to its developer it remains "as viable as when [it was] first advanced" in other contexts.
   One factor that limits the benefits of food fortification is that isolated nutrients added back into a processed food that has had many of its nutrients removed, does not always result in the added nutrients being as bioavailable as they would be in the original, whole food. An example is skim milk that has had the fat removed, and then had vitamin A and vitamin D added back. Vitamins A and D are both fat-soluble and non-water-soluble, so a person consuming skim milk in the absence of fats may not be able to absorb as much of these vitamins as one would be able to absorb from drinking whole milk. On the other hand, the nutrient added as a fortificant may have a higher bioavailability than from foods, which is the case with folic acid used to increase folate intakes.
   The drinking water contribution to mineral nutrients intake is also unclear. Inorganic minerals generally enter surface water and ground water via storm water runoff or through the Earth's crust. Treatment processes also lead to the presence of some minerals. Examples include calcium, zinc, manganese, phosphate, fluoride and sodium compounds. Water generated from the biochemical metabolism of nutrients provides a significant proportion of the daily water requirements for some arthropods and desert animals, but provides only a small fraction of a human's necessary intake. There are a variety of trace elements present in virtually all potable water, some of which play a role in metabolism. For example, sodium, potassium and chloride are common chemicals found in small quantities in most waters, and these elements play a role in body metabolism. Other elements such as fluoride, while beneficial in low concentrations, can cause dental problems and other issues when present at high levels.
   It is vital for organisms to be able to maintain their fluid levels in very narrow ranges. The goal is to keep the interstitial fluid, the fluid outside the cell, at the same concentration as the intracellular fluid, fluid inside the cell. This condition is called isotonic and occurs when the same level of solutes are present on either side of the cell membrane so that the net water movement is zero. If the interstitial fluid has a higher concentration of solutes than the intracellular fluid it will pull water out of the cell. This condition is called hypertonic and if enough water leaves the cell it will not be able to perform essential chemical functions. If the interstitial fluid becomes less concentrated the cell will fill with water as it tries to equalize the concentrations. This condition is called hypotonic and can be dangerous because it can cause the cell to swell and rupture. One set of receptors responsible for thirst detects the concentration of interstitial fluid. The other set of receptors detects blood volume.
   BULLET::::3. Storage – Water from rivers may also be stored in bankside reservoirs for periods between a few days and many months to allow natural biological purification to take place. This is especially important if treatment is by slow sand filters. Storage reservoirs also provide a buffer against short periods of drought or to allow water supply to be maintained during transitory pollution incidents in the source river.
   Section::::Formation of urine.:Reabsorption.
  
  Tubular reabsorption is the process by which solutes and water are removed from the tubular fluid and transported into the blood. It is called "reabsorption" (and not "absorption") both because these substances have already been absorbed once (particularly in the intestines) and because the body is reclaiming them from a postglomerular fluid stream that is well on its way to becoming urine (that is, they will soon be lost to the urine unless they are reclaimed).
   Section::::Nutrients.:Fat.
  
  A molecule of dietary fat typically consists of several fatty acids (containing long chains of carbon and hydrogen atoms), bonded to a glycerol. They are typically found as triglycerides (three fatty acids attached to one glycerol backbone). Fats may be classified as saturated or unsaturated depending on the chemical structure of the fatty acids involved.
  
  Section::::Nutrients.:Fiber.
   Groundwater makes up about twenty percent of the world's fresh water supply, which is about 0.61% of the entire world's water, including oceans and permanent ice. Global groundwater storage is roughly equal to the total amount of freshwater stored in the snow and ice pack, including the north and south poles. This makes it an important resource that can act as a natural storage that can buffer against shortages of surface water, as in during times of drought.
  
  Groundwater is naturally replenished by surface water from precipitation, streams, and rivers when this recharge reaches the water table.
  
presuppositions: Our bodies need to store compounds and nutrients equally. Our bodies do not store vitamins.
corrections: Our body does not need to store all compounds and nutrients. Our bodies do store vitamins that are fat soluble.
raw_presuppositions: Our bodies need to store compounds and nutrients equally. Our bodies do not store vitamins.
raw_corrections: Our body does not need to store all compounds and nutrients. Our bodies do store vitamins that are fat soluble.