Hydration is the body's ability to absorb water and to maintain the correct balance of fluid and electrolytes and is essential to human health. Dehydration can adversely affect metabolism, thermoregulation, digestion, and neurological, kidney, and heart function. Aging as well as disease and medications affect water and electrolyte levels in the body and can lead to dehydration in older adults. In this review, we discuss factors contributing to dehydration in older adults, how hydration is measured, and strategies to improve hydration status. We close with a summary of the different areas of focus related to hydration research. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( https://creativecommons.org/licenses/by/4.0/ ). Abstract Hydration is the body’s ability to absorb water and to maintain the correct balance of fluid and electrolytes and is essential to human health. Dehydration can adversely affect metabolism, thermoregulation, digestion, and neurological, kidney, and heart function. Aging as well as disease and medications affect water and electrolyte levels in the body and can lead to dehydration in older adults. This is sometimes defined as above a specific percentage loss of body mass, such as greater than 2% [ 14 ]. Water plays an active role in human physiology—acting as a solvent, a carrier/transporter of nutrients and waste products, and as an essential contributor to thermoregulation [ 12 , 15 ]. Hydration in older adults promotes mobility by reducing the risk of falls and improving orthopedic rehabilitation outcomes [ 16 ]. It helps maintain muscle function and lowers the risk of exhaustion, urinary infections, and gastrointestinal complications [ 17 ]. Thus, the total daily water inputs for an adult are estimated to be in the range of 2–3 L [ 12 ] or 3–4 L [ 33 ]. The rate of water absorption within the digestive tract changes with the speed of digestion depending on nutrients within the food and beverages consumed, gastric volume, and energy density. When water is ingested alone, it can start being absorbed in as quickly as 5 min and is mostly absorbed within 20 min [ 45 ]. Different beverages can have different transit times. Half-emptying rates of liquids in the stomach have been found to range from 10 min to 1 h, while solid foods take longer, such as from 50 to 115 min depending on the food [ 46 ]. It is unclear how gastric emptying changes with aging or what effect this has on water absorption. Some research indicates that the stomach may empty faster in older adults for liquids, while other studies found increased times for both solids and liquids, and yet others found no differences at all [ 46 ]. Frailty may alter the rate of gastric emptying and further explain differences between study findings. In addition to water from nutritional consumption, to aid digestion, the gastrointestinal tract also secretes roughly 7.5 L of fluid daily, which is also reabsorbed. Assessing individual hydration habits is also important, as psychological barriers may influence fluid intake [ 128 ]. Reminders, including mobile apps, can help prompt regular drinking, especially in hot or dry environments. Flavoring water may also encourage individuals to increase fluid consumption. One study, involving 50 patients, assessed fluid intake before and after an intervention that included offering flavored beverages. The postintervention group consistently consumed more fluids than the preintervention group over a week [ 107 ]. 5.2. The need for fluid replacement depends on factors such as the length of exercise, intensity, and environmental conditions. In other terms, unless sweating leads to a fluid loss equating to about 2% or more of body weight, research shows minimal performance benefits from drinking during activity. After physical activity, it is important to rehydrate by replacing both fluids and electrolytes. A general guideline is to drink about 1.5 times the amount of fluid lost during exercise; tracking weight before and after can help estimate this loss. The council recommends using only electrolyte beverages during extreme (“salty”) sweating [ 127 ]. 5.3. Electrolytes Electrolytes such as sodium, potassium, magnesium, and chloride play a crucial role in maintaining fluid balance and retention in the body. Drinks rich in electrolytes are particularly important during prolonged physical activity or in hot environments where significant sweat loss occurs. Sports drinks are recommended over fruit juices and high-sugar beverages for this purpose [ 37 , 131 ]. Additionally, tablet or powder stick packs containing electrolytes mixed into water have been reported to improve the BHI and noted to be well-tolerated with daily intake [ 132 , 133 ]. Sodium enhances water absorption in the small intestine and helps the body retain fluid [ 37 ]. However, aging may alter the effects of electrolytes on increasing fluid retention. Recent studies evaluating hydration characteristics of different beverages in young and old populations have shown that sodium was more effective in maintaining hydration in younger vs. older adults [ 38 , 39 ]. 5.4. Carbohydrates Small amounts of carbohydrates, such as glucose, are included in hydration solutions like sports drinks to provide energy, improve water and electrolyte absorption, and add flavor to encourage voluntary drinking [ 37 ]. While many recommendations have been made on how to encourage better hydration practices for older adults living in different home environments, with varying degrees of support, a standard methodology is needed to better track hydration and to evaluate different interventions more consistently. Additional research is needed to understand how different comorbidities alter hydration needs and what dietary changes and novel beverage delivery systems may improve hydration in this vulnerable population.

digestion . Eating between meals, and excess in eating, lead to stomach derangement. "The best remedy for acidity of the stomach is hot -water drinking When alcohol is given during collapse there is often no absorption until reaction occurs, and then the quantity accumulated speedily produces intoxication. It was the same with opium; he had found pills unchanged in the stomach for hours. He recommended hot drinks; he had tried every kind of medicine and had little faith in it. The nursing was very important, and it was important that the nurses should abstain. "Dr. In the case of the young man, it is probable that the difficulty with the oatmeal was the lack of sufficient cooking. Oatmeal made into gruel, well cooked, and diluted with a large quantity of scalded milk is easy of digestion. Eating between meals, and excess in eating, lead to stomach derangement. "The best remedy for acidity of the stomach is hot-water drinking. Two or three glasses should be taken as hot as can be sipped, one hour before each meal, and half an hour before going to bed. The effect of the hot water is to wash out the stomach, and so remove any fermenting remains of the previous meal. Richardson was addressed to a lady who had sought the great physician's advice on the subject:— "25 Manchester Square, W., July 18, 1896. " Dear Madam : There is no substance which acts as a substitute for alcohol, nor is anything like it wanted. The human body is a water engine, as I have often described it, and alcohol plays no part in its natural motion. The idea that when it begins to fail, a stimulant has to be called for, springs merely from habit, and if, whenever any of the symptoms of fainting you speak of occur, the person merely lies down on the side or back and drinks a glass of hot water, or hot milk and water, all that can be done is done. "When the faint has been recovered from, but the hemorrhage continues so much that it is feared another faint may occur, and, perhaps, be fatal, it may be warded off by drinking any hot liquid; if Liebig's extract of meat, or strong beef tea, is at hand and can be given hot, there is nothing better." Heart Disease :—Dr. Ridge says: "I trench here on a delicate subject, because, when there is real disease of the heart, medical advice will of course have been obtained, and very probably a doctor may have said that some alcoholic liquor is essential. During the disease a simple but nutritious diet should be allowed, but stimulants of all kinds should be prohibited ." "It is wholly unnecessary, and dangerous as well, to give whisky to bring out the eruption."— Dr. I. N. Quimby , Jersey City. "Any hot drink, such as ginger tea or hot lemonade, may be used to hasten the eruption, if delayed."   ​ Malaria :—Observers of this disease in such regions as the gold coast of Africa have noted the fact that malarial attacks are generally preceded by impaired digestion. The disease is said to be due to animal parasites. These parasites are supposed to generate in the soil of certain regions, and thence, through the drinking water, or otherwise, find entrance to the human body. "A healthy stomach is able to destroy germs of all sorts, hence the best protection from malaria is the boiling of all drinking water, and the maintenance of sound digestion and purity of blood by an aseptic dietary." Dr. J. H. Kellogg says in The Voice :— "It must be understood, however, that fruit in malarial regions, especially watermelons, may be thickly covered with malarial parasites and the parasites may sometimes find entrance to the fruit when it becomes over-ripe, so that the skin is broken. Neuralgia :—" The principal cause of neuralgia is defective nutrition of the nerves. Disorders of digestion are very often accompanied by neuralgia in various parts of the body. It may also result from taking cold, from loss of sleep, from dissipation, and also from the use of tobacco, alcohol, tea and coffee. "The patient's general health must be improved by a wholesome, simple diet, and the employment of tonic baths, as a daily sponge bath, and massage in feeble cases. Sun-baths and exercise in the open air are of first importance. Ordinary neuralgia may almost always be relieved by either moist or dry heat. In some cases, cold applications give more relief than hot. "It is not a very fatal disease in young and healthy subjects, but in weak children, old persons and habitual drinkers, it is a very fatal malady." Nature Cure recommends a vapor bath immediately upon the appearance of the first symptoms, together with copious drinking of hot lemonade, and a good supply of pure fresh air in the room, together with the application of alternating hot and cold compresses, and no drugs . Dr. Kellogg says:— "Cool compresses or ice-bags, alternated every three hours by hot fomentations for ten minutes, should be applied to the chest, particularly to the affected side, the seat of pain. "The latest researches in regard to alcohol reveal it as a poison to the human system in whatever way it may be diluted or disguised. Its effect is always the same in proportion to the amount taken. It is impossible to habitually use it in any form, even in small quantities, without disease and degeneration resulting therefrom. When taken into the stomach the action is the same as with any other narcotic; the meaning of this word is to become torpid . A mustard plaster on the back of the neck, or over the stomach, and hot water or hot bottles to the feet, are never out of place, while vinegar, or smelling salts, or dilute ammonia to the nostrils is reviving."— Ezra M. Hunt, M. D. , late secretary of New Jersey State Board of Health. "Both the popular and professional beliefs in the efficacy of alcoholic liquids for relieving exhaustion, faintness, shock, etc. are equally fallacious. All these conditions are temporary, and rapidly recovered from by simply the recumbent position, and free access to fresh air.