The information explosion in the science of nutrition very often creates the impression that available information is contradictory. Consequently, it is no longer easy to distinguish between fact, misinformation and fiction. The Nutrition Information Centre of the University of Stellenbosch (NICUS) was established to act as a reliable and independent source of nutrition information.
Water is often referred to as the neglected nutrient, but it hasvery important functions in the body. Water maintains the structure of large molecules such as protein and glycogen in the body and thus forms part of the structure of all cells. It also participates in metabolic reactions, forms the basis of all body fluids, serves as the solvent of all digestive products and contributes to the transport of nutrients, oxygen, waste products and carbon dioxide. It also acts as an essential lubricant and cushion around joints e.g. inside the eye and the spinal cord, while saliva lubricates food to make swallowing easier. It is a cleanser (e.g. tears washing away dirt from the eyes), aids in regulating body temperature and maintains blood volume, which in turns influences blood pressure.
The body must excrete a minimum volume of water every day in order to wash away the waste products produced by the metabolic processes - this is referred to as the compulsory water loss through the kidneys and amounts to about 500 - 600 ml per day. Waste products that have to be excreted via the urine include urea from body protein breakdown, ketones from body fat breakdown, phosphates, sulfates and other minerals. Thus, if protein or fat stores are being broken down, the need for water increases to produce more urine to excrete the extra waste products - urea or ketones, respectively. Beyond this compulsory water loss, excretion adjusts according to intake and body requirements and this is referred to as the facultative water excretion by the kidneys.
Maintaining fluid balance in the body is one of the primary functions of the healthy kidneys. During kidney failure excretion of fluid is impaired, resulting in disturbances in the body’s fluid balance and the development of fluid overload. This can become a burden to especially the heart which has to work harder and the lungs, causing shortness of breath. It can also cause a sudden increase in body weight and swelling, all signs that your fluid intake is too much for your kidneys.
It is important to be aware of the amount of fluid that is consumed and how much urine is passed. In the early stages of kidney failure it is usually not necessary to restrict fluid intake. Total fluid intake depends on physical activity levels, the climate one lives in, one’s health status, clothing and various other physiological factors. A doctor or dietitian will prescribe the amount of fluid to be consumed per day based on the kidney function, urine output and individual factors such as body weight, activity levels and the background diet.
Fluids in the diet: what is regarded as fluid?
Most of the water in the diet comes from water and other beverages, as well as from solid food (most fruit and vegetables contain up to 90% water, whilst many meats and cheeses contain at least 50% water). Depending on the type and amount of food that you eat, solid foods can supply as much as 500 – 1000 ml of fluid per day (or an average of 800ml/day). Water is however also generated inside the body by metabolic reactions. When nutrients are broken down for energy, their carbons and hydrogen combine with oxygen to form carbon dioxide (CO2) and water (H2O). This water, termed metabolic water, provides about 25% of the daily requirements of a sedentary person.
Which food items can be regarded as fluids?
· Water, large amounts of ice (crushed ice or ice cubes)
· Coffee, tea
· Milk, buttermilk
· Cooldrink, fruit juice
· Yoghurt, ice cream
· Jelly, custard
· Wine, beer or other alcoholic drinks
General recommendations for healthy drink choices:
In most developed Western societies diets are reported to provide an excess of total energy, which is associated with obesity and the related profiles of the so-called diseases of lifestyle. Although plain water is ideal and fulfils almost all the fluid needs of healthy adults, individual preferences, perceived needs, taste, cultural, social and other factors have led to the availability of a great variety of beverages on the market. Some of these beverages may contribute significantly to the total daily energy intake and the intake of other nutrients. Indeed, depending on the frequency and amounts consumed, the intake of energy and/or other nutrients may become inappropriately high.
- Sweetened cool drinks, such as carbonated cool drink drinks should be limited to avoid excess fluid and energy intake. Dark drinks such as cola and beer may contain large amounts of phosphate and should be restricted by patients with kidney failure.
- Fruit and vegetable juices (100% juices) and sports drinks should be limited to avoid excess energy intake as well as excess electrolytes such as potassium and sodium.
- Diet- or artificially-sweetened cool drinks could replace sweetened drinks in a varied diet to avoid excess energy intake, especially for overweight and obese patients (see previous comment on dark cool drinks).
- Coffee and tea may be enjoyed in moderation and in accordance with the daily fluid limits. Preferably, these should be with fat free or low fat milk (from the total daily milk allowance) and no sugar in the case of overweight and obese patients.
- Milk or soy beverages (enriched with calcium): milk is high in phosphate and adults with chronic renal failure should generally limit their intake of fat free and low fat milk to 1 portion or ½ cup per day (in consultation with the dietitian and depending on the stage of renal failure).
Practical hints to control thirst
· Restrict your salt intake
· Suck on an ice cube
· Spread your fluids evenly throughout the day
· Chewing sugar-free gum
· Use an aerosol mouth freshener
(Acknowledgement: RenalSmart Nutritional Information System. Guidelines on Fluid Restriction. References from the scientific literature used to compile this document are available on request.
For further, personalised and more detailed information, please contact NICUS (e-mail: firstname.lastname@example.org or phone 021-933 1408) or contact a dietician registered with the Health Professions Council of South Africa.
- (NICUS, Health24, June 2010)
Your guide to renal nutrition