Both acute and chronic dehydration can have important implications for human behaviour and health. Young children, non-autonomous individuals and the elderly are at a greater risk of dehydration. Mild hypertonic dehydration could be related to less efficient cognitive and physical performance and has been reported to be associated with frequently occurring pathological conditions, especially nephrolithiasis. The assessment of hydration status in a large sample appears to be of interest for conducting epidemiological and large clinical studies aimed at improving preventive and curative care. Especially in large-population studies, methods that are used have to be accurate, cheap, quick and require no technical expertise. Body weight change is widely used to determine acute hydration changes, but seems to be insufficiently accurate in longitudinal studies. Bioimpedance analysis methods enable the assessment of total body water content, but their use is still under debate. Because plasma osmolality directly reflects intracellular osmolality, it constitutes a good marker to assess acute hydration changes, but not chronic hydration status because it changes constantly. Moreover, venepuncture is considered to be invasive and is not suitable for a large-sample study, especially in children. Urinary markers appear to be good alternatives for assessing hydration status in large populations. Collection of urine samples is non-invasive and cheap. High technical expertise is not required to perform urinary marker measurements and these measurements can be carried out quickly. Thus, methods based on urinary markers are very well suited for field studies. Urine colour is probably the least sensitive marker despite its high specificity. Urine osmolality and especially urine specific gravity could be easily used for determining hydration status in large-sample studies.

Little is known about the impact of habitual fluid intake on physiology. Specifically, biomarkers of hydration status and body water regulation have not been adequately explored in adults who consume different fluid volumes in everyday conditions, without prolonged exercise or environmental exposure. The purpose of the present study was to compare adults with habitually different fluid intakes with respect to biomarkers implicated in the assessment of hydration status, the regulation of total body water and the risk of kidney pathologies. In the present cross-sectional study, seventy-one adults (thirty-two men, thirty-nine women, age 25–40 years) were classified according to daily fluid intake: thirty-nine low drinkers (LD; ≤ 1·2 litres/d) and thirty-two high drinkers (HD; 2–4 litres/d). During four consecutive days, urinary parameters (first morning urine (FMU) on day 1 and subsequent 24 h urine (24hU) collections), blood parameters, and food and beverage intake were assessed. ANOVA and non-parametric comparisons revealed significant differences between the LD and HD groups in 24hU volume (1·0 (se 0·1) v. 2·4 (se 0·1) litres), specific gravity (median 1·023 v. 1·010), osmolality (767 (se 27) v. 371 (se 33) mOsm/kg) and colour (3·1 (se 0·2) v. 1·8 (se 0·2)). Similarly, in the FMU, the LD group produced a smaller amount of more concentrated urine. Plasma cortisol, creatinine and arginine vasopressin concentrations were significantly higher among the LD. Plasma osmolality was similar between the groups, suggesting physiological adaptations to preserve plasma osmolality despite low fluid intake. The long-term impact of adaptations to preserve plasma osmolality must be examined, particularly in the context of renal health.

A centrifugal method for extracting apoplastic sap from roots of lupin (Lupinus angustifolius) and pea (Pisum sativum) plants, and a method to analyse malic dehydrogenase in the sap using capillary electrophoresis, are described. Osmolality of apoplastic sap was relatively constant at relative centrifugal forces (RCFs) between 600 and 3000 g for lupin, and between 600 and 4000 g for pea. Electropherograms of a marker enzyme (malic dehydrogenase) and other components in apoplastic and symplastic saps revealed that contamination occurred at 7000 g. It is suggested that apoplastic sap expelled from plant roots at RCF between 600 and 3000 g is free from symplastic contamination, and is regarded as being of apoplastic origin. The proposed method was used to measure apoplastic pH changes in the plant roots in response to external pH, ammonium, nitrate and vanadate.

In patients who require enteric tube-feeding the osmolality of the formulas is assumed to play an important role. There is the dilemma that osmolality increases as the digestibility of formulas is enhanced by means of degradation of the nutrients. Hitherto there have been no reports of whether there are differences in nutrient absorption and water fluxes between iso-osmotic polymeric and hyperosmotic oligomeric diets. We therefore investigated absorption of nutrients and net fluxes of water during perfusion of a 1·5 m jejunal segment with oligomeric, polymeric and commercial oligopeptide diets either in the absence of pancreatic juice or with concomitant infusion of pancreatic enzymes. In the absence of pancreatic juice the absorption rates of the polymeric diet and the commercial oligopeptide diets reached 58·0 and 84·5 % respectively of that of a completely-hydrolysed hyperosmotic oligomeric diet. The concomitant infusion of pancreatic enzymes with the polymeric and oligopeptide diets significantly increased the absorption rates of nutrients and energy. The highest absorption rate of energy occurred with the commercial formula Survimed® (Fresenius, Bad Homburg, Germany), probably due to an optimal composition of the macronutrients. The increase in absorption due to the degradation of nutrients by pancreatic enzymes was associated with an increase in net water secretion and flow-rate, reaching similar values to those with the hyperosmotic oligomeric diet. It may be concluded that iso-osmotic oligopeptide formulas require further pancreatic hydrolysis for optimum absorption. In patients with normal pancreatic secretion, oligopeptide formulas have no advantage over polymeric diets. In patients with reduced pancreatic secretion, either completely-hydrolysed hyperosmotic oligomeric diets or polymeric diets supplemented with pancreatic enzymes are appropriate.

The phospholipid composition and fatty acid characteristics of diacyl and plasmalogen forms ofphosphatidylcholine and phosphatidylethanolamine of sperm of five marine and five freshwater speciesof fish have been determined. There was considerable variation in composition within each group offish, which obscured any differences which are related to the osmotic environment. Since such factors as temperature, nutrition, etc. probably greatly influence the lipid composition, we failed to determine a relationship between osmolality and phospholipid composition. We have also found certain phospho-lipid peculiarities, characteristic of sperm of various species of fish.Nevertheless, as we have found previously, the molar ratio of cholesterol to phospholipids in fishsperm is determined by the salinity of the habitat.

The site where osmotically active substances act to depress food intake was determined in sheep. After 5 × 5 h of food deprivation, solutions of sodium chloride or polyethylene glycol-200 (PEG-200) were added to either the reticulo-rumen or the abomasum. The sheep were then immediately offered pelleted lucerne (Medicago sativa). Water was withheld during the first 60 min of feeding but was available from 60 to 90 min. There was a linear inhibition in food intake in the first 10 min after loading 2.37, 6.25, 12.5, 25.0 or 50.0g NaCI into the rumen according to a 5.5 Latin square design (P = 0.0001). The intake reduction was 3.49 g food/g NaCI. An osmotic load of PEG-200 equivalent to 50 g NaCI also significantly inhibited food intake in the first 10 min of the meal compared with a control treatment. The inhibition of food intake after loading 55 g NaCI into the rumen was not affected by injecting lidocaine hydrochloride into the reticulum immediately before NaCI loading. NaCI injected into the abomasum did not significantly affect food intake in the first 10 min of feeding even though the tonicity of abomasal digesta was increased to unphysiological levels. There was no consistent relationship between food intake and the change in the tonicity of jugular plasma following solute loading and drinking. The sensing site of hypertonicity was localized to the wall of the reticulo-rumen where neuronal receptors appear to be capable of detecting osmotic pressure within the physiological range to depress food intake. These receptors should be identified and characterized because of their possible significance in limiting food intake by ruminants.