The clinical potential of sphingolipid-based therapeutics

The era of sphingolipid-based therapeutics is upon us. A large body of work has been accumulating that demonstrates the distinct biological roles of sphingolipids in maintaining a homeostatic environment and in responding to environmental stimuli to regulate cellular processes. It is thus necessary to further investigate alterations in sphingolipid-metabolism in pathological conditions and, in turn, try to exploit altered sphingolipid-metabolizing enzymes and their metabolites as therapeutic targets. This review will examine how advances in the fields of drug delivery, drug discovery, synthetic chemistry, enzyme replacement therapy, immunobiology, infectious disease and nanotechnology have delivered the potential and promise of utilizing and/or targeting sphingolipid metabolites as therapies for diverse diseases.

Estimation of iron absorption in humans using compartmental modelling

OBJECTIVE

To evaluate whether a compartmental model could estimate iron absorption as accurately as the well-validated technique of plasma area under the curve using labelled test meals.

DESIGN

The study is a randomised cross-sectional intervention.

SETTING

The study was carried out at the Human Nutrition Unit at the Institute of Food Research, Norwich, UK.

SUBJECTS

A total of nine female volunteers, aged 33+/-8 y.

INTERVENTIONS

Volunteers were given an oral dose (approximately 5 mg) of Fe-57 as iron sulphate in an orange juice test drink and simultaneously infused Fe-58 (approximately 200 microg) as iron citrate over 90 min. Multiple blood samples were taken for the following 6 h. The samples were analysed by mass spectrometry and iron absorption was estimated using a mathematical model based on the appearance of Fe isotopes in plasma and the area under the curve technique.

RESULTS

The geometric mean (-1 s.d., +1 s.d.) absorption of the model estimate is 16% (9, 31) and the area under curve estimate is 18% (8, 29).

CONCLUSIONS

Results indicate that a compartmental model can be used to estimate labelled iron absorption although it is unlikely that this new method will be used in favour of an existing one. Further studies are now needed with unlabelled iron to assess whether the technique could have application in the assessment of total (haem+nonhaem) iron absorption from food.

Bioavailability of selenium from fish, yeast and selenate: a comparative study in humans using stable isotopes

OBJECTIVE

To measure the bioavailability of selenium from cooked and raw fish in humans by estimating and comparing apparent absorption and retention of selenium in biosynthetically labelled fish with labelled selenate and biosynthetically labelled selenium in brewers yeast.

DESIGN

The intervention study was a parallel, randomised, reference substance controlled design carried out at two different centres in Europe.

SETTING

The human study was carried out at the Institute of Food Research, Norwich, UK and at TNO Nutrition and Food Research, Zeist, The Netherlands.

SUBJECTS

In all, 35 male volunteers aged 18-50 y were recruited; 17 subjects were studied in Norwich (UK) and 18 in Zeist (Netherlands). All of the recruited subjects completed the study.

INTERVENTIONS

Biosynthetically labelled trout fish (processed by two different methods), biosynthetically labelled brewers yeast and isotopically labelled selenate were used to estimate selenium apparent absorption and retention by quantitative analysis of stable isotope labels recovered in faeces and urine. Subjects consumed the labelled foods in four meals over two consecutive days and absorption was measured by the luminal disappearance method over 10 days. Urinary clearance of isotopic labels was measured over 7 days to enable retention to be calculated.

RESULTS

Apparent absorption of selenium from fish was similar to selenate and there was no difference between the two processing methods used. However, retention of fish selenium was significantly higher than selenate (P<0.001). Apparent absorption and retention of yeast selenium was significantly different (P<0.001) from both fish selenium and selenate.

CONCLUSION

Fish selenium is a highly bioavailable source of dietary selenium. Cooking did not affect selenium apparent absorption or retention from fish. Selenium from yeast is less bioavailable.

Use of the stable isotope (106)Cd for studying dietary cadmium absorption in humans

Hydroponically grown wheat was intrinsically labelled with the stable isotope 106cadmium (106Cd) and the flour made into a porridge. The abundance of the isotope in the porridge was approximately 30 times the natural abundance, but the total level of Cd in the porridge was 0.03 mg/kg fresh weight, which was the same as expected in a normal diet. Cadmium measurements were made using inductively coupled plasma-mass spectrometry (ICP-MS). The porridge was eaten at breakfast by adult and infant volunteers. Bulked faecal collections were analysed for unabsorbed Cd. Initial results suggest that the apparent absorption of Cd may be higher than 5% as commonly quoted, but longer faecal collection times may be necessary to confirm this.

Bioavailability of iron glycine as a fortificant in infant foods

The bioavailability of iron glycine added to a vegetable infant weaning food was compared with ferrous sulfate. Stable, isotopically labeled compounds (57Fe or 58Fe) were mixed into the midday meal (1.4 mg added Fe/serving) and fed to 9-mo-old infants on alternate days for 8 d. Bioavailability, expressed as a percentage of the dose consumed, was measured from isotopic enrichment of hemoglobin 14 d after the last test meal. There was no difference between iron glycine and ferrous sulfate (x+/-SEM): 9.0+/-0.7% and 9.9+/-0.8%, respectively. The effect of chelation was examined by measuring iron bioavailability of iron glycine and ferrous sulfate added to a high-phytate (310 mg/100 g) whole-grain cereal weaning food and comparing it with a lower-phytate (147 mg/100 g) vegetable food, as used in the first study. Both iron compounds had lower bioavailability from the high-phytate food, 5.2+/-0.5% for iron glycine and 3.8+/-0.9% for ferrous sulfate, than the lower-phytate food, 9.8+/-1.5% for iron glycine and 9.1+/-1.3% for ferrous sulfate. The results showed no significant difference in bioavailability between the two forms of iron when added to infant weaning foods, suggesting that the glycine complex was fully or partially dissociated in the gastrointestinal tract. It is concluded that chelation does not improve the bioavailability of iron in the presence of dietary inhibitors.

Factors affecting iron stores in infants 4-18 months of age

OBJECTIVES

To determine the effects of dietary, physiological or environmental factors on body iron levels in infants aged 4-18 months.

DESIGN

The daily iron intake of the infants was measured from a diet history obtained by interview using a standardised question sheet, previously validated against weighed intake (minimum 3 days) in an independent sample of 8 and 18 month old infants. Capillary blood samples were analyzed for haemoglobin, mean cell volume, haematocrit, zinc protoporphyrin and plasma ferritin concentration. Ferritin values were log-transformed prior to analysis to give a better approximation to the normal distribution and forward stepwise multiple linear regression was carried out using SPSS.

SETTING

The city of Norwich, UK and some of its suburbs.

SUBJECTS

One hundred and eighty-one healthy infants in age groups 4, 8, 12 and 18 months.

RESULTS

Main determinants of iron stores in the 4 month old infants were birth weight (+ve (P < 0.001)) and body weight (-ve (P < 0.005)). In the 8 month old infants intake of cow's milk (-ve (P < 0.05)), belonging to a smoking household (-ve (P < 0.05)) and quantity of commercial babyfood consumed (+ve (P < 0.05)) were significant. In this age group there was a gender effect (girls > boys (P < 0.01)) and the gender effect remained at 12 months (girls > boys (P < 0.05)), but at 18 months only non-haem iron intake was a significant factor (-ve (P < 0.05)).

CONCLUSIONS

At 4 months of age birth weight and body weight exert the greatest influence on iron stores, whereas by 8 months components of the weaning diet have an effect (commercial babyfood (+ve), cow's milk (-ve)); there is also a gender effect (girls > boys), possibly reflecting the different growth rate between boys and girls. At 12 and 18 months the only significant factors are gender (girls > boys) and non-haem iron intake (-ve) respectively.

Zinc absorption in infants fed iron-fortified weaning food

The effect of fortification iron (reduced iron) on zinc absorption from a commercial vegetable-based weaning food was assessed in 11 9-mo-old infants. Each infant was fed a test meal of unfortified or iron-fortified product, labeled extrinsically with 1 mg 67Zn or 70Zn (as citrate), and the next day was fed the second product labeled with the other isotope. A complete fecal collection was carried out for 3-4 d, and the amount of unabsorbed isotope measured by thermal-ionization quadrupole mass spectrometry. Apparent zinc absorption (isotope intake minus fecal excretion, expressed as the % of dose administered) was 31.1 +/- 8.3% (x +/- SD) from the iron-fortified food and 28.6%28.6 +/- 10.5% from the unfortified food. These values were not significantly different, thus iron fortification of the weaning food did not reduce zinc absorption.

The validity of extrinsic stable isotopic labeling for mineral absorption studies in rats

The use of extrinsic stable and radioisotopic labels (Fe, Zn, Cu and Se) was compared with the use of intrinsic labels by measuring label retention in rats. Saccharomyces cerevisiae (Hansen strain CBS 1171) was prepared intrinsically enriched with a stable isotope of iron, zinc, copper or selenium, and unenriched freeze-dried yeast was extrinsically labeled with the appropriate stable and/or radioisotope. Male Wistar rats, weighing 80-100 g and fed a purified diet, were given a test meal of one of the above labeled yeasts. Isotopic retention was determined by fecal monitoring. Retention of the stable isotopes was determined by thermal ionization quadruple mass spectrometry (TIQMS) and retention of the radioisotopes by counting feces in a whole-body counter. The results indicated that the behavior of the labels differed among the minerals, with copper as the only one in which the intrinsic and extrinsic stable isotopes were comparably retained. With zinc, retention of the extrinsic radiolabel and intrinsic label was similar, but retention of the extrinsic stable isotope label was higher. With iron, the intrinsic label had a significantly lower retention than the two extrinsic labels; with selenium, retention of all three labels was different, but these differences were not of a sufficient magnitude to conclude that extrinsic stable isotopic labelling is not valid. These results demonstrate that an extrinsic stable isotope label can be used for copper, selenium and inorganic iron, but that such a label is not valid for studies on zinc.

Assessment of zinc bioavailability: studies in rats on zinc absorption from wheat using radio- and stable isotopes

Absorption from wheat intrinsically and extrinsically labelled with 67Zn and extrinsically labelled with 65Zn was measured from 67Zn faecal excretion and 65Zn whole-body retention in rats. There were significant differences between the extrinsically- and intrinsically-labelled 67Zn (P < 0.001), but not between the extrinsically-labelled 65Zn and intrinsically-labelled 67Zn. The effect of chicken meat on the absorption of Zn from intrinsically-labelled wheat was also studied in the rat. Mean Zn absorption from wheat and chicken meat fed separately was 18.5 and 68.2% respectively, and from a mixture of the two containing the same level of Zn was 50.1%. The apparent absorption of Zn from the composite meal was significantly higher than predicted from the results of the foods on their own (P < 0.001).

The measurement of exchangeable pools of zinc using the stable isotope 70Zn

The present study was designed to assess the feasibility of using small doses of a stable isotope of Zn to follow plasma kinetics over a 10 d period and, hence, make deductions about Zn turnover and body pool sizes. At the beginning of the 10 d metabolic balance, two adults, consuming their habitual diet, were given an intravenous injection of 70Zn. There was a fourfold difference in the administered dose between the two subjects (0.445 and 2.078 mg). Blood samples were taken at regular intervals and plasma enrichment with 70Zn measured by thermal ionization mass spectrometry. Urine and faeces were collected and analysed for Zn and 70Zn. Kinetic analysis of the plasma 70Zn decay by several different methods was undertaken. It was apparent from both deconvolution analysis of the short-term (0-90 min) decay data and four-compartment modelling of the longer-term (0-24 h) data that isotopic Zn very rapidly equilibrates with the plasma Zn and with a rapidly exchanging non-plasma pool, probably located within the liver. This latter pool appears to contain less than 10 mg Zn and the peak of isotope enrichment occurs at about 20 min post injection. The later decay of plasma Zn enrichment appears to be dictated by exchange with a much larger pool of approximate size 350 mg.

Zinc absorption in adult men from a chicken sandwich made with white or wholemeal bread, measured by a double-label stable-isotope technique

Eleven fasted adult men consumed a chicken meat sandwich made with white or wholemeal bread, extrinsically labelled with 2 mg 67Zn, on two different occasions. Immediately after eating the sandwich they were given an intravenous injection of 1.5 mg 70Zn. True Zn absorption (which was approximately 7% higher than apparent absorption) was determined by the faecal balance technique by making an allowance for endogenous excretion from measurements of faecal excretion of 70Zn. There was no significant difference in mean true Zn absorption from the white or wholemeal bread sandwich, 33.6 and 25.4% respectively. It was concluded that the substitution of wholemeal for white bread does not reduce Zn absorption from meat-based sandwiches.

A preliminary study of the bioavailability of iron- and zinc-glycine chelates

Groups of rats were fed diets containing marginal levels of Fe and Zn as glycine chelates (tradename 'Chelazome', Albion Laboratories, Verona, New Jersey, USA), or the same level of mineral as ferrous sulphate or zinc carbonate. The Fe diets were fed to weanling rats for 4 weeks and the Zn diets to young adult rats for 5 weeks. Blood Hb concentrations were significantly higher in the group fed Fe-chelazome than ferrous sulphate, 149 and 128 g/l respectively (P less than 0.001), but PCV and liver Fe concentrations were similar between the two groups. No difference in plasma Zn, pancreas, testes or femur Zn concentrations were observed between the two Zn groups, indicating that Zn-chelazome has no advantage over zinc carbonate. The results of this preliminary study indicate that Fe-chelazome has a higher bioavailability than ferrous sulphate and merits further study.

Apparent zinc absorption by rats from foods labelled intrinsically and extrinsically with 67Zn

A variety of foods (peas (Pisum sativum), chicken meat, eggs, goat's milk, human milk) enriched with the stable isotope 67Zn were prepared by means of intrinsic- and extrinsic-labelling procedures. They were fed to rats and apparent absorption of 67Zn determined from faecal excretion measurements using thermal ionization mass spectrometry. There were significant differences in the absorption of the extrinsic and intrinsic label which differed in magnitude between the foods tested. The extrinsic 67Zn was less well absorbed in peas, chicken meat, eggs, and human milk than intrinsic 67Zn, but in goat's milk the extrinsic 67Zn was better absorbed than the intrinsic label. These results demonstrate that extrinsically-added stable Zn isotopes do not fully exchange with endogenous Zn in many foods, and illustrate the need for caution when using extrinsic labels for Zn bioavailability studies.

Intrinsic labelling of different foods with stable isotope of zinc (67Zn) for use in bioavailability studies

Intrinsically-labelled foods are required to validate extrinsic-labelling techniques used to study the bioavailability of trace elements. Wheat (Triticum aestivum), peas (Pisum sativum), goat's milk, human milk, eggs and chicken meat were selected for intrinsic-labelling studies with 67Zn. Peas were grown hydroponically in enriched nutrient solution and wheat was grown in sand and watered with enriched nutrient solution. Some of the wheat plants were also given stem injections of 67Zn solution. Eggs and chicken meat were prepared by administering 67Zn intravenously to chickens, and human milk was collected after an oral dose of 67Zn in a cola drink. All the foods investigated were sufficiently enriched with 67Zn for Zn absorption studies except wheat prepared by the sand and water-culture method.

Fat consumption and factor VII coagulant activity in middle-aged men. An association between a dietary and thrombogenic coronary risk factor

Diet was measured by 5-day weighed inventory to search for an association between fat intake in the general population and factor VII coagulant activity (VIIc), a strong predictor of coronary heart disease. Of 275 men aged 40-59 years registered with a medical practice, 203 (74%) participated and 170 (62%) provided a satisfactory record. After allowance for the increase in fat intake with body size, a statistically significant and positive association was found between dietary fat and VIIc (r = 0.18; P less than 0.05). The correlation coefficient was increased to 0.24 when adjusted for the effect of day-to-day variability in individual fat intake, thereby providing an improved estimate of the true strength of association. The mean difference in VIIc of 12% of standard between men in the highest and lowest quarters of the distribution of fat intake was similar to that reported between men experiencing coronary heart disease and those remaining free. The results support previous experimental fat-feeding studies and suggest that a high fat diet has adverse consequences for blood coagulability and coronary thrombosis.

Dietary intake of men in Wembley, London, by weighed inventory: comparison with national recommendations, with particular emphasis on fat intake

To assess the extent to which current dietary habits conform with the national nutritional guidelines aimed at reducing the high national mortality from coronary heart disease (CHD), 203 of 275 men (74 per cent) aged 40-59 years in a London community provided a weighed record of all food and drink consumed over 5 d. Satisfactory records were returned by 170 men weighing at least 75 per cent of their intakes, including 43 on medical or self-imposed diets. Mean (s.d.) energy intake was 2476 (590) kcal (10.38 MJ), comprising 37 per cent fat, 42 per cent carbohydrate, 14.7 per cent protein and 6.3 per cent alcohol. Butter or butter spreads were used by 53 per cent, sunflower margarine by 29 per cent and low fat spreads by 13 per cent; lard or dripping was used by 23 per cent of households as the main cooking fat. Intakes of vitamins and trace elements were generally above recommended amounts. These results are closely comparable with the only similar studies for the UK undertaken in Wales and Scotland. Compared with national guidelines, only 32 per cent of men met the short-term goals for fat intake, and even fewer the long-term goals. Carbohydrate and fibre intake remained too low. Alcohol intake exceeded the short-term goal (5 per cent of energy of less) in 36 per cent of men. Greater public health effort will be needed to reduce fat intake further for primary prevention of CHD, particularly as habitual intake is probably underestimated by the weighed dietary record.