National nutrition and public health policies: issues related to bioavailability of nutrients when developing dietary reference intakes

Dietary reference intakes (DRI), like its predecessor, the recommended dietary allowances (RDA) and the Recommended Nutrient Intakes (RNIs), are reference values, based on the best scientific evidence available. They serve as reference amounts of specific nutrients and food components for use in assessing the adequacy of and in planning for nutritious diets. They have been used for over 50 y as the basis for national nutrition monitoring and intervention programs in the United States, Canada, and other countries and as the basis for dietary guidance developed for both individuals and for targeted groups of people. Thus, although not developed for specific policy applications, they have represented the best scientific perspectives regarding what should be the basis for nutrition and public health policy related to foods and supplements. In determining DRIs, as was the case with the RDA, significant attention must be paid to the form of the nutrient or food component that is evaluated. Research conducted to determine how much of a nutrient is needed must evaluate the chemical form provided, the matrix in which it is given and the effect of other food components on absorption and/or utilization. Because the DRI recommendations will be used in population-wide policy development, assumptions must be made explicitly about what is expected for all of these factors in a typical diet. At the same time, where data exist relative to nontypical but potentially very significant effects on bioavailability, these must also be delineated to be of use in a variety of settings. Finally, one of the most important aspects of determining bioavailability in developing reference intakes is that as new information emerges, new complexities enter into the process. As more chemical complexes of nutrients and food components become available in the marketplace, new bioavailability factors may need to be established. Examples of such changes exist in the DRI reports already published for vitamin B-12 and folate and in previous RDA for iron and protein. It is often the different assumptions related to bioavailability that alter the reference intakes used as the basis for public health policy in different countries, rather than the basic science from which the recommendation is derived.

Dietary Reference Intakes: the new basis for recommendations for calcium and related nutrients, B vitamins, and choline

Dietary Reference Intakes (DRIs) represent the new approach adopted by the Food and Nutrition Board to providing quantitative estimates of nutrient intakes for use in a variety of settings, replacing and expanding on the past 50 years of periodic updates and revisions of the Recommended Dietary Allowances (RDAs). The DRI activity is a comprehensive effort undertaken to include current concepts about the role of nutrients and food components in long-term health, going beyond deficiency diseases. The DRIs consist of 4 reference intakes: the RDA, which is to be used as a goal for the individual; the Tolerable Upper Intake Level (UL), which is given to assist in advising individuals what levels of intake may result in adverse effects if habitually exceeded; the Estimated Average Requirement (EAR), the intake level at which the data indicate that the needs for 50% of those consuming it will not be met; and the Adequate Intake (AI), a level judged by the experts developing the reference intakes to meet the needs of all individuals in a group, but which is based on much less data and substantially more judgment than that used in establishing an EAR and subsequently the RDA. When an RDA cannot be set, an AI is given. Both are to be used as goals for an individual. Two reports have been issued providing DRIs for nutrients and food components reviewed to date: these include calcium and its related nutrients: phosphorus, magnesium, vitamin D, and fluoride; and most recently, folate, the B vitamins, and choline. The approaches used to determine the DRIs, the reference values themselves, and the plans for future nutrients and food components are discussed.

Variables affecting antisense RNA inhibition of gene expression

We have sought to determine what variables affect the extent of inhibition of gene expression by stable nuclear-derived antisense RNAs. Myosin heavy chain gene II (MHCII) expression in Dictyostelium was used as a model system, because previous results have shown that nearly complete inhibition of expression can be achieved under appropriate conditions. Various fragments of the myosin gene were inserted into several transformation vectors in both sense and antisense orientation, and the effects on expression of protein and RNA from the endogenous MHC II gene were assayed. The results indicate that the critical factor was the particular fragment of the gene used to produce the antisense RNA. Some fragments produced complete inhibition of expression, whereas others gave only slight inhibition. The fragments that produced the greatest inhibition were from the tail region of the gene. In addition, cells were capable of overcoming the inhibition while still expressing the antisense RNA. We hypothesize that the three-dimensional topology of the antisense and sense RNAs determines their accessibility for interstrand hybridization. Transformation with sense fragments of the myosin gene also caused inhibition of the endogenous myosin gene. The inhibition seen with sense expression is not as dramatic as that for antisense, but it had the same phenotypic consequences for the cells. This phenomenon, which has now been documented in other systems, may be mechanistically similar to inhibition by antisense RNA in our system.

Calmodulin dependence of transferrin receptor recycling in rat reticulocytes

Kinetic analysis of transferrin receptor properties in 6-8 day rat reticulocytes showed the existence of a single class of high-affinity receptors (Kd 3-10 nM), of which 20-25% were located at the cell surface and the remainder within an intracellular pool. Total transferrin receptor cycling time was 3.9 min. These studies examined the effects of various inhibitors on receptor-mediated transferrin iron delivery in order to define critical steps and events necessary to maintain the functional integrity of the pathway. Dansylcadaverine inhibited iron uptake by blocking exocytic release of transferrin and return of receptors to the cell surface, but did not affect transferrin endocytosis; this action served to deplete the surface pool of transferrin receptors, leading to shutdown of iron uptake. Calmidazolium and other putative calmodulin antagonists exerted an identical action on iron uptake and receptor recycling. The inhibitory effects of these agents on receptor recycling were overcome by the timely addition of Ca2+/ionomycin. From correlative analyses of the effects of these and other inhibitors, it was concluded that: (1) dansylcadaverine and calmodulin antagonists inhibit iron uptake by suppression of receptor recycling and exocytic transferrin release, (2) protein kinase C, transglutaminase, protein synthesis and release of transferrin-bound iron are not necessary for the functional integrity of the iron delivery pathway, (3) exocytic transferrin release and concomitant receptor recycling in rat reticulocytes is dependent upon Ca2+/calmodulin, (4) dansylcadaverine, dimethyldansylcadaverine and calmidazolium act on iron uptake by interfering with calmodulin function, and (5) the endocytotic and exocytotic arms of the iron delivery pathway are under separate regulatory control.

Resting metabolic rate: measurement reliability

Resting metabolic rate (RMR) usually is measured by indirect calorimetry for a brief interval, and the results are then extrapolated to 24-hr resting energy production. The aim of the current study was to examine the validity of this approach by measuring the within- and between-day variability in RMR. The RMR was measured hourly in 14 healthy adults from 8:00 am to 4:00 pm after an overnight fast on two separate days. The results indicated that, during day 1, RMR remained unchanged from early morning to late afternoon, and there was no significant difference between RMR measurements 1 through 8. Based upon analysis of variance and intraclass correlation coefficients, the averages of the first and the last three RMR measurements were found to be unreliable. RMR reliability was improved by averaging all measurements taken during the day, and maximum reliability was obtained by averaging the middle three RMR measurements. A similar pattern of results was observed during day 2, although the overall trend was for the reliability of RMR measurements to improve relative to day 1. The average RMRs on days 1 and 2 were not significantly different. These results indicate that the current practice of establishing a patient's RMR based upon a single measurement potentially can lead to large errors in determining energy needs. Measurement reliability can be improved by serially measuring RMR, eliminating the initial measurement, and averaging the remaining two to three values.