The effects of different levels of dietary restriction on aging and survival in the Sprague-Dawley rat: implications for chronic studies.

A study was undertaken to determine the effects of incremental levels of dietary restriction (DR) in rats. Survival, growth, reproductive, and dietary intake (DI) variables were monitored in a chronic study in which male Sprague Dawley (SD) rats (NCTR colony) were fed their ration ad libitum (AL), or DR. The main objectives were to determine if low levels of DR could be used to increase the survival rate of SD rats in the chronic bioassay, and to identify the survival characteristics of a long-lived SD rat strain (NCTR colony). The average life span of AL rats was 115 months. At 104 weeks on study (110 weeks of age), the survival rate for the AL and 10%, 25%, and 40% DR groups was 63.4, 87.5, 87.5, and 97.5%, respectively. The largest increase in survival (24.1%) occurred between AL and 10% DR, indicating that very low levels of DR have a significant effect on survival. Whole-body, liver, prostate, and epididymis weights and body length were decreased by DR, whereas brain weight, testicular weight, and skull length were not altered by DR. Rats from the NCTR colony were found to be ideal for chronic studies because they are much longer-lived than other SD stocks. Although the 104-week survival rate for these SD, non-obese AL rats exceeds the FDA's "Redbook" survival guideline (> 50%) for chronic bioassays, the use of DR is advocated because it reduces individual variability in body weight.

ICCVAM evaluation of the murine local lymph node assay. Conclusions and recommendations of an independent scientific peer review panel.

The validation status of the murine local lymph node assay (LLNA), a method for assessing the allergic contact dermatitis potential of chemicals, was evaluated by an independent peer review panel (Panel) convened by the Interagency Coordinating Committee on the Validation of Alternative Methods (ICCVAM). The LLNA measures lymphocyte proliferation using incorporation of radioactive thymidine or iododeoxyuridine into cells of the draining lymph nodes of mice topically exposed to a test article. The Panel concluded that the assay performed as well as currently accepted guinea pig methods [guinea pig maximization test (GPMT)/Buehler assay (BA)] for the hazard identification of strong to moderate chemical sensitizing agents, but that it might not correctly identify all weak sensitizers or metals (potential false negative response) or all strong irritants (potential false positive response). The Panel concluded also that the LLNA involves less pain and distress than conventional guinea pig methods. The Panel unanimously recommended the LLNA as a stand-alone alternative for contact sensitization hazard assessment, provided that certain protocol modifications were made. These included collection of individual, rather than pooled, animal response data; the inclusion of a concurrent positive control; and consideration of dose-response information and statistical analyses. A standardized LLNA protocol is provided.

Food and Drug Administration Proposed Guidelines for Neurotoxicological Testing of Food Chemicals.

The fact that some chemicals may adversely affect the nervous system is certainly not a new concept in regulatory toxicology. In 1982, the FDA issued testing guidelines for the safety evaluation of proposed direct food and color additives which included the assessment of nervous system toxicity as part of the general toxicological profile. However, these guidelines provide only broad and nonspecific recommendations as to how this assessment may best be carried out. The information derived from toxicity screening studies conducted according to these guidelines enable little more than the detection of clearly evident adult nervous system toxicity associated with general neuropathology and overt neurological dysfunction. Little consistent or systematically documented information is typically developed about other equally important types of neurotoxic effects including, for example, behavioral dysfunction and developmental neurotoxicity. Concerns about these more subtle types of neurotoxic effects have become a prominent public health issue and have resulted in demands for an increasing level of assurance that efforts are being made to minimize even further the risks of neurotoxicity from human exposure to chemical substances. In an effort to address these concerns, the FDA is including specific attention to neurotoxicity in a proposed revision of its toxicity testing guidelines for food additives. These proposed guidelines focus on a more careful evaluation of structural and functional measures of neurotoxicity as a routine component of safety assessment. This focus will enable the development of the type of information needed for a more effective assessment of the full spectrum of neurotoxic hazards. The revised guidelines for neurotoxicity testing will be discussed in terms of the FDA's overall approach to safety assessment.

Evaluation of vitamin A toxicity.

Toxicity has been associated with abuse of vitamin A supplements and with diets extremely high in preformed vitamin A. Consumption of 25,000-50,000 IU/d for periods of several months or more can produce multiple adverse effects. The lowest reported intakes causing toxicity have occurred in persons with liver function compromised by drugs, viral hepatitis, or protein-energy malnutrition. Certain drugs or other chemicals may markedly potentiate vitamin A toxicity in animals. Especially vulnerable groups include children, with adverse effects occurring with intakes as low as 1,500 IU.kg-1.d-1, and pregnant women, with birth defects being associated with maternal intakes as low as approximately 25,000 IU/d. The maternal dose threshold for birth defects cannot be identified from present data. An identifiable fraction of the population surveyed consumes vitamin A supplements at 25,000 IU/d and a few individuals consume much more. beta-Carotene is much less toxic than vitamin A.

FDA's priority-based assessment of food additives. III. Specific toxicity parameters.

In this third paper, we focus on a subset of 58 additives in the Priority-Based Assessment of Food Additives (PAFA) Project that have been associated in toxicity studies with specific toxic effects of inherently high concern. Additives producing these effects are assigned to categories based on chemical structure. A probability distribution of "R values" is calculated. We examine whether any apparent correlation exists between chemical structure category assignment and a tendency for compounds in a category to produce certain types of toxic response. Many chemical structure categories include additives having little or no toxicological data. Although this circumstance limits the scope of conclusions that may be drawn about structure-activity relationships, certain "trends" in the data appear to be worth additional future investigation.

FDA's priority-based assessment of food additives. II. General toxicity parameters.

We examine in greater depth some general toxicity parameters that have been compiled for food additives in the Priority-Based Assessment of Food Additives project. Correlations between chemical structure categories and basic parameters such as lowest effect level, highest no-effect level, and study duration indicate (among other things) trends in these parameters that are consistent in magnitude and direction with the Concern Level break points proposed in the FDA's Redbook. Certain data on the nature and extent of observed toxic effects of additives, when related to corresponding estimated levels of human intake, help to provide a profile of toxicological characteristics of a number of the regulated additives. The resulting breadth of such a profile reinforces the need for systematic priority setting in the allocation of resources to potential food additive safety concerns.

FDA's priority-based assessment of food additives. I. Preliminary results.

The principles outlined in the Bureau of Foods' recently published Toxicological Principles for the Safety Assessment of Direct Food Additives and Color Additives Used in Food (the Redbook) have been systematically applied to a Bureau of Foods food additive data base comprising nearly 1600 food and color additives and synthetic flavoring substances. The present publication is the first of several papers reviewing and analyzing the results of this work. This paper describes the content and organization of the scientific data base that the Bureau has compiled, and presents some preliminary general conclusions regarding the processing of these data according to Redbook principles.