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Meal-Feeding Rodents and Toxicology Research Gale B. Carey* and Lisa C. Merrill Department of Molecular, Cellular & Biomedical Sciences, University of New Hampshire, Durham, New Hampshire 03824, United States ABSTRACT: Most laboratory rodents used for toxicology studies are fed ad libitum, with unlimited access to food. As a result, ad libitum-fed rodents tend to overeat. Research demonstrates that ad libitum-fed rodents are physiologically and metabolically different from rodents fed controlled amounts of food at scheduled times (meal-fed). Ad libitum-fed rodents can develop hypertriglyceridemia, hypercholesterolemia, diet-induced obesity, nephropathy, cardiomyopathy, and pituitary, pancreatic, adrenal, and thyroid tumors, conditions likely to affect the results of toxicology research studies. In contrast, meal-feeding synchronizes biological rhythms and leads to a longer life span, lower body weight, lower body temperature, hypertrophy of the small intestine, and synchronization of hepatic and digestive enzymes. The circadian rhythms present in nearly all living organisms are entrained by light intensity and food intake, and peripheral clocks in all organs of the body, especially the GI tract and liver, are particularly sensitive to food intake. Feeding schedule has been demonstrated to alter the toxicity and metabolism of drugs including sodium valproate, chloral hydrate, acetaminophen, gentamicin, and methotrexate. Feeding schedule alters the expression of genes that code for Phase I, II, and III proteins, thereby altering the rate and amplitude of drug disposition. Rhythms of plasma insulin and glucagon that fluctuate with food ingestion are also altered by feeding schedule; ad libitum feeding promotes hyperinsulinemia which is a precursor for developing diabetes. The emerging field of chronopharmacology, the interaction of biological rhythms and drugs, will lead to optimizing the design and delivery of drugs in a manner that matches biological rhythms, but it is wise for toxicology researchers to consider feeding schedule when designing these experiments. It has been 10 years since the Society for Toxicologic Pathology voiced its position that feeding schedule is an important variable that should be controlled in toxicology experiments, and research continues to underscore this position.
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CONTENTS
Introduction Circadian Clocks, Meal Feeding, and Chronopharmacology Meal Feeding and Drug Metabolism Pharmacokinetics Drug-Metabolizing Enzymes Meal Feeding and Metabolic Homeostasis Conclusion and Implications for Toxicology Research Author Information Acknowledgments Abbreviations References
The physiologic, metabolic, and biochemical interiore milleu of ad libitum-fed rodents dramatically alters their sensitivity to xenobiotics, compared to rodents fed restricted levels of food.1 This suggests that the meal-fed rodent is physiologically and metabolically different from its ad libitum-fed counterpart. It is precisely these differences that led Keenan to describe ad libitum feeding as the “most poorly controlled variable [in rodent bioassays] and it adversely affects every physiological process and anatomical structure to the molecular level.”2 Keenan has observed hypertriglyceridemia, hypercholesterolemia, and dietary-induced obesity in ad libitum-fed rats compared to that in rats fed a restricted diet. He has observed the development of spontaneous nephropathy, cardiomyopathy, and degenerative changes in multiple organs, as well as early development and progression of pituitary, pancreas, adrenal, and thyroid tumors in ad libitum-fed compared to restricted-fed rats. One outcome of this work is the suggestion that the increasing study-to-study variability associated with rodent bioassays since 1999, particularly in toxicity and carcinogenicity studies, could be explained primarily by the practice of ad libitum feeding.2−5 Indeed, unlimited access to high-energy food, along with a trend toward the genetic
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INTRODUCTION The nutritional status of laboratory rodents used for toxicology studies is rarely controlled. In contrast to canines, pigs, and nonhuman primates, who are fed controlled amounts of food at scheduled times, laboratory rodents are typically fed ad libitum, with constant and unlimited access to food. Given this freedom, rodents will eat intermittently throughout the dark cycle and continue to consume small amounts of food during the light cycle. As a result, ad libitum-fed rodents tend to overeat. © 2012 American Chemical Society
Received: March 12, 2012 Published: May 29, 2012 1545
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responsible. These findings were confirmed by Damiola et al.,32 by measuring mRNA expression of several clock genes and transcription factors in tissues of mice under time-restricted feeding conditions. Their findings were consistent with those of Stokkan et al.34 and were extended to demonstrate that the response of the liver was most abrupt, followed by the kidney, heart, and pancreas and that even in mice kept under constant darkness, restricted feeding uncoupled the response of peripheral clock genes from the SCN, confirming that feeding time does not entrain SCN neurons. Damiola et al. propose that the role of circadian gene expression in the liver is to respond to the processing of food.32 How does this relate to toxicology? It has long been known that the toxicity, efficacy, metabolism, and elimination of many drugs change over a 24-h period.35 For example, Kim and Lee reported that a single 400 mg/kg i.p. dose of acetaminophen was hepatotoxic to mice at 20:00 h but not at 8:00 or 14:00.36 More recent knowledge about the clock genes and other molecular mechanisms that underlie these circadian changes has given rise to the field of chronopharmacology, the study of the interaction of biological rhythms and drugs, and chronopharmaceutics, the design and evaluation of delivering drugs in a time-controlled manner and rhythm that matches biological requirements.30 The inherent circadian rhythms of many diseases, including asthma, arthritis, cancer, diabetes, hypertension, and hypercholesterolemia, make chronopharmaceutics a promising field for the health care industry.30
selection of animals that grow faster and exhibit increased fecundity, has resulted in increased rodent weights.1 Martin et al.,6 in their excellent review on the standard control rodent as metabolically morbid, note that “the beneficial effects of some drugs in animal models might result from their effects on processes associated with an unhealthy lifestyle rather than a specific effect of the drug on the disease process. This is a critical issue that should be addressed in future studies.” One way to control the food intake of rodents in a way that mimics that in the wild, and the focus of this review, is meal feeding. Meal feeding, also referred to as time-limited feeding, time-restricted feeding, or timed feeding, grants animals unlimited access to food during a defined time span. The meal time can vary from 2 to 4 h every 24 h for smaller animals to 8 h every 48 h for larger animals.7 This method of feeding laboratory rodents, predominantly rats, has been employed since the 1950s in nutrition research. Meal feeding is an important synchronizer of many behaviors and biological rhythms, including food anticipatory activity and organization of metabolism.8−16 Meal feeding restricts weight gain in experimental rodents compared to that in ad libitum-fed rodents: ad libitum feeding causes rapid weight gain, a trend that continues throughout the life of the animal.17−19 Other physiologic and metabolic adaptations that occur in the mealfed versus the ad libitum-fed rodent are a longer life span, lower body temperature, decreased mitotic activity, hypertrophy of the stomach and small intestines, delayed gastric emptying, alteration of circadian rhythms, and synchronization of numerous hepatic and digestive enzymes.8−10,20−28 Does the feeding protocol of rodents used in toxicological studies influence research findings? Findings from studies conducted over the past 40 years would suggest that it does. This perspective will examine many of these findings, with particular attention to how meal feeding affects chronopharmacology, drug metabolism, and metabolic homeostasis. The goal of this perspective is to stimulate the thinking of toxicologists to consider feeding protocol when designing experiments.
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MEAL FEEDING AND DRUG METABOLISM Given, then, that drug metabolism is subject to circadian rhythm and that meal feeding can entrain peripheral circadian rhythms, a logical question is this: does the pattern of food intake alter the metabolism of drugs? Nakano et al.37 were the first to report that feeding schedule could influence the circadian rhythm of drug action and kinetics. Mice fed for 8 h during the light period demonstrated a 12-h shift in their toxic response to phenobarbital, compared to that by ad libitum-fed mice. Song et al. found that mice injected with 100 mg/kg i.p. methotrexate38 or 180 mg/kg s.c. gentamicin39 and fed for 8 h during the light period had a higher mortality, higher plasma drug concentration, and lower clearance of the drug during the dark (nonfeeding) period compared to that in mice fed ad libitum. Feeding schedule has been demonstrated to alter the toxicity and metabolism of sodium valproate,40 chloral hydrate,41 and acetaminophen.42 It is clear, then, that controlled food intake, in comparison to the usual practice of ad libitum feeding, has striking effects on the toxicity, efficacy, and processing of drugs. Pharmacokinetics. Song et al. examined the effect of feeding schedule on the pharmacokinetics of gentamicin, an antibiotic used to treat gram-negative bacterial infections,39 and methotrexate, a potent anticancer agent.40 Six-week old ICR mice were meal-fed for 8 h during the light cycle, or fed ad libitum. After 14 days, mice were injected with 180 mg of gentamicin/kg s.c. or 100 mg of methotrexate/kg i.p. The plasma gentamicin concentration 30 min after dosing and total body clearance of gentamicin was shifted by 12 h as a result of the meal feeding, demonstrating that feeding schedule modifies gentamicin pharmacokinetics. Methotrexate was even more sensitive to meal feeding, in which plasma levels at critical times of the day were from 50% to 130% higher compared to that in ad libitum-fed mice.
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CIRCADIAN CLOCKS, MEAL FEEDING, AND CHRONOPHARMACOLOGY The master circadian clock in mammals resides in the suprachiasmatic nucleus (SCN) located in the ventral part of the anterior hypothalamus.29,30 The SCN is entrained primarily by the 24-h variation in light intensity31 and is relatively resistant to the effects of food intake. The SCN signals secondary clocks in the form of clock genes, including Clock, Bmal1, Per1−3, and Cry1−2, and their protein products in both the SCN and brain as well as peripheral tissues such as the liver, heart, muscle, kidney, pancreas, adipose tissue, leukocytes, smooth muscle, and lung.32,33 While light synchronizes the master circadian clock, the feeding−fasting cycle can shift clock gene oscillations in the periphery.32,34 Using transgenic mice in which a luciferase reporter was linked to the Per1 gene, Stokkan et al.34 demonstrated that meal-feeding animals for a 4 h-period each day during the light cycle caused a 12-h shift in hepatic clock gene expression after only two days. The response of the lung tissue was slower and less pronounced, shifting six hours after seven days of meal feeding. There was no change in clock expression in the SCN, which remained phase-locked to the light−dark cycle. The liver and lung shifts did not respond to the corticosterone injection, suggesting that other feeding signals such as taste of food, stomach distension, metabolite, or insulin levels may be 1546
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Ohdo et al.40 examined the influence of meal feeding (8 h during the light phase of 0700−1900) of 5-week old male ICR mice on the pharmacokinetics and efficacy of valproic acid (VA), an antiepileptic drug. When VA was administered as a 600 mg/kg oral bolus at 0900, the plasma level of VA 30 min later was the highest in meal-fed mice (1550 ug/mL) but lowest in ad libitum-fed mice (500 ug/mL), with the curves inverse to each other. When VA was constantly infused at 1072.6 ug/h at 0900, plasma VA was identical for ad libitumand meal-fed mice at 0900 (approximately 33 ug/mL). However, the levels peaked at 55 ug/mL for ad libitum-fed mice at 1700, then declined to 25 ug/mL at 0500, while levels remained low at 25 ug/mL for meal-fed mice at 1700, then rose to 55 ug/mL at 0500. The authors conclude that VA action and kinetics are altered by feeding schedule regardless of the light− dark cycle, suggesting feeding may be a stronger synchronizer of VA kinetics and response. Moreover, short-term manipulation of feeding, that is, fasting the ad libitum-fed mice for 12 h before the experiment, did not overcome the rhythm created by the feeding schedule. The authors speculate that regional blood flow, which is altered by food intake, may be a controlling factor by which food intake affects drug kinetics and activity. Moreover, the authors note that short-term manipulations of feeding do not overcome metabolic shifts induced by meal feeding. Drug-Metabolizing Enzymes. The metabolism of drugs is conducted by three groups of phase I, II, and III proteins. Phase I proteins are the microsomal cytochrome P450 superfamily of enzymes with oxidase, reductase, and hydrolase activities that can inhibit or activate drugs, while phase II proteins contain the sulfotransferases, UDP-glucuronotransferases, glutathione-Stransferases, and N-acetyltransferases that can conjugate drugs to deactivate them or alter their hydrophilicity. Phase III transporters contain the solute carrier and ATP-binding cassette proteins that transport drugs across biological membranes, into or from the cells of the body. The genes responsible for expressing phase I, II, and III proteins are subject to circadian rhythm. Using male C57BL/6 mice on a 12 h light−dark cycle (lights on 5 a.m.) and fed ad libitum, Zhang et al.43 demonstrated that phase I enzymes are more highly expressed during the dark phase, while the phase II enzyme expression pattern suggests more glutathione conjugation in the early light phase, glucuronidation in the latelight phase, and sulfation in the early dark phase. A key question that arises from this work is as follows: Is the circadian expression of drug-metabolizing enzymes, which can be eliminated in SCN-lesioned rats, influenced by peripheral clocks that respond to food intake? Hirao et al.44 addressed this question by examining the effect of meal feeding on total hepatic phase I enzyme activity. Male F344/DuCrj rats were divided into two groups: ad libitum fed or meal fed for 8 h during the light period. After an 8-day adaptation, the left lateral lobes of the livers were removed and assayed for hepatic 7-alkoxycoumarin O-dealkylase activity which reflects global phase I activity. Meal feeding during the light cycle inverted the circadian rhythm of hepatic alkoxycoumarin O-dealkylase activity, demonstrating that it is circadian factors in peripheral, not central, organs that drive hepatic phase I enzyme rhythms. In mice, the commonly used analgesic acetaminophen is bioactivated to N-acetyl-p-benzoquinone imine (NAPQI), which can deplete hepatic glutathione and cause acetaminophen toxicity. This bioactivation occurs via the cytochrome
P450 enzymes CYP 1A2 and CYP2E1, while detoxification occurs by conjugation to glucuronic acid and sulfate. Given that glutathione levels demonstrate circadian rhythm and that fasting can alter the 24-h rhythm of acetaminophen toxicity, Matsunaga et al.42 sought to determine the 24-h rhythm of CYP 2E1 activity under meal-feeding conditions. Male 5-wk old ICR mice that were meal fed for 8 h during the light cycle (0700−1900) or ad libitum fed were injected daily with 0, 300, or 600 mg/kg acetaminophen i.p.42 After two weeks of treatment, hepatic CYP2E1 activity was measured at 4-h intervals over a 24-h period. In the absence of drug treatment, CYP2E1 activity in the livers of ad libitum-fed mice peaked at 9 a.m., at a level that was nearly 50% of the peak in livers of meal-fed mice, which peaked at 9 p.m. Moreover, mealfed mice had a striking circadian rhythm in CYP 2E1 activity that was absent in the ad libitum mice. The 24-h rhythm in hepatic glutathione levels in meal-fed mice was inverse and had greater amplitude than that in ad libitum-fed mice. The authors conclude that the 24-h rhythm of glutathione levels and CYP2E1 activity, which are altered by feeding regimen, may dictate the hepatotoxicity of acetominophen.
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MEAL FEEDING AND METABOLIC HOMEOSTASIS Just as meal feeding entrains circadian rhythms associated with the metabolism of drugs, it can entrain rhythms associated with the metabolism of food. This entrainment can improve the health and well being of the animal as well as the accuracy of experimental data; both of these are important considerations for toxicology researchers. The major dietary sources of energy in animals are carbohydrates and lipids, whose primary constituents are glucose and fatty acids, respectively. Once ingested, these constituents are either oxidized immediately for energy or stored as glycogen (liver and muscle) and triacylglycerol (adipose tissue) for mobilization at a later time when energy is needed. The master conductors of this metabolic dance between “using it now” vs “storing and using it later” are insulin and glucagon. The release of these two hormones is hard-wired into the metabolic machinery of all animals in order to maintain normal blood glucose levels, essential for brain functioning and thus life. When dietary carbohydrates are digested and absorbed, the rise in blood glucose levels and the resulting release of insulin from the β cells of the pancreas promotes the uptake and conversion of glucose to glycogen and fatty acids by the liver. Glucose not removed by the liver passes into the peripheral blood where insulin stimulates its uptake into skeletal muscle and adipose tissue for oxidation, conversion to fatty acids, and storage as triacylglycerol, or storage as glycogen. When dietary lipids are ingested, fatty acids are either oxidized directly by most tissues or esterified and stored as triacylglycerol in adipose tissue for later use. In contrast, when an animal does not eat, the metabolic dance shifts to mobilizing stored energy. A drop in blood glucose and the resulting release of glucagon from the α cells of the pancreas, promotes liver glycogen breakdown and the de novo synthesis of glucose from endogenous substrates. This hepatic response provides glucose to the blood to maintain normal blood glucose levels. A rise in glucagon, along with a drop in insulin, promotes the mobilization of triacylglycerol from adipose tissue, releasing fatty acids and glycerol into peripheral blood. Fatty acids can then serve as an energy source, and glycerol can serve as a gluconeogenic precursor. The liver circumvents the inability of free fatty acids to 1547
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nearly double those of adipocytes from meal-fed animals. By the 20-h time point, the absolute variability in both groups of animals had decreased to values that were well below those measured immediately after feeding, suggesting that time from food ingestion impacts experimental results. These findings confirm that food intake is a major source of variation in the measurement of adipocyte glucose oxidation and that insulin sensitivity of adipose tissue from meal-fed rats is greater than that of ad libitum-fed rats. The heightened insulin sensitivity of the meal-fed animal and thus a reduced need for insulin was demonstrated by Sugden et al.52 Four weeks of meal feeding resulted in a plasma insulin concentration of 15 μU/mL before food intake and a rise to 30 μU/mL 2 h after the provision of food (p < 0.5); the insulin level returned to baseline 4 h after the removal of food. In contrast, peak plasma insulin concentration in ad libitum-fed rats was nearly 47% higher than that of meal-fed rats, at 44 μU/ mL. Twelve weeks of meal feeding demonstrated that the baseline serum insulin level in meal-fed rats was 65% that of ad libitum-fed rats (18.1 versus 28.0 μU/mL, p < 0.05) 12 to 14 h after food removal.19 Two separate studies, varying in length of feeding time, confirmed these findings. The first, a 3-week study conducted on adult, male Wistar rats by Diaz-Munoz et al.,53 showed that prior to feeding a 2-h meal, the serum insulin level was 50% of the ad libitum-fed control group (p < 0.01). After feeding, insulin levels increased to above control, remaining higher than the control 4 h postprandial, then gradually returning to 50% of ad libitum-fed levels. The second study, lasting 20 months, revealed plasma insulin levels that were consistently and significantly lower in meal-fed rats than in ad libitum-fed rats (p < 0.05) throughout the entire study at all measured time points.46 Thus, data supports the notion that meal feeding promotes insulin sensitivity and that ad libitum feeding promotes hyperinsulinemia, a precursor for the development of diabetes. In summary, the physiological, metabolic, enzymatic, and genetic responses of rodents to a meal-feeding regimen can have dramatic effects on the disposition of drugs and animal health. What should be done with this information?
penetrate the blood−brain barrier by converting fatty acids to ketone bodies, which are soluble and penetrate the blood− brain barrier. Ketone body synthesis reduces the brain’s reliance on glucose and lessens the demand on the liver to produce glucose. Thus, the meal-fed animal is metabolically distinct from its ad libitum-fed counterpart. When food availability is restricted, the meal-fed animal must store fuel quickly and mobilize it slowly throughout the day. For example, when Escobar et al.45 restricted the feeding of rats to a daily two-hour period for three weeks, hepatic glycogen content in meal-fed rats was nearly depleted one hour prior to the scheduled feeding time, and free fatty acid and ketone body levels were significantly elevated compared to ad libitum-fed rats, whose hepatic glycogen was still present and free fatty acids, and ketone bodies were not elevated. An example of tightened metabolic homeostasis with meal feeding is that throughout a day, mealfed animals maintain plasma glucose concentrations approximately 17 mg/dL below their ad libitum-fed counterparts (p < 0.001).46 The only time this is not the case is after the meal-fed animals are provided with food, at which time, their plasma glucose levels approach those of the ad libitum-fed rats. Indeed, the rise in plasma glucose following glucose ingestion is dampened, and the values return to basal more quickly, in mealfed rats compared to that in the ad libitum-fed rats.47−49 This suggests a greater insulin sensitivity of adipose and muscle tissues. The suggestion that insulin sensitivity of adipose tissue in meal-fed rats was greater than that in ad libitum-fed rats was tested in our laboratory.50 Young, male Wistar rats were either fed ab libitum or maintained on a two-hour meal-feeding schedule for approximately three weeks. Experiments on epididymal fat were performed immediately after feeding, 10 h after feeding, and 20 h after feeding, using the method of Rodbell51 in which adipocytes were isolated and incubated with 14 C-glucose in the absence and presence of insulin. When adipocytes from ad libitum-fed rats were incubated with 14Cglucose plus 10−7 M insulin, glucose oxidation increased 2-fold, while oxidation in adipocytes from meal-fed rats in the presence of insulin increased by 3- to 4- fold (Figure 1). Moreover, the standard deviation and coefficient of variation for glucose oxidation in adipocytes isolated from ad libitum-fed rats are
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CONCLUSION AND IMPLICATIONS FOR TOXICOLOGY RESEARCH Meal-fed rodents adapt, both physiologically and metabolically, to having limited access to food. They live longer, are less disease-prone, and are more insulin-sensitive than their ad libitum-fed counterparts. The enhanced insulin sensitivity of the meal-fed rodent suggests that toxicology researchers using an ad libitum model are unknowingly using a diabetic-type model. Meal feeding is associated with less experimental variation than that observed with ad libitum feeding: by controlling the timing of food intake, toxicologists can more accurately and reproducibly determine an animal’s temporal and metabolic handling of the drug, compared to the ad libitum-fed animal, in which time from eating is unknown. From a practical standpoint, timed feeding is attainable. Toxicology experiments could be conducted under a reverse light−dark cycle (lights on 7 a.m.−7 p.m.) in which rodents are fed between 9 a.m. and 5 p.m. Feeding can be done manually or via a timed-feeding device. An alternative approach to reduce animal morbidity has been used by National Toxicology Program studies, the NTP-2000 diet. This diet replaces NIH-
Figure 1. Effect of time of day on fold-increase of insulin-stimulated glucose oxidation over basal in adipocytes from ad libitum (AD, blue)and 2-hr meal-fed (MF, red) rats. Results are expressed as means ± SD, n = 4−6. Coefficient of variation around the mean at 11 a.m., 9 p.m., and 7 a.m. is 47%, 61%, and 21% for ad libitum-fed rats (unpublished data), and 27%, 27%, and 17% for meal-fed rats,50 respectively. 1548
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(11) Ahlers, I., Ahlersova, E., and Smajda, B. (1983) Circadian rhythm of serum and tissue lipids in the rat: the effect of limited access to food. Physiol. Bohemoslov. 32, 64−72. (12) Ahlers, I., Ahlersova, E., Datelinka, I., and Toropila, M. (1984) Effect of food synchronization on the circadian rhythm of bone marrow and thymus lipids in rats. Physiol. Bohemoslov. 33, 511−514. (13) Comperatore, C. A., and Stephan, F. K. (1987) Entrainment of duodenal activity to periodic feeding. J. Biol. Rhythms 2, 227−242. (14) White, W., and Timberlake, W. (1995) Two meals promote entrainment of rat food-anticipatory and rest-activity rhythms. Physiol. Behav. 57, 1067−1074. (15) Stokkan, K., Yamasaki, S., Tei, H., Sakaki, Y., and Menaker, M. (2001) Entrainment of the circadian clock in the liver by feeding. Science 291, 490−493. (16) Satoh, Y., Kawai, H., Kudo, N., Kawashima, Y., and Mitsumoto, A. (2006) Time-restricted feeding entrains daily rhythms of energy metabolism in mice. Am. J. Physiol. Regul. Integr. Comp. Physiol. 290, R1276−1283. (17) DiGirolamo, M., and Rudman, D. (1968) Variations in glucose metabolism and sensitivity to insulin of the rat’s adipose tissue, in relation to age and body weight. Endocrinology 82, 1133−1141. (18) Salans, L. B., and Dougherty, J. W. (1971) The effect of insulin upon glucose metabolism by adipose cells of different size. J. Clin. Invest. 50, 1399−1410. (19) Reiser, S., and Hallfrisch, J. (1977) Insulin sensitivity and adipose tissue weight of rats fed starch or sucrose diets ad libitum or in meals. J. Nutr. 107, 147−155. (20) Tepperman, J., and Tepperman, H. M. (1958) Effects of antecedent food intake pattern on hepatic lipogenesis. Am. J. Physiol. 193, 55−64. (21) Holeckova, E., and Fabry, P. (1959) Hyperplasia and gastric hypertrophy in rats adapted to intermittent starvation. Br. J. Nutr. 13, 260−266. (22) Hollifield, G., and Parson, W. (1962) Metabolic adaptations to a “stuff and starve” feeding program. I. Studies of adipose tissue and liver glycogen in rats limited to a short daily feeding period. J. Clin. Invest. 41, 245−249. (23) Bolles, R. C., Duncan, P. M., Grossen, N. E., and Matter, C. F. (1968) Relationship between activity level and body temperature in the rat. Psychol. Rep. 23, 991−994. (24) Saito, M., Murakami, E., and Suda, M. (1976) Circadian rhythms in disaccharidases of rat small intestine and its relation to food intake. Biochim. Biophys. Acta 421, 177−179. (25) Lanza-Jacoby, S., Stevenson, N. R., and Kaplan, M. L. (1986) Circadian changes in serum and lipid metabolites and liver lipogenic enzymes in ad libitum-and meal-fed, lean and obese Zucker rats. J. Nutr. 116, 1798−1809. (26) Nelson, W., and Halberg, F. (1986) Meal-timing, circadian rhythms and life span of mice. J. Nutr. 116, 2244−2253. (27) Kohsaka, A., and Bass, J. (2006) A sense of time: how molecular clocks organize metabolism. Trends Endocrinol. Metab. 18, 4−11. (28) Varady, K. A., Roohk, D. J., and Hellerstein, M. E. (2007) Dose effects of modified alternate-day fasting regimens on in vivo cell proliferation and plasma insulin-like growth factor-1 in mice. J. Appl. Physiol. 103, 547−551. (29) Mendoza, J. (2007) Circadian clocks: setting time by food. J. Neuroendocrinol. 19, 127−137. (30) Youan, B. B. (2004) Chronopharmaceutics: gimmick or clinically relevant approach to drug delivery? J. Controlled Release 98, 337−353. (31) Ohdo, S., Koyanagi, S., Matsunaga, N., and Hamdan, A. (2011) Molecular basis of chronopharmaceutics. J. Pharm. Sci. 100, 3560− 3576. (32) Damiola, F., Minh, N. L., Preitner, N., Kornmann, B., FleuryOlela, F., and Schibler, U. (2000) Restricted feeding uncouples circadian oscillators in peripheral tissues from the central pacemaker in the suprachiasmatic nucleus. Genes Dev. 14, 2950−2961. (33) Balsalobre, A. (2002) Clock genes in mammalian peripheral tissues. Cell Tissue Res. 309, 193−199.
07, as certain components of NIH-07 such as protein content, calcium-to-phosphorus ratio, and vitamin D concentration, were thought to contribute to age-related illnesses. Rodents fed NTP-2000 showed significantly increased survival time and reduced nephropathy and cardiomyopathy.54 It has been 10 years since the publication of the Society for Toxicologic Pathology position paper noting feeding as an important variable that should be controlled in toxicology experiments.5 Data support the notion that circadian clocks and their gene products including CYPs, respond to food intake. Therefore, it is crucial that feeding regimen be carefully considered in designing toxicology experiments.
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AUTHOR INFORMATION
Corresponding Author
*Phone: (603) 862-4628. Fax: (603) 862-1148. E-mail: gale.
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Lauren Hufnagle for conducting experiments on ad libitum-fed rats and the NH Agricultural Experiment Station for financial support, contributing to Figure 1.
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ABBREVIATIONS CYP, cytochrome P450; SCN, suprachiasmatic nucleus; VA, valproic acid
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REFERENCES
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dx.doi.org/10.1021/tx300109x | Chem. Res. Toxicol. 2012, 25, 1545−1550
