A human model of dietary saturated fatty acid induced insulin resistance.

BACKGROUND: Increased consumption of high-fat diets is associated with the development of insulin resistance and type 2 diabetes. Current models to study the mechanisms of high-fat diet-induced IR in humans are limited by their long duration or low efficacy. In the present study we developed and characterized an acute dietary model of saturated fatty acid-enriched diet induced insulin resistance. METHODS: High caloric diets enriched with saturated fatty acids (SFA) or carbohydrates (CARB) were evaluated in subjects with normal and impaired glucose tolerance (NGT or IGT). Both diets were compared to a standard eucaloric American Heart Association (AHA) control diet in a series of crossover studies. Whole body insulin resistance was estimated as steady state plasma glucose (SSPG) concentrations during the last 30min of a 3-h insulin suppression test. RESULTS: SSPG was increased after a 24-h SFA diet (by 83±74% vs. control, n=38) in the entire cohort, which was comprised of participants with NGT (92±82%, n=22) or IGT (65±55%, n=16) (all p<0.001). SSPG was also increased after a single SFA breakfast (55±32%, p=0.008, n=7). The increase in SSPG was less pronounced after an overnight fast following a daylong SFA diet (24±31%, p=0.04, n=10), and further attenuated 24h after returning to the control diet (19±35%, p=0.09, n=11). SSPG was not increased after a 24-h CARB diet (26±50%, p=0.11, n=12). CONCLUSIONS: A short-term SFA-enriched diet induced whole body insulin resistance in both NGT and IGT subjects. Insulin resistance persisted overnight after the last SFA meal and was attenuated by one day of a healthy diet. This model offers opportunities for identifying early mechanisms and potential treatments of dietary saturated fat induced insulin resistance.

Preclinical Studies of a Kidney Safe Iodinated Contrast Agent.

BACKGROUND AND PURPOSE: Contrast-induced acute kidney injury (CI-AKI) is a serious complication of the use of iodinated contrast agents. This problem is particularly acute in interventional neurology and interventional cardiology, probably due to the intra-arterial route of injection, high contrast volumes, and preexisting risk factors of these patients. In an attempt to develop a contrast agent that is less damaging to the kidneys, we have studied the effects of adding a small amount of the substituted cyclodextrin, sulfobutyl-ether-ß-cyclodextrin (SBECD), to iohexol in rodent models of renal toxicity. METHODS: Renally compromised mice and rats were injected with iohexol and iohexol-SBECD via the tail vein. The renal pathology, creatinine clearance, and survival benefits of iohexol-SBECD were studied. The safety of direct intra-arterial injection of the iohexol-SBECD formulation was studied in a dog heart model system. Mechanism of action studies in cell culture model using a human kidney cell line was performed using flow cytometry. RESULTS: Nephrotoxicity was significantly reduced using iohexol-SBECD compared to iohexol alone, at mole ratios of iohexol:SBECD of 1:0.025. SBECD increased survival from 50% to 88% in a rat survival study. In the dog heart model, iohexol-SBECD was safe. Cell culture studies suggest that SBECD interferes with the early stages of contrast-induced apoptosis in a human renal cell line. CONCLUSION: We have shown that the addition of a small amount of SBECD (one molecule of SBECD per 40 iohexol molecules) significantly protects rodent kidneys from CI-AKI. Further development of this new formulation of iodinated contrast is warranted.

Relationship of circulating adipokines to body composition in pregnant women.

Circulating adipokines are associated with physiological and pathophysiological processes in both obesity and pregnancy. Obesity in pregnancy increases the risk of pregnancy complications and the majority of research uses body mass index (BMI) to assess fatness. Specific fat compartments are associated with obesity-induced health risks yet it is not known how abdominal fat mass in pregnancy is related to circulating adipokines. Plasma leptin, resistin, visfatin, and adiponectin were measured by immunoassay in healthy pregnant women of normal weight (BMI 18.5-24.9; n = 17) and overweight/obese pregnant women (BMI 25.0-40, n = 21) in the third trimester. Total body and abdominal subcutaneous and visceral fat mass were measured at 1-3 weeks postpartum. Overweight/obese women had greater total body fat (t = -6.210, P < 0.001) and abdominal subcutaneous fat (t = -5.072, P < 0.001) than normal-weight women while there was no difference in abdominal visceral fat. Overweight/obese women had higher leptin (66.3 ± 34.2 vs. 35.7 ± 19.3 ng/mL, P < 0.001) compared to normal-weight women. Leptin was associated with total body fat (r = 0.782, P < 0.001) and resistin was associated with abdominal visceral fat (r = 0.452, P = 0.045). No significant correlations were observed between adiponectin or visfatin and any measure of body composition. In pregnant women, resistin has the potential to be a circulating biomarker for visceral fat, an ectopic fat compartment. These observational data may provide insight for the pathophysiological roles of adipokines and the impact of visceral fat in pregnant women.

Abdominal visceral adiposity influences CD4+ T cell cytokine production in pregnancy.

Women with pre-gravid obesity are at risk for pregnancy complications. While the macrophage response of obese pregnant women categorized by body mass index (BMI) has been documented, the relationship between the peripheral CD4(+) T cell cytokine profile and body fat compartments during pregnancy is unknown. In this study, third trimester peripheral CD4(+) T cell cytokine profiles were measured in healthy pregnant women [n=35; pre-pregnancy BMI: 18.5-40]. CD4(+) T cells were isolated from peripheral blood mononuclear cells and stimulated to examine their capacity to generate cytokines. Between 1 and 3weeks postpartum, total body fat was determined by dual-energy X-ray absorptiometry and abdominal subcutaneous and visceral fat masses were determined by magnetic resonance imaging. Pearson's correlation was performed to assess relationships between cytokines and fat mass. Results showed that greater abdominal visceral fat mass was associated with a decrease in stimulated CD4(+) T cell cytokine expression. IFN-gamma, TNF-alpha, IL-12p70, IL-10 and IL-17A were inversely related to visceral fat mass. Chemokines CCL3 and IL-8 and growth factors G-CSF and FLT-3L were also inversely correlated. Additionally, total body fat mass was inversely correlated with FGF-2 while abdominal subcutaneous fat mass and BMI were unrelated to any CD4(+) T cell cytokine. In conclusion, lower responsiveness of CD4(+) T cell cytokines associated with abdominal visceral fat mass is a novel finding late in gestation.

Typical prenatal vitamin D supplement intake does not prevent decrease of plasma 25-hydroxyvitamin D at birth.

OBJECTIVE: The objective of this longitudinal study was to determine what typical vitamin D predictors influence the change in vitamin D status from mid-pregnancy to birth. METHODS: Plasma 25-hydroxyvitamin D [25(OH)D] was determined at mid-pregnancy (8-20 weeks gestation) and following birth (n = 193). Usual predictors of vitamin D status [body mass index (BMI), race, season] in addition to prenatal supplemental vitamin D intake and docosahexaenoic acid (DHA) status at delivery were assessed for their interaction on the change on plasma 25(OH)D concentration between the two time points. RESULTS: Forty-nine percent of women had inadequate vitamin D status [categorized as deficient (<30 nmol/L) or insufficient (30-49.9 nmol/L) by IOM guidelines] at mid-pregnancy and 82% were deficient or insufficient at birth. Plasma 25(OH)D concentration dropped 61% from mid-pregnancy to birth. Season of birth (F = 7.86, P = 0.006) and mid-pregnancy plasma 25(OH)D concentration (F = 6.17, P = 0.014) were significant variables in the change of vitamin D status while BMI, race, DHA status, and typical vitamin D intake (334 IU/day) from prenatal supplements did not have an effect. Women who delivered in summer and fall had a 1.5-fold greater plasma 25(OH)D concentration than women who delivered in winter in spring (41.1 ± 23.1 and 40.7 ± 20.5 nmol/L summer and fall, respectively, versus 27.7 ± 17.9 and 29.3 ± 21.4 nmol/L in winter and spring, respectively). CONCLUSIONS: Typical supplemental vitamin D intake during pregnancy did not prevent precipitous drops in maternal plasma 25(OH)D concentration. Clinicians and dietitians should be aware of the risk of inadequate vitamin D status in pregnant women in the United States relative to their initial vitamin D status and the season of birth.

Streptozotocin-induced diabetes partially attenuates the effects of a high-fat diet on liver and brain fatty acid composition in mice.

The current study addresses the effects of a high-fat diet on liver and brain fatty acid compositions and the interaction of that diet with diabetes in a type 1 mouse model. Adult, male, normal and streptozotocin-induced diabetic C57BL/6 mice were fed standard (14 % kcal from fat) or high-fat (54 % kcal from fat, hydrogenated vegetable shortening and corn oil) diets for 8 weeks. Liver and whole brain total phospholipid fatty acid compositions were then determined by TLC/GC. In the liver of non-diabetic mice, the high-fat diet increased the percentages of 18:1n-9, 20:4n-6, and 22:5n-6 and decreased 18:2n-6 and 22:6n-3. Diabetes increased 16:0 in liver, and decreased 18:1n-7 and 20:4n-6. The effects of the high-fat diet on liver phospholipids in diabetic mice were similar to those in non-diabetic mice, or were of smaller magnitude. In the brain, the high-fat diet increased 18:0 and 20:4n-6 of non-diabetic, but not diabetic mice. Brain 22:5n-6 acid was increased by the high-fat diet in both non-diabetic and diabetic mice, but this increase was smaller in diabetic mice. Diabetes alone did not alter the percentage of any individual fatty acid in brain. This indicates that the effects of a high-fat diet on liver and brain phospholipid fatty acid compositions are partially attenuated by concomitant hyperglycemia with hypoinsulinemia.

Differences in methylphenidate dose response between periadolescent and adult rats in the familiar arena-novel alcove task.

Methylphenidate is a psychostimulant widely used in the treatment of attention deficit hyperactivity disorder. In this study, the effects of two nonstereotypy-inducing doses of methylphenidate (2.5 and 5.0 mg/kg s.c.) were examined in periadolescent [postnatal days (P) 35 and 42] and young adult (P70), male Long-Evans rats using a three-period locomotor activity paradigm that affords inferences about exploration, habituation, and attention to a novel stimulus (an "alcove") in a familiar environment in a single test session. In the first test period, P35 and P42 rats were more active than P70 rats, and methylphenidate increased locomotion in a dose-related manner. The introduction of a novel spatial stimulus in the third test period revealed a significant interaction of dose and age such that P70 rats exhibited dose-related increases in distance traveled, but P35 rats did not. Furthermore, methylphenidate dose-relatedly disrupted the rats' tendency to spend increasing amounts of time in the alcove across the test period at P70 but not at P35. Brain and serum methylphenidate concentrations were significantly lower at P35 than at P70, with intermediate levels at P42. Developmental differences in dopaminergic neurochemistry were also observed, including increased dopamine content in the caudate-putamen, nucleus accumbens, and frontal cortex and decreased densities of D(1)-like receptors in the frontal cortex in P70 than in P42 rats. These results raise the possibility that children and adults may respond differently when treated with this drug, particularly in situations involving response to novelty and that these effects involve developmental differences in pharmacokinetics and dopaminergic neurochemistry.

Dopamine receptor alterations in female rats with diet-induced decreased brain docosahexaenoic acid (DHA): interactions with reproductive status.

Decreased tissue levels of n-3 (omega-3) fatty acids, particularly docosahexaenoic acid (DHA), are implicated in the etiologies of non-puerperal and postpartum depression. This study examined the effects of a diet-induced loss of brain DHA content and concurrent reproductive status on dopaminergic parameters in adult female Long-Evans rats. An alpha-linolenic acid-deficient diet and breeding protocols were used to produce virgin and parous female rats with cortical phospholipid DHA levels 20-22% lower than those fed a control diet containing adequate alpha-linolenic acid. Decreased brain DHA produced a significant main effect of decreased density of ventral striatal D(2)-like receptors. Virgin females with decreased DHA also exhibited higher density of D(1)-like receptors in the caudate nucleus than virgin females with normal DHA. These receptor alterations are similar to those found in several rodent models of depression, and are consistent with the proposed hypodopaminergic basis for anhedonia and motivational deficits in depression.

Decreased brain docosahexaenoic acid content produces neurobiological effects associated with depression: Interactions with reproductive status in female rats.

Decreased tissue levels of docosahexaenoic acid (DHA; 22:6n-3) are implicated in the etiologies of non-puerperal and postpartum depression. With the aim of determining neurobiological sequelae of decreased brain DHA content, this study examined the effects of a loss of brain DHA content and concurrent reproductive status in adult female Long-Evans rats. An alpha-linolenic acid-deficient diet and breeding protocols were used to produce virgin and parous female rats with cortical phospholipid DHA levels 23-26% lower than virgin and parous rats fed a control diet containing adequate alpha-linolenic acid. Parous dams were tested/euthanized at weaning (postnatal day 20) of the second litter; virgin females, during diestrus. Decreased brain DHA was associated with decreased hippocampal BDNF gene expression and increased relative corticosterone response to an intense stressor, regardless of reproductive status. In virgin females with decreased brain DHA, serotonin content and turnover in frontal cortex were decreased compared to virgin females with normal brain DHA. In parous dams with decreased brain DHA, the density of 5-HT(1A) receptors in the hippocampus was increased, corticosterone response to an intense stressor was increased, and the latency to immobility in the forced swim test was decreased compared to parous dams with normal DHA. These findings demonstrate neurobiological alterations attributable to decreased brain DHA or an interaction of parous status and brain DHA level. Furthermore, the data are consistent with findings in depressed humans, and thus support a role for DHA as a factor in the etiologies of depressive illnesses, particularly postpartum depression.

Diet (n-3) polyunsaturated fatty acid content and parity affect liver and erythrocyte phospholipid fatty acid composition in female rats.

The fatty acid composition of membrane phospholipids affects the physicochemical properties of the membrane and thus influences cell function. In this study, the effects of 1-4 sequential pregnancies on the phospholipid fatty acid compositions of the maternal liver and erythrocytes were determined in female rats fed diets containing alpha-linolenic acid (ALA), ALA and preformed docosahexaenoic acid (DHA; ALA+DHA), or minimal ALA (low ALA). Virgin females, fed the diets for commensurate durations, served as a control for reproduction. Liver and erythrocyte total phospholipid compositions were determined at weaning by TLC/GC. In both tissues, significant main effects of diet and reproductive status were detected for many fatty acids, but a significant interaction of diet, reproductive status, and duration of treatment (no. of reproductive cycles or equivalent time period for virgins) was detected only for DHA, 22:6(n-3). Primiparous dams fed the ALA and low ALA diet had decreased liver DHA content of 31% and 74%, respectively, compared with virgin females fed the ALA diet. Liver DHA did not decrease further after additional reproductive cycles. Liver DHA content was unchanged in parous dams fed the ALA+DHA diet, but virgin females fed this diet exhibited a 50% increase in liver DHA after 13 wk of treatment. Changes in erythrocyte DHA were of similar magnitude and time course to those observed in liver, suggesting that this tissue may serve as a marker for liver DHA status.

Maternal parity and diet (n-3) polyunsaturated fatty acid concentration influence accretion of brain phospholipid docosahexaenoic acid in developing rats.

The long-chain PUFA, docosahexaenoic acid [22:6(n-3), DHA], a major component of neuronal membrane phospholipids, accumulates in brain during late prenatal and early neonatal development and is essential for optimal attentional and cognitive function. Because all nutrition is supplied to the developing fetus/neonate by the mother and maternal DHA status is affected by parity, this study examined the effects of maternal diet and parity on DHA accretion in the developing brain. Whole brain total phospholipid fatty acid composition was determined by TLC and GC in weanling male Long-Evans rats (n = 5) from the 1st, 2nd, 3rd, or 4th litters of dams fed diets containing alpha-linolenic acid (ALA), containing ALA and preformed DHA (ALA + DHA), or lacking ALA (low-ALA). First-litter low-ALA offspring exhibited a decrease in phospholipid fatty acid DHA content to 68% of 1st-litter ALA pups. DHA in 2nd-litter low-ALA pups was further decreased to 55% of 1st-litter ALA pups, but further decreases were not observed in subsequent litters. DHA levels increased 15-20% in 2nd to 4th-litter ALA + DHA pups and 11% in 4th-litter ALA pups compared with 1st-litter ALA pups. These findings demonstrate that maternal diet and parity interact to affect offspring brain DHA status and suggest that maternal multiparity may place offspring at greater risk of decreased accretion of brain DHA if the maternal diet contains insufficient (n-3) PUFA.

Specific brain regions of female rats are differentially depleted of docosahexaenoic acid by reproductive activity and an (n-3) fatty acid-deficient diet.

Low tissue levels of (n-3) PUFA, particularly docosahexaenoic acid [DHA, 22:6(n-3)], are implicated in postpartum depression. Brain DHA content is depleted in female rats undergoing pregnancy and lactation when the diet supplies inadequate (n-3) PUFA. In this study, the effects of DHA depletion as a result of reproductive activity and an (n-3) PUFA-deficient diet were examined in 8 specific brain regions of female rats after undergoing 2 sequential reproductive cycles. Virgin females, fed the alpha-linolenic acid (ALA)-containing or deficient (low-ALA) diets for a commensurate duration (13 wk) served as a control for reproduction. Total phospholipid composition of each brain region was determined at weaning (postnatal d 21) by TLC/GC. The regional PUFA composition of ALA virgins was similar to that previously measured in male rats. All brain regions examined were affected by reproductive activity and/or the low-ALA diet; however, the magnitude of the loss of DHA and compensatory incorporation of docosapentaenoic acid [(n-6) DPA, 22:5(n-6)] varied among brain regions. In low-ALA parous dams, frontal cortex (77% of ALA virgin) and temporal lobe (83% of ALA virgin), regions involved in cognition and affect, were among those exhibiting the greatest depletion of DHA. Caudate-putamen also exhibited significant depletion of DHA (82% of ALA virgin), whereas only (n-6) DPA levels were altered in ventral striatum, hypothalamus, hippocampus, and cerebellum. This pattern of changes in regional DHA and (n-6) DPA content suggests that specific neuronal systems may be differentially affected by depletion of brain DHA in the postpartum organism.

Sex-specific effects of brain LC-PUFA composition on locomotor activity in rats.

Insufficient availability of n-3 polyunsaturated fatty acids (PUFAs) during pre- and neonatal development decreases accretion of docosahexaenoic acid (DHA, 22:6n-3) in the developing brain and is associated with sub-optimal sensory and cognitive function in humans, altered behavior in animals, and may contribute to neurodevelopmental disorders such as attention deficit hyperactivity disorder and schizophrenia. This study examined the effects of variation in dietary availability of n-3 PUFAs on brain fatty acid composition and the consequent effects on locomotor activity in male and female Long-Evans rats. Rats were raised from conception using purified diets and breeding protocols designed to produce four groups with distinct brain phospholipid compositions varying in DHA content and/or the proportion of n-3 and n-6 PUFAs. Locomotor behavior was measured for a 2-h period on postnatal days 28, 42, 56, and 70. In males, decreased brain DHA produced alterations in activity that were most pronounced post-adolescence and with the greatest decrease in DHA. However, the behavioral effects in males were not linearly related to brain DHA level. In contrast, no significant effects of variation in brain fatty acid composition were observed in females. This suggests that variation in brain DHA content produces sex-specific alterations in locomotor activity and that the neurochemical alterations underlying the observed behavioral changes vary depending on the degree of DHA depletion.

Differential effects of modulation of docosahexaenoic acid content during development in specific regions of rat brain.

Variation in brain FA composition, particularly decreased DHA (22:6n-3), affects neurodevelopment, altering visual, attentional, and cognitive functions, and is implicated in several neuropsychiatric disorders. To further understand how specific brain processes and systems are affected by variation in brain DHA content, we sought to determine whether specific brain regions were differentially affected by treatments that alter brain DHA content. Adult male Long-Evans rats were raised from conception using diet/breeding treatments to produce four groups with distinct brain phospholipid compositions. Total phospholipid FA composition was determined in whole brain and 15 brain regions by TLC/GC. Brain regions exhibited significantly different DHA contents, with the highest levels observed in the frontal cortex and the lowest in the substantia nigra/ventral tegmental area. Increased availability of DHA resulted in increased DHA content only in the olfactory bulb, parietal cortex, and substantia nigra/ventral tegmental area. In contrast, treatment that decreased whole-brain DHA levels decreased DHA content in all brain regions except the thalamus, dorsal midbrain, and the substantia nigra/ventral tegmental area. Alterations in DHA level were accompanied by changes in docosapentaenoic acid (n-6 DPA, 22:5n-6) content; however, the change in DHA and n-6 DPA was nonreciprocal in some brain regions. These findings demonstrate that the FA compositions of specific brain regions are differentially affected by variation in DHA availability during development. These differential effects may contribute to the specific neurochemical and behavioral effects observed in animals with variation in brain DHA content.

Diet (n-3) polyunsaturated fatty acid content and parity interact to alter maternal rat brain phospholipid fatty acid composition.

Low tissue levels of (n-3) polyunsaturated fatty acids (PUFAs), particularly docosahexaenoic acid [DHA, 22:6(n-3)], are implicated in postpartum depression. The effects of 1-4 sequential reproductive cycles on maternal brain phospholipid fatty acid composition were determined in female rats fed diets containing alpha-linolenic acid (ALA), containing ALA and pre-formed DHA (ALA+DHA), or lacking ALA (low-ALA). Virgin females, fed the diets for commensurate durations served as a control for reproduction. Whole-brain total phospholipid composition was determined at weaning by TLC/GC. A single reproductive cycle on the low-ALA diet decreased brain DHA content by 18% compared to ALA primiparas (P < 0.05), accompanied by incorporation of docosapentaenoic acid ((n-6) DPA, 22:5(n-6)) to 280% of ALA primiparas (P < 0.05). DHA was not further decreased after subsequent cycles; however, there was an additional increase in (n-6) DPA after the second cycle (P < 0.05). Brain DHA of virgin females fed the low-ALA diet for 27 wk decreased 15% (P < 0.05), but was accompanied by a more modest increase in (n-6) DPA than in parous low-ALA dams (P < 0.05). Virgin females and parous dams fed the diet containing ALA+DHA exhibited only minor changes in brain fatty acid composition. These observations demonstrate that brain DHA content of adult animals is vulnerable to depletion under dietary conditions that supply inadequate (n-3) PUFAs, that this effect is augmented by the physiological demands of pregnancy and lactation, and that maternal diet and parity interact to affect maternal brain PUFA status.

All-trans-retinoic acid rapidly induces glycine N-methyltransferase in a dose-dependent manner and reduces circulating methionine and homocysteine levels in rats.

Glycine N-methyltransferase (GNMT) regulates the methyl group supply for S-adenosylmethionine-dependent transmethylation reactions. Retinoids have been shown to perturb methyl group metabolism by increasing the abundance and activity of GNMT, thereby leading to the loss of methyl groups. Previous studies used pharmacologic doses (30 micro mol/kg body weight) of various retinoids administered daily for a total of 10 d. Here, we examined the dose- and time-dependent relationships between all-trans-retinoic acid (ATRA) administration and induction of GNMT, as well as determining additional indices of methyl group and folate metabolism. For the dose-response study, rats were administered 0, 1, 5, 10, 15 or 30 micro mol ATRA/kg body weight for 10 d. For the time-course study, rats were given 30 micromol ATRA/kg body weight for 0, 1, 2, 4, or 8 d. A significant increase (105%) in GNMT activity was observed with doses as low as 5 micromol/kg body weight, whereas maximal induction (231%) of GNMT activity was achieved at 30 micromol/kg body weight. Induction of hepatic GNMT by ATRA was rapid, exhibiting a 31% increase after a single dose (1 d) and achieving maximal induction (95%) after 4 d. Plasma methionine and homocysteine concentrations were decreased 42 and 53%, respectively, in ATRA-treated rats compared with controls. In support of this finding, the hepatic activity of methionine synthase, the folate-dependent enzyme required for homocysteine remethylation, was elevated 40% in ATRA-treated rats. This work demonstrates that ATRA administration exerts a rapid effect on hepatic methyl group, folate and homocysteine metabolism at doses that are within the therapeutic range used by humans.

Activation and induction of glycine N-methyltransferase by retinoids are tissue- and gender-specific.

Glycine N-methyltransferase (GNMT) is a key protein in the liver that functions to regulate S-adenosylmethionine (SAM) and the SAM/S-adenosylhomocysteine ratio. Significant GNMT expression is also present in the kidney and pancreas. Inappropriate regulation of GNMT may have negative consequences on methyl group and folate metabolism. We have demonstrated that retinoid compounds significantly elevated hepatic GNMT activity and abundance (approximately 2-fold) in male rats. However, pancreatic GNMT activity and abundance were not altered by retinoid treatment. Likewise, retinoid administration was without effect on renal GNMT activity. Hepatic GNMT activity was also elevated in female rats treated with all-trans-retinoic acid, but to a lesser extent compared to males. Collectively, these results indicate that the modulation of methyl group metabolism by retinoids is tissue- and gender-specific, and may compromise the availability of methyl groups for SAM-dependent transmethylation reactions. In support of this, SAM-dependent synthesis of creatinine was significantly reduced 21% following all-trans-retinoic acid treatment.