Dietary l-carnitine can be metabolized by intestinal microbiota to trimethylamine, which is absorbed by the gut and further oxidized to trimethylamine N-oxide (TMAO) in the liver. TMAO plasma levels have been associated with atherosclerosis development in ApoE(-/-) mice. To better understand the mechanisms behind this association, we conducted in vitro and in vivo studies looking at the effect of TMAO on different steps of atherosclerotic disease progression. J774 mouse macrophage cells were used to evaluate the effect of TMAO on foam cell formation. Male ApoE(-/-) mice transfected with human cholesteryl ester transfer protein (hCETP) were fed l-carnitine and/or methimazole, a flavin monooxygenase 3 (FMO3) inhibitor that prevents the formation of TMAO. Following 12 week treatment, l-carnitine and TMAO plasma levels, aortic lesion development, and lipid profiles were determined. TMAO at concentrations up to 10-fold the Cmax reported in humans did not affect in vitro foam cell formation. In ApoE(-/-)mice expressing hCETP, high doses of l-carnitine resulted in a significant increase in plasma TMAO levels. Surprisingly, and independently from treatment group, TMAO levels inversely correlated with aortic lesion size in both aortic root and thoracic aorta. High TMAO levels were found to significantly correlate with smaller aortic lesion area. Plasma lipid and lipoprotein levels did not change with treatment nor with TMAO levels, suggesting that the observed effects on lesion area were independent from lipid changes. These findings suggest that TMAO slows aortic lesion formation in this mouse model and may have a protective effect against atherosclerosis development in humans.

Trimethylamine-N-oxide (TMAO) is generated in a microbial-mammalian co-metabolic pathway mainly from the digestion of meat-containing food and dietary quaternary amines such as phosphatidylcholine, choline, betaine, or L-carnitine. Fish intake provides a direct significant source of TMAO. Human observational studies previously reported a positive relationship between plasma TMAO concentrations and cardiometabolic diseases. Discrepancies and inconsistencies of recent investigations and previous studies questioned the role of TMAO in these diseases. Several animal studies reported neutral or even beneficial effects of TMAO or its precursors in cardiovascular disease model systems, supporting the clinically proven beneficial effects of its precursor, L-carnitine, or a sea-food rich diet (naturally containing TMAO) on cardiometabolic health. In this review, we summarize recent preclinical and epidemiological evidence on the effects of TMAO, in order to shed some light on the role of TMAO in cardiometabolic diseases, particularly as related to the microbiome.

Given its pivotal role in fatty acid oxidation and energy metabolism, l-carnitine has been investigated as ergogenic aid for enhancing exercise capacity in the healthy athletic population. Early research indicates its beneficial effects on acute physical performance, such as increased maximum oxygen consumption and higher power output. Later studies point to the positive impact of dietary supplementation with l-carnitine on the recovery process after exercise. It is demonstrated that l-carnitine alleviates muscle injury and reduces markers of cellular damage and free radical formation accompanied by attenuation of muscle soreness. The supplementation-based increase in serum and muscle l-carnitine contents is suggested to enhance blood flow and oxygen supply to the muscle tissue via improved endothelial function thereby reducing hypoxia-induced cellular and biochemical disruptions. Studies in older adults further showed that l-carnitine intake can lead to increased muscle mass accompanied by a decrease in body weight and reduced physical and mental fatigue. Based on current animal studies, a role of l-carnitine in the prevention of age-associated muscle protein degradation and regulation of mitochondrial homeostasis is suggested.

Progressive decline in skeletal muscle mass and function are growing concerns in an aging population. Diet and physical activity are important for muscle maintenance but these requirements are not always met. This highlights the potential for nutritional supplementation. As a primary objective, we sought to assess the effect of a novel combination of L-Carnitine, creatine and leucine on muscle mass and performance in older subjects. Forty-two healthy older adults aged 55–70 years were randomized to receive either a novel L-Carnitine (1500 mg), L-leucine (2000 mg), creatine (3000 mg), Vitamin D3 (10 μg) (L-Carnitine-combination) product (n = 14), L-Carnitine (1500 mg) (n = 14), or a placebo (n = 14) for eight weeks. We evaluated body mass by DXA, upper and lower strength by dynamometry, and walking distance by a 6-min walk test at baseline and after eight weeks of intervention. These measures, reflecting muscle mass, functional strength and mobility have been combined to generate a primary composite score. Quality of life, blood safety markers, and muscle biopsies for protein biomarker analysis were also conducted at baseline and the end of the study. The primary composite outcome improved by 63.5 percentage points in the L-Carnitine-combination group vs. placebo (P = 0.013). However, this composite score did not change significantly in the L-Carnitine group (P = 0.232), and decreased slightly in the placebo group (P = 0.534). Participants supplemented with the L-Carnitine-combination showed a 1.0 kg increase in total lean muscle mass (P = 0.013), leg lean muscle mass (0.35 kg, P = 0.005), and a 1.0 kg increase in lower leg strength (P = 0.029) at week 8. In addition, these increases were significant when compared to the placebo group (P = 0.034, P = 0.026, and P = 0.002, respectively). Total mTOR protein expression was increased in participants in the L-Carnitine-combination group at the end of the study compared to the baseline (P = 0.017). This increase was also significant when compared to the placebo (P = 0.039), suggesting that the increase in muscle mass and strength was due to new protein synthesis and mTOR pathway activation. The trial did reach its primary objective. L-Carnitine combined with creatine and L-leucine significantly improved the composite score which reflects muscle mass and strength, at the end of the study compared to placebo. The combination showed an increase in mTOR protein level, a driver for increased muscle mass which translated to an improvement in muscle strength. This new combination may provide a potential nutritional intervention to promote muscle growth and improved physical functioning in older adults.

Promising evidence suggests beneficial health effects of arabinogalactan, but little is known about the effect of this non-digestible carbohydrate on the gut microbiota, a crucial mediator of human health. The objective of this study was to investigate the effect of an arabinogalactan product (ResistAid) on the fecal microbiome and short-chain fatty acids and gastrointestinal tolerance in healthy adults in a randomized, double-blind, crossover trial. Thirty adults were randomly assigned to consume 15 g/d maltodextrin (control) or ResistAid for 6 wk. At week 6, compared to placebo, ResistAid supplementation led to a significant decrease in the ratio of fecal Firmicutes to Bacteroidetes, driven by an increase in Bacteroidetes and a decrease in Firmicutes. Moreover, the relative abundance of Bifidobacterium tended to increase with ResistAid supplementation. Additionally, ResistAid significantly decreased the α-diversity of the fecal microbiome. Predicted functional abundances based on 16S rRNA sequences showed that ResistAid supplementation increased the gene abundance of the gut microbiome for α-l-rhamnosidase, β-fructosidase, and levanase, as well as tricarboxylic acid and vitamin B₆ biosynthesis pathways. Fecal isovaleric, valeric, and hexanoic acids were significantly lower after ResistAid consumption. There were no statistically significant changes in bowel habit, stool consistency, gastrointestinal tolerance symptoms, chemistry profile, metabolic panel, or vitals, suggesting that consumption of 15 g daily ResistAid over 6 wk is safe. These results demonstrate that the gut microbiome composition and predicted functions can be modulated by ResistAid consumption, perhaps suggesting a mechanistic explanation on its reported benefits in metabolic parameters and the immune system.

Oral administration of active pharmaceutical ingredients, nutraceuticals, enzymes or probiotics requires an appropriate delivery system for optimal bioactivity and absorption. The harsh conditions during the gastrointestinal transit can degrade the administered products, hampering their efficacy. Enteric or delayed-release pharmaceutical formulations may help overcome these issues. In a Simulator of Human Intestinal Microbial Ecosystem model (SHIME) and using caffeine as a marker for release kinetics and L. acidophilus survivability as an indicator for protection, we compared the performance of ten capsule configurations, single or DUOCAP® combinations. The function of L. acidophilus and its impact on the gut microbiota was further tested in three selected capsule types, combinations of DRcaps® capsule in DRcaps® capsule (DR-in-DR) and DRcaps® capsule in Vcaps® capsule (DR-in-VC) and single Vcaps® Plus capsule under colonic conditions. We found that under stomach and small intestine conditions, DR-in-DR and DR-in-VC led to the best performance both under fed and fasted conditions based on the slow caffeine release and the highest L. acidophilus survivability. The Vcaps® Plus capsule however, led to the quickest caffeine and probiotic release. When DR-in-DR, DR-in-VC and single Vcaps® Plus capsules were tested through the whole gastrointestinal tract, including under colonic conditions, caffeine release was found to be slower in capsules containing DRcaps® capsules compared to the single Vcaps® capsules. In addition, colonic survival of L. acidophilus was significantly increased under fasted conditions in DR-in-DR or DR-in-VC formulation compared to Vcaps® Plus capsule. To assess the impact of these formulations on the microbial function, acetate, butyrate and propionate as well as ammonia were measured. L. acidophilus released from DR-in-DR or DR-in-VC induced a significant increase in butyrate and a decrease in ammonia, suggesting a proliferation of butyrate-producing bacteria and reduction in ammonia-producing bacteria. These data suggest that L. acidophilus included in DR-in-DR or DR-in-VC reaching the colon is viable and functional, potentially contributing to changes in colonic microbiota composition and diversity.