Postprandial spillover of dietary lipid into plasma is increased with moderate amounts of ingested fat and is inversely related to adiposity in healthy older men

Tracers and Imaging of Fatty Acid and Energy Metabolism of Human Adipose Tissues

White adipose tissue and brown adipose tissue (WAT and BAT) regulate fatty acid metabolism and control lipid fluxes to other organs. Dysfunction of these key metabolic processes contributes to organ insulin resistance and inflammation leading to chronic diseases such as type 2 diabetes, metabolic dysfunction-associated steatohepatitis, and cardiovascular diseases. Metabolic tracers combined with molecular imaging methods are powerful tools for the investigation of these pathogenic mechanisms. Herein, I review some of the positron emission tomography and magnetic resonance imaging methods combined with stable isotopic metabolic tracers to investigate fatty acid and energy metabolism, focusing on human WAT and BAT metabolism. I will discuss the complementary strengths offered by these methods for human investigations and current gaps in the field.

The Roles of Lipid Metabolism in the Pathogenesis of Chronic Diseases in the Elderly

Lipid metabolism plays crucial roles in cellular processes such as hormone synthesis, energy production, and fat storage. Older adults are at risk of the dysregulation of lipid metabolism, which is associated with progressive declines in the physiological function of various organs. With advancing age, digestion and absorption commonly change, thereby resulting in decreased nutrient uptake. However, in the elderly population, the accumulation of excess fat becomes more pronounced due to a decline in the body's capacity to utilize lipids effectively. This is characterized by enhanced adipocyte synthesis and reduced breakdown, along with diminished peripheral tissue utilization capacity. Excessive lipid accumulation in the body, which manifests as hyperlipidemia and accumulated visceral fat, is linked to several chronic lipid-related diseases, including cardiovascular disease, type 2 diabetes, obesity, and nonalcoholic fatty liver disease. This review provides a summary of the altered lipid metabolism during aging, including lipid digestion, absorption, anabolism, and catabolism, as well as their associations with age-related chronic diseases, which aids in developing nutritional interventions for older adults to prevent or alleviate age-related chronic diseases.

Increased postprandial nonesterified fatty acid efflux from adipose tissue in prediabetes is offset by enhanced dietary fatty acid adipose trapping

The mechanism of increased postprandial nonesterified fatty acid (NEFA) appearance in the circulation in impaired glucose tolerance (IGT) is due to increased adipose tissue lipolysis but could also be contributed to by reduced adipose tissue (AT) dietary fatty acid (DFA) trapping and increased "spillover" into the circulation. Thirty-one subjects with IGT (14 women, 17 men) and 29 with normal glucose tolerance (NGT, 15 women, 14 men) underwent a meal test with oral and intravenous palmitate tracers and the oral [¹⁸F]-fluoro-thia-heptadecanoic acid positron emission tomography method. Postprandial palmitate appearance (Ra_palmitate) was higher in IGT versus NGT (P < 0.001), driven exclusively by Ra_palmitate from obesity-associated increase in intracellular lipolysis (P = 0.01), as Ra_palmitate from DFA spillover was not different between the groups (P = 0.19) and visceral AT DFA trapping was even higher in IGT versus NGT (P = 0.02). Plasma glycerol appearance was lower in IGT (P = 0.01), driven down by insulin resistance and increased insulin secretion. Thus, we found higher AT DFA trapping, limiting spillover to lean organs and in part offsetting the increase in Ra_palmitate from intracellular lipolysis. Whether similar findings occur in frank diabetes, a condition also characterized by insulin resistance but relative insulin deficiency, requires further investigation (Clinicaltrials.gov: NCT04088344, NCT02808182).NEW & NOTEWORTHY We found higher adipose tissue dietary fatty acid trapping, limiting spillover to lean organs, that in part offsets the increase in appearance rate of palmitate from intracellular lipolysis in prediabetes. These results point to the adaptive nature of adipose tissue trapping and dietary fatty acid spillover as a protective mechanism against excess obesity-related palmitate appearance rate from intracellular adipose tissue lipolysis.

100th anniversary of the discovery of insulin perspective: insulin and adipose tissue fatty acid metabolism

Insulin inhibits systemic nonesterified fatty acid (NEFA) flux to a greater degree than glucose or any other metabolite. This remarkable effect is mainly due to insulin-mediated inhibition of intracellular triglyceride (TG) lipolysis in adipose tissues and is essential to prevent diabetic ketoacidosis, but also to limit the potential lipotoxic effects of NEFA in lean tissues that contribute to the development of diabetes complications. Insulin also regulates adipose tissue fatty acid esterification, glycerol and TG synthesis, lipogenesis, and possibly oxidation, contributing to the trapping of dietary fatty acids in the postprandial state. Excess NEFA flux at a given insulin level has been used to define in vivo adipose tissue insulin resistance. Adipose tissue insulin resistance defined in this fashion has been associated with several dysmetabolic features and complications of diabetes, but the mechanistic significance of this concept is not fully understood. This review focusses on the in vivo regulation of adipose tissue fatty acid metabolism by insulin and the mechanistic significance of the current definition of adipose tissue insulin resistance. One hundred years after the discovery of insulin and despite decades of investigations, much is still to be understood about the multifaceted in vivo actions of this hormone on adipose tissue fatty acid metabolism.

Effect of Paraplegia on the Time Course of Exogenous Fatty Acid Incorporation Into the Plasma Triacylglycerol Pool in the Postprandial State

Spinal cord injury (SCI) results in disordered fat metabolism. Autonomic decentralization might contribute to dyslipidemia in SCI, in part by influencing the uptake of dietary fats through the gut-lymph complex. However, the neurogenic contributions to dietary fat metabolism are unknown in this population. We present a subset of results from an ongoing registered clinical trial (NCT03691532) related to dietary fat absorption. We fed a standardized (20 kcal⋅kgFFM^–1) liquid meal tolerance test (50% carb, 35% fat, and 15% protein) that contained stable isotope lipid tracer (5 mg⋅kgFFM^–1 [U-¹³C]palmitate) to persons with and without motor complete thoracic SCI. Blood samples were collected at six postprandial time points over 400 min. Changes in dietary fatty acid incorporated into the triacylglycerol (TAG) pool (“exogenous TAG”) were used as a marker of dietary fat absorption. This biomarker showed that those with paraplegia had a lower amplitude than non-injured participants at Post₂₄₀ (52.4 ± 11.0 vs. 77.5 ± 16.0 μM), although this failed to reach statistical significance (p = 0.328). However, group differences in the time course of absorption were notable. The injury level was also strongly correlated with time-to-peak exogenous TAG concentration (r = −0.806, p = 0.012), with higher injuries resulting in a slower rise in exogenous TAG. This time course documenting exogenous TAG change is the first to show a potential neurogenic alteration in SCI dietary fat absorption.

Chylomicron-Derived Fatty Acid Spillover in Adipose Tissue: A Signature of Metabolic Health?

Context and Objectives

Spillover of fatty acids (FAs) into the plasma nonesterified fatty acid (NEFA) pool, because of an inability of adipose tissue (AT) to accommodate sufficient fat uptake, has been suggested to contribute to obesity-related insulin resistance. Using specific labeling techniques, we compared the proportion of spillover-derived NEFA across a range of adiposity.

Participants and Methods

Seventy-one healthy men and women were fed a mixed meal (40 g fat) containing [U13C]palmitate to assess the contribution of chylomicron-derived spillover FAs. To investigate subcutaneous abdominal-specific spillover, arteriovenous difference and stable-isotope methodologies were used in substudy (six men, six women).

Results

Chylomicron-derived FA spillover was higher in individuals with a BMI <25 kg/m2 (n = 18) compared with those with a BMI ≥25 kg/m2 (n = 53) (22.2 ± 1.6% vs 18.6 ± 0.7%, P = 0.02). Women had higher chylomicron-derived FA spillover than age- and BMI-matched men (21.9 ± 1.1% vs 15.0 ± 1.6%, P = 0.001). Assessing spillover across subcutaneous abdominal AT showed higher proportions in women than in men (28.5 ± 6.1% vs 9.9 ± 1.3%, P = 0.01).

Conclusion

There is a considerable degree of spillover FA into the systemic NEFA pool in the postprandial state; this process is greater and more dynamic in lean individuals and women. Contrary to general perception, spillover of chylomicron-derived FA into systemic circulation is a physiologically normal feature most easily observed in people with a higher capacity for clearance of plasma triglycerides, but does not appear to be a pathway providing excess NEFA in obesity.

Out of the frying pan: dietary saturated fat influences nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is characterized by excess accumulation of fat in the liver. In some cases, NAFLD is also accompanied by insulin resistance, resulting in metabolic dysfunction. Dietary fat content probably influences both NAFLD and insulin resistance; however, the immediate effects of fat consumption have not been fully explored. In this issue of the JCI, Hernández et al. evaluated hepatic glucose and lipid metabolism in humans and mice following a single oral dose of saturated fat. This one bolus of fat resulted in a measurable increase in insulin resistance, hepatic triglycerides, and gluconeogenesis. In mice, the saturated fat bolus resulted in the induction of several NAFLD-associated genes. Together, the results of this study indicate that saturated fat intake has immediate effects on metabolic function.

Digestion and Postprandial Metabolism in the Elderly

The elderly are an increasing segment of the population. Despite the rapid gains in medical knowledge and treatments, older adults are more likely to experience chronic illnesses that decrease quality of life and accelerate mortality. Nutrition is a key modifiable lifestyle factor which greatly impacts chronic disease risk. Yet despite the importance of nutrition, relatively little is known of the impact of advancing age on the gastrointestinal function, the digestive responses, and the post-meal metabolic adaptations that occur in response to ingested food. Knowledge of the age-related differences in digestion and metabolism in the elderly is essential to the development of appropriate nutritional recommendations for the maintenance of optimal health and prevention of disease.

Fatty acid sources and their fluxes as they contribute to plasma triglyceride concentrations and fatty liver in humans

PURPOSE OF REVIEW

Different sources of fatty acids (FA) used for VLDL-triglyceride synthesis include dietary FA that clear to the liver via chylomicron uptake, FA synthesized de novo in the liver from carbohydrates, nonesterified fatty acids derived from adipose tissue, nonesterified fatty acids derived from the spillover of chylomicron-triglyceride in the fasted and fed states, and FA stored in liver lipid droplets.

RECENT FINDINGS

Data have amassed on the contributions of each of these sources to liver-triglyceride accrual, VLDL-triglyceride synthesis, and hypertriglyceridemia. Discussed here is the timing of use of FA from each of these sources for synthesis of VLDL-triglyceride. Secondly, as all of these FA sources have been shown to contribute significantly to nonalcoholic fatty liver disease (NAFLD), data are presented demonstrating how poor handling of FA and glucose in the periphery can contribute to NAFLD. Lastly, we highlight how the stress of excess FA availability on the liver can be corrected by reduction of dietary intake of sugars and fats, weight loss, and increased physical activity.

SUMMARY

A better understanding of how lifestyle factors improve FA flux will aid in the development of improved treatments for the devastating condition of NAFLD.

Clinical considerations and mechanistic determinants of postprandial lipemia in older adults

The typical diet of individuals in Western societies results in metabolic responses associated with fed-state fat metabolism for most of the daily life of the individual. This fat metabolism is characterized specifically by an increase in the concentration of plasma lipids, primarily triglycerides. Increased postprandial lipemia, which is typically observed in older individuals (i.e., >65 y old), has now emerged as an important correlate of cardiovascular disease risk. An understanding of the mechanisms contributing to the increased postprandial lipemia in older individuals becomes, therefore, of particular clinical importance in any effort to explain and address the well-documented increase in cardiovascular disease risk as individuals age. Current evidence points to an increase in the accumulation of ingested lipid in lipoprotein particles of hepatic origin, together with an overall accumulation of lipid in these lipoproteins during the postprandial period, as primary contributors to the postprandial lipemia in older persons. When this evidence is considered together with the evidence suggesting large atherogenic potential of lipoproteins of hepatic origin, this can, at least in part, explain the increased risk of cardiovascular disease in older individuals. Understanding changes in the metabolism of ingested fat in the immediate postprandial period with advancing age, and how lifestyle interventions such as diet and physical exercise can ameliorate the increase in postprandial lipemia in older individuals, is important in order to address the increased cardiovascular disease risk in this particularly affected and growing segment of the population.

Sexual dimorphism in the effects of exercise on metabolism of lipids to support resting metabolism

Exercise training is generally a healthful activity and an effective intervention for reducing the risk of numerous chronic diseases including cardiovascular disease and diabetes. This is likely both a result of prevention of weight gain over time and direct effects of exercise on metabolism of lipids and the other macronutrient classes. Importantly, a single bout of exercise can alter lipid metabolism and metabolic rate for hours and even into the day following exercise, so individuals who regularly exercise, even if not performed every single day, overall could experience a substantial change in their resting metabolism that would reduce risk for metabolic diseases. However, resting metabolism does not respond similarly in all individuals to exercise participation, and indeed gender or sex is a major determinant of the response of resting lipid metabolism to prior exercise. In order to fully appreciate the metabolic effects and health benefits of exercise, the differences between men and women must be considered. In this article, the differences in the effects of exercise on resting metabolic rate, fuel selection after exercise, as well as the shuttling of triglyceride and fatty acids between tissues are discussed. Furthermore, concepts related to sex differences in the precision of homeostatic control and sex differences in the integration of metabolism between various organs are considered.

Postprandial triglyceride and free fatty acid metabolism in obese women after either endurance or resistance exercise

We investigated the effects of two exercise modalities on postprandial triglyceride (TG) and free fatty acid (FFA) metabolism. Sedentary, obese women were studied on three occasions in randomized order: endurance exercise for 60 min at 60-65% aerobic capacity (E), ~60 min high-intensity resistance exercise (R), and a sedentary control trial (C). After exercise, a liquid-mixed meal containing [U-(13)C]palmitate was consumed, and subjects were studied over 7 h. Isotopic enrichment (IE) of plasma TG, plasma FFA, and breath carbon dioxide compared with meal IE indicated the contribution of dietary fat to each pool. Total and endogenously derived plasma TG content was reduced significantly in both E and R compared with C (P < 0.05), with no effect of exercise on circulating exogenous (meal-derived) TG content. Exogenous plasma FFA content was increased significantly following both E and R compared with C (P < 0.05), whereas total and endogenous FFA concentrations were elevated only in E (P < 0.05) compared with C. Fatty acid (FA) oxidation rates were increased significantly after E and R compared with C (P < 0.05), with no difference between exercise modalities. The present results indicate that E and R may be equally effective in reducing postprandial plasma TG concentration and enhancing lipid oxidation when the exercise sessions are matched for duration rather than for energy expenditure. Importantly, tracer results indicated that the reduction in postprandial lipemia after E and R exercise bouts is not achieved by enhanced clearance of dietary fat but rather, is achieved by reduced abundance of endogenous FA in plasma TG.