Brain insulin action on peripheral insulin sensitivity in women depends on menstrual cycle phase

Expanding the understanding of insulin resistance in brain and periphery

Insulin resistance is a central feature of metabolic disorders such as type 2 diabetes (T2D). While studies on this disorder have largely been linked to glucose metabolism and intracellular signaling, recent advances reveal that insulin resistance extends beyond traditional glucose regulatory pathways, impacting multiple organs including the brain, contributing to cognitive dysfunction and neurodegenerative diseases such as Alzheimer's disease (AD). This opinion revisits insulin resistance through molecular, cellular, and systemic perspectives, emphasizing the intersection between peripheral and brain insulin resistance (BIR), the role of the blood-brain barrier (BBB), and emerging biomarkers. Furthermore, we integrate insights from multi-omics and neuroimaging studies to refine our understanding, advocating for a broader perspective that informs early detection and intervention in metabolic and neurodegenerative diseases.

Repeated net-tDCS of the hypothalamus appetite-control network enhances inhibitory control and decreases sweet food intake in persons with overweight or obesity

BACKGROUND

Reduced inhibitory control is associated with obesity and neuroimaging studies indicate that diminished prefrontal cortex activity influence eating behavior and metabolism. The hypothalamus regulates energy homeostasis and is functionally connected to cortical and subcortical regions especially the frontal areas.

OBJECTIVES

We tested network-targeted transcranial direct current stimulation (net-tDCS) to influence the excitability of brain regions involved in appetite control.

METHODS

In a randomized, double-blind parallel group design, 44 adults with overweight or obesity (BMI 30.6 kg/m2, 52.3 % female) received active (anodal or cathodal) or sham 12-channel net-tDCS on the hypothalamus appetite-control network for 25 min on three consecutive days while performing a Stop-Signal-Task to measure response inhibition. Before and after stimulation, state questionnaires assessed changes in desire to eat and food craving. Directly after stimulation, participants received a breakfast buffet to evaluate ad-libitum food intake. An oral glucose tolerance test was conducted at follow-up. Resting-state functional MRI was obtained at baseline and follow-up.

RESULTS

The Stop-Signal Reaction Time (SSRT) was shorter in both active groups versus sham, indicating improved response inhibition. Additionally, a stronger increase in hypothalamic functional connectivity was associated with shorter SSRT. Caloric intake of sweet food was lower in the anodal group versus sham, but no main effects between groups were observed on total and macronutrient intake, food craving ratings and desire to eat. At follow-up, no differences were observed between groups on peripheral metabolism.

CONCLUSION

Our study suggests that modulating hypothalamic functional network connectivity patterns via net-tDCS may improve food choice and inhibitory control.

Glycemic Control Across the Menstrual Cycle in Women with Type 1 Diabetes Using the MiniMed 780G Advanced Hybrid Closed-Loop System: The 780MENS Prospective Study

Objective: To evaluate whether the MiniMed™ 780G advanced hybrid closed-loop (AHCL) system maintains similar glycemic control across two different phases of the menstrual cycle in women with type 1 diabetes (T1D) and to analyze the system's performance in these situations. Methods: Continuous glucose monitoring (CGM) and insulin delivery metrics from 12 participants with T1D using the MiniMed™ 780G AHCL were analyzed throughout 3 prospectively recorded, consecutive menstrual cycles (36 cycles in total). Mixed models were used to compare the different variables between the early follicular phase and late luteal phase. Results: A higher average glucose was found throughout the late luteal phase compared with the early follicular phase (139.5 [133.5, 145.2] vs. 131.5 [126.8, 140.2] mg/dl, respectively, P = 0.002), together with an increase in total daily insulin dose (37.2 ± 11.9 vs. 33.6 ± 12.2 IU, P < 0.001). However, similar values between phases were observed for most of the remaining CGM metrics, including time in range (83.0 [76.0, 87.5] vs. 85.0 [79.8, 89.0] %, P = 0.101). Conclusion: Our results suggest that differences in glycemic control may be found across the menstrual cycle in women with T1D using the MiniMed™ 780G AHCL. Although higher average glucose levels may be expected in the late luteal phase, the deterioration of glycemic control during this phase may be mild with the MiniMed™ 780G AHCL, given the similarities for most of the CGM metrics with respect to the early follicular phase.

Exploring the effects of an insulin challenge on neuroimaging outcomes: A scoping review

Emerging evidence demonstrates that insulin has a modulating effect on metabolic and cognitive function in the brain, highlighting the potential role of aberrant brain insulin signaling in the pathogenesis of various neuropsychiatric illnesses. Neuroimaging paradigms using intranasal insulin (INI) as a pharmacological challenge have allowed us to study the effects of insulin in the human brain. In this scoping review, we conducted a systematic database search to identify relevant research studies that employed an INI-based neuroimaging assay of brain insulin signaling. Thirty-six studies met inclusion criteria for this review. INI was found to significantly modulate activity and cerebral blood flow in brain regions related to homeostatic/hedonic control of food intake, as well as cognition. This review highlights the putative role of insulin signaling in the brain and the potential therapeutic value of INI in patients with mental health, addiction, and co-morbid metabolic disorders.

A short-term, high-caloric diet has prolonged effects on brain insulin action in men

Brain insulin responsiveness is linked to long-term weight gain and unhealthy body fat distribution. Here we show that short-term overeating with calorie-rich sweet and fatty foods triggers liver fat accumulation and disrupted brain insulin action that outlasted the time-frame of its consumption in healthy weight men. Hence, brain response to insulin can adapt to short-term changes in diet before weight gain and may facilitate the development of obesity and associated diseases.

Non-caloric sweetener effects on brain appetite regulation in individuals across varying body weights

Sucralose, a widely used non-caloric sweetener, provides sweet taste without calories. Some studies suggest that non-caloric sweeteners stimulate appetite, possibly owing to the delivery of a sweet taste without the post-ingestive metabolic signals that normally communicate with the hypothalamus to suppress hunger. In a randomized crossover trial (ClinicalTrials.gov identifier: NCT02945475 ), 75 young adults (healthy weight, overweight or with obesity) consumed a drink containing sucralose, sweetness-matched sucrose or water. We show that acute consumption of sucralose versus sucrose stimulates hypothalamic blood flow (P < 0.018) and greater hunger responses (P < 0.001). Sucralose versus water also increases hypothalamic blood flow (P < 0.019) but produces no difference in hunger ratings. Sucrose, but not sucralose, increases peripheral glucose levels, which are associated with reductions in medial hypothalamic blood flow (P < 0.007). Sucralose, compared to sucrose and water, results in increased functional connections between the hypothalamus and brain regions involved in motivation and somatosensory processing. These findings suggest that non-caloric sweeteners could affect key mechanisms in the hypothalamus responsible for appetite regulation.

Continuous Glucose Monitoring Underreports Blood Glucose During a Simulated Ultraendurance Run in Eumenorrheic Female Runners

PURPOSE

Continuous-glucose-monitoring (CGM) sensors provide near-real-time glucose data and have been introduced commercially as a tool to inform nutrition decisions. The aim of this pilot study was to explore how factors such as the menstrual phase, extended running duration, and carbohydrates affect CGM outcomes among trained eumenorrheic females in an outdoor simulated ultraendurance running event.

METHODS

Twelve experienced female ultrarunners (age 39 [6] y) participated in this crossover study. Participants completed an ultraendurance simulation run of 4 hours in the midfollicular and midluteal phases of their menstrual cycle, which consisted of a 3-hour fasted outdoor run (FASTED) followed by a 1-hour treadmill run (TREAD), where 3 standardized 20-g oral glucose doses were provided.

RESULTS

Using a mixed linear model, the menstrual phase was statistically significant for differences in glucose measurements from CGM compared with capillary glucose sampling during TREAD (P = .02) but not FASTED. Additionally, the CGM sensor reported glucose levels with an average of -0.43 mmol·L-1 (95% CI, - 0.86 to -0.005) and -1.02 mmol·L-1 (95% CI, -1.63 to -0.42) lower in fasted and fed scenarios, respectively, when compared with capillary glucose.

CONCLUSION

CGM underreports capillary glucose during fasted and fed exercise. Factors contributing to this underreporting between the sampling methods (CGM vs capillary) were dependent on a combination of exogenous glucose availability, individual biological differences, and the menstrual phase.

Liver-specific actions of GH and IGF1 that protect against MASLD

Metabolic dysfunction-associated steatotic liver disease (MASLD; also known as nonalcoholic fatty liver disease) is a chronic condition associated with metabolic syndrome, a group of conditions that includes obesity, insulin resistance, hyperlipidaemia and cardiovascular disease. Primary growth hormone (GH) deficiency is associated with MASLD, and the decline in circulating levels of GH with weight gain might contribute to the development of MASLD. Raising endogenous GH secretion or administering GH replacement therapy in the context of MASLD enhances insulin-like growth factor 1 (IGF1) production and reduces steatosis and the severity of liver injury. GH and IGF1 indirectly control MASLD progression by regulating systemic metabolic function. Evidence supports the proposal that GH and IGF1 also have a direct role in regulating liver metabolism and health. This Review focuses on how GH acts on the hepatocyte in a sex-dependent manner to limit lipid accumulation, reduce stress, and promote survival and regeneration. In addition, we discuss how GH and IGF1 might regulate non-parenchymal cells of the liver to control inflammation and fibrosis, which have a major effect on hepatocyte survival and regeneration. Development of a better understanding of how GH and IGF1 coordinate the functions of specific, individual liver cell types might provide insight into the aetiology of MASLD initiation and progression and suggest novel approaches for the treatment of MASLD.

Diabetes mellitus-Progress and opportunities in the evolving epidemic

Diabetes, a complex multisystem metabolic disorder characterized by hyperglycemia, leads to complications that reduce quality of life and increase mortality. Diabetes pathophysiology includes dysfunction of beta cells, adipose tissue, skeletal muscle, and liver. Type 1 diabetes (T1D) results from immune-mediated beta cell destruction. The more prevalent type 2 diabetes (T2D) is a heterogeneous disorder characterized by varying degrees of beta cell dysfunction in concert with insulin resistance. The strong association between obesity and T2D involves pathways regulated by the central nervous system governing food intake and energy expenditure, integrating inputs from peripheral organs and the environment. The risk of developing diabetes or its complications represents interactions between genetic susceptibility and environmental factors, including the availability of nutritious food and other social determinants of health. This perspective reviews recent advances in understanding the pathophysiology and treatment of diabetes and its complications, which could alter the course of this prevalent disorder.

The insulin resistant brain: impact on whole-body metabolism and body fat distribution

Insulin exerts its actions not only on peripheral organs but is also transported into the brain where it performs distinct functions in various brain regions. This review highlights recent advancements in our understanding of insulin's actions within the brain, with a specific emphasis on investigations in humans. It summarises current knowledge on the transport of insulin into the brain. Subsequently, it showcases robust evidence demonstrating the existence and physiological consequences of brain insulin action, while also introducing the presence of brain insulin resistance in humans. This pathophysiological condition goes along with an impaired acute modulation of peripheral metabolism in response to brain insulin action, particularly in the postprandial state. Furthermore, brain insulin resistance has been associated with long-term adiposity and an unfavourable adipose tissue distribution, thus implicating it in the pathogenesis of subgroups of obesity and (pre)diabetes that are characterised by distinct patterns of body fat distribution. Encouragingly, emerging evidence suggests that brain insulin resistance could represent a treatable entity, thereby opening up novel therapeutic avenues to improve systemic metabolism and enhance brain functions, including cognition. The review closes with an outlook towards prospective research directions aimed at further elucidating the clinical implications of brain insulin resistance. It emphasises the critical need to establish feasible diagnostic measures and effective therapeutic interventions.

Impact of Physiological Fluctuations of Sex Hormones During the Menstrual Cycle on Glucose Metabolism and the Gut Microbiota

Diabetes mellitus is one of the most prevalent chronic diseases. Previous studies have shown differences in glucose metabolism between males and females. Moreover, difficulties in medication adherence have been reported in females with type 2 diabetes. These observations are believed to be caused by fluctuations in sex hormone concentrations during the menstrual cycle. Furthermore, gut microbiota is linked to female host metabolism and sex hormone production. Understanding the interactions between fluctuating hormone concentrations during the menstrual cycle, gut microbiota, and glucose metabolism in humans is significant because of the increasing prevalence of diabetes and the consequent need to expand preventive efforts. A literature search was performed to determine and summarize the existing evidence, deduce future research needs to maintain female health, and investigate the relationship between the physiological menstrual cycle and glucose metabolism. Studies from 1967 to 2020 have already examined the relationship between variations during the menstrual cycle and glucose metabolism in healthy female subjects using an oral-glucose tolerance test or intravenous glucose tolerance test. However, the overall number of studies is rather small and the results are contradictory, as some studies detected differences in glucose concentrations depending on the different cycle phases, whereas others did not. Some studies reported lower glucose levels in the follicular phase than in the luteal phase, whereas another study detected the opposite. Data on gut microbiota in relation to the menstrual cycle are limited. Conflicting results exist when examining the effect of hormonal contraceptives on the gut microbiota and changes in the course of the menstrual cycle. The results indicate that the menstrual cycle, especially fluctuating sex hormones, might impact the gut microbiota composition.The menstrual cycle may affect the gut microbiota composition and glucose metabolism. These results indicate that glucose tolerance may be the greatest in the follicular phase; however, further well-conducted studies are needed to support this assumption.

The Influence of Menstrual Cycle Phases on Maximal Strength Performance in Healthy Female Adults: A Systematic Review with Meta-Analysis

Maximal strength is a significant factor in achieving peak performance and injury prevention in athletes. In individualization strategies for the efficient development of athletes, it is necessary to consider the respective components separately. The purpose of this study was to systematically examine the effects of the different cycle phases on isometric, isokinetic, and dynamic maximum strength. A systematic literature review was conducted; databases were searched from January 1960 to September 2023. The included studies focused on the expression of maximal strength in the earlier follicular phase as well as at least one comparative phase. Of the initial 707 articles identified, 22 met the selection criteria and were included. The studies considered a total of 433 subjects. Our results revealed medium effects (weighted mean standardized mean difference (SMD) = 0.60; seven studies) for isometric maximal strength in favor of the late follicular phase, small effects (weighted mean SMD = 0.39; five studies) for isokinetic maximal strength in favor of the ovulation phase, and small effects (weighted mean SMD = 0.14; three studies) for dynamic maximal strength in favor of the late follicular phase. The results indicate that the early follicular phase is unfavorable for all strength classes. Peak performance in isometric strength is seen in the late follicular phase, whereas isokinetic strength peaks during ovulation. Dynamic strength is optimal in the late follicular phase.