Metabolism & Nutrient Sensing

Caloric restriction is a sustained reduction in energy intake, typically 10–30% below ad libitum, without malnutrition. It activates conserved nutrient-sensing pathways including AMPK and sirtuins while suppressing mTOR and insulin/IGF-1 signaling. In many rodent models it extends lifespan, though effects vary by strain, sex, age at onset, and protocol; non-human primate trials gave divergent results (Wisconsin vs. NIA). In humans, the CALERIE-2 trial (~12% achieved restriction, below the 25% target) improved cardiometabolic markers and reduced inflammation.

A continuous glucose monitor (CGM) is a wearable sensor, typically inserted into subcutaneous tissue, that measures interstitial glucose every few minutes, typically about 7 to 14 days for transcutaneous sensors (system-dependent), and up to a year for implantable devices such as Eversense 365. It generates trend data on fasting, postprandial, and nocturnal glucose, time-in-range, and glycemic variability. CGMs are increasingly used in non-diabetic adults to personalize nutrition and inform longevity-oriented lifestyle adjustments.

CRON is a structured form of caloric restriction in which energy intake is reduced by roughly 20–30% while micronutrient density (vitamins, minerals, essential fatty acids, protein quality) is deliberately maximized. The approach was developed by Roy Walford, Lisa Walford, and Brian M. Delaney, with The Anti-Aging Plan (1994) as a key reference, and later promoted by the CR Society. The aim is to capture metabolic benefits—improved insulin sensitivity, lower inflammation, favorable lipids—without inducing nutrient deficiencies.

The fasting-mimicking diet is a 5-day low-calorie, low-protein, plant-based regimen developed by Valter Longo's group that reproduces metabolic effects of water-only fasting—reduced IGF-1, glucose, and insulin; elevated ketones—while still providing some food. Pivotal trials used 3 cycles spaced one month apart and reported improvements in cardiometabolic risk markers and abdominal adiposity. Suggested benefits on biological age estimates rest on secondary analyses (Brandhorst et al. 2024) and should be considered preliminary.

Glucose variability quantifies the magnitude and frequency of blood glucose fluctuations over hours and days, typically expressed as standard deviation, coefficient of variation, or mean amplitude of glycemic excursions (MAGE). High variability is implicated in oxidative stress, endothelial dysfunction, and diabetic complications independent of mean glucose. In non-diabetic adults, lower variability correlates with better metabolic health, and continuous glucose monitoring increasingly tracks it as a longevity-relevant biomarker.

HbA1c (glycated hemoglobin) reflects the proportion of hemoglobin stably bound to glucose, providing an integrated estimate of average blood glucose over approximately the prior 2 to 3 months, with the most recent ~30 days contributing roughly half of the signal per published kinetic models. It is the primary biomarker for diagnosing and monitoring type 2 diabetes (threshold 6.5%) and prediabetes (5.7 to 6.4%). HbA1c is influenced by erythrocyte lifespan, anemia, and hemoglobin variants.

HOMA-IR (Homeostatic Model Assessment of Insulin Resistance) is a fasting blood index calculated as (fasting insulin in µU/mL × fasting glucose) / k, where k = 22.5 if glucose is in mmol/L (SI units) or k = 405 if glucose is in mg/dL (US conventional units). It estimates whole-body insulin resistance from fasting measures, predominantly reflecting hepatic insulin action, as a low-cost surrogate for clamp methods. Cutoffs are population- and assay-dependent, with no universal threshold; values are commonly cited around 2 to 2.9 in adults.

Insulin resistance is a state in which target tissues respond poorly to insulin, prompting the pancreas to secrete more to maintain glucose homeostasis. Driven by visceral adiposity, ectopic fat in liver and muscle, chronic inflammation, and inactivity, it underlies prediabetes, type 2 diabetes, metabolic syndrome, and metabolic dysfunction-associated steatotic liver disease (MASLD, formerly NAFLD). Insulin resistance is also associated with accelerated cardiovascular aging, cognitive decline, and shortened healthspan.

Insulin sensitivity describes how effectively cells, especially in muscle, liver, and adipose tissue, respond to insulin to take up glucose and suppress hepatic glucose output. Higher sensitivity allows lower circulating insulin to maintain normoglycemia, reducing strain on pancreatic beta cells. It is improved by physical activity, sleep, low visceral fat, and dietary fiber. Robust insulin sensitivity is a hallmark of metabolic health and longevity-relevant resilience.

Intermittent fasting is an umbrella term for eating patterns that alternate normal intake with extended fasting windows, including 16:8 time-restricted eating, alternate-day fasting, and 5:2 protocols. Fasting periods lower insulin and glycogen, trigger lipolysis and ketogenesis, and induce autophagy. Clinical trials show modest improvements in body composition, glycemic control, and blood pressure; meta-analyses suggest results are broadly comparable to matched continuous calorie reduction, though some trials report small advantages for visceral fat or insulin sensitivity.

The ketogenic diet is a very-low-carbohydrate (typically <50 g/day), high-fat, moderate-protein eating pattern that drives the body into sustained nutritional ketosis. Originally developed to treat refractory pediatric epilepsy, it is now studied for type 2 diabetes, obesity, and neurodegenerative conditions. Mechanisms include lower insulin, improved metabolic flexibility, and ketone-mediated signaling. Long-term effects on lipid profiles, kidney function, and adherence remain under active investigation.

Ketone bodies are three water-soluble molecules—β-hydroxybutyrate, acetoacetate, and acetone—produced in hepatocyte mitochondria from acetyl-CoA derived from fatty acid β-oxidation when carbohydrate availability is low. Beyond serving as efficient ATP fuel for brain and heart, β-hydroxybutyrate is an endogenous signaling molecule that inhibits class I histone deacetylases, dampens NLRP3 inflammasome activity, and may improve mitochondrial efficiency, mechanisms relevant to fasting biology and longevity research.

Ketosis is a metabolic state in which the liver converts fatty acids into ketone bodies—β-hydroxybutyrate, acetoacetate, and acetone—that serve as alternative fuel for brain, heart, and muscle when glucose is scarce. It is induced by fasting, prolonged exercise, or very-low-carbohydrate diets, with blood β-hydroxybutyrate typically rising above the 0.5 mmol/L nutritional-ketosis threshold (Volek/Phinney). β-hydroxybutyrate also acts as a signaling molecule, inhibiting class I HDACs and modulating inflammation.

Metabolic flexibility is the capacity of cells and the whole organism to switch efficiently between fuel sources—primarily glucose and fatty acids—in response to feeding, fasting, and physical activity. It depends on intact mitochondrial function, insulin sensitivity, and hormonal signaling. Loss of flexibility, marked by impaired fasting fat oxidation and postprandial glucose handling, is a hallmark of insulin resistance, obesity, and aging, and is a key target of fasting and exercise interventions.

Postprandial glucose refers to blood glucose levels after a meal, often peaking around 60 minutes (typically within 1 to 2 hours) before returning toward fasting baseline. The size and duration of the spike reflect carbohydrate quantity and quality, gastric emptying, insulin response, and tissue uptake. Recurrent large excursions (a value rarely exceeded in non-diabetics is roughly 140 mg/dL) are associated, in a graded fashion, with vascular risk and cardiovascular disease, making postprandial control a key target in metabolic and longevity-oriented nutrition.

Prolonged fasting refers to fasting periods of roughly 48 hours up to several days during which only water, electrolytes, and sometimes minimal calories are consumed. After glycogen depletion, the body shifts to fatty acid oxidation and ketogenesis, suppresses IGF-1 and mTOR, and upregulates autophagy. Stem-cell-based regeneration has been demonstrated in rodents; human translation remains limited. Risks include electrolyte disturbances and refeeding syndrome, so prolonged fasting should be medically supervised.

Time-restricted eating (TRE) confines daily food intake to a consistent window of typically 6–10 hours, leaving 14–18 hours of fasting. The concept emerged from Satchin Panda's circadian biology lab at the Salk Institute. Some trials report improved insulin sensitivity, lipids, and blood pressure independent of calorie reduction; others, including Liu et al. (NEJM 2022), found no advantage of TRE over caloric restriction. Early-window TRE may be preferable, but the evidence remains preliminary.