Insulin is the body’s master fuel–partitioning hormone. Made by pancreatic beta cells, it signals muscles and fat to take up glucose, tells the liver to store rather than make sugar, enables protein building, and coordinates lipid storage. Because of this, it influences energy, brain function, vascular health, reproductive hormones, and how we respond to meals.On a fasting lab, insulin normally sits in a low, narrow range and rises after eating as it clears glucose from the bloodstream. In people with healthy glucose levels, optimal fasting values tend to be toward the low end of normal, reflecting good insulin sensitivity. During pregnancy, baseline insulin runs higher due to placental hormones.When values are low, they can mean either excellent insulin sensitivity or inadequate insulin production. If too low for the body’s glucose load, blood sugar rises and ketones appear: thirst, frequent urination, weight loss, fatigue, and blurry vision are typical. In children and teens, this often signals type 1 diabetes; in adults it can reflect autoimmune beta‑cell failure (LADA) or pancreatic injury. In pregnancy, insufficient insulin leads to maternal hyperglycemia that can affect fetal growth.When values are high, they usually reflect insulin resistance—pancreas working overtime to keep glucose normal. This often precedes high glucose by years and links to sleepiness after carb‑heavy meals, increasing waist, skin tags or darkened neck/underarms, fatty liver, high triglycerides, lower HDL, and higher blood pressure. Women may see ovarian androgen excess and irregular cycles (PCOS); men may notice reduced testosterone and erectile issues. Large swings can cause shakiness and hunger from reactive hypoglycemia.Big picture: insulin sits at the crossroads of muscle, liver, fat, brain, and vascular systems. Its pattern over time predicts risks for type 2 diabetes, cardiovascular disease, kidney disease, and fatty liver, and it interacts with thyroid, cortisol, and growth hormone to shape long‑term metabolic health.
