Excess fat is body fat that has accumulated beyond what your body needs for normal functioning. Every person requires a baseline amount of fat for insulation, organ protection, hormone regulation, and energy reserves. When stored fat consistently exceeds those needs, it begins to change how your body operates, driving up blood pressure, blood sugar, and cholesterol while increasing the risk of serious disease. The threshold varies by age and sex, but generally, body fat above about 23% for men or 27% for women in their twenties is considered dangerously high.
What Body Fat Actually Does
Fat tissue is not just a passive storage depot. It is an active organ that secretes hormones controlling appetite, metabolism, and inflammation. White fat cells, the kind that makes up most of your body fat, store energy, insulate you from temperature extremes, and cushion organs. They also release signaling molecules that help regulate how hungry you feel, how your body uses sugar, and how aggressively your immune system responds to threats.
Brown fat cells serve a different purpose entirely. They generate heat, a function especially important in newborns who can’t shiver to warm themselves. Adults retain small amounts of brown fat, mostly around the neck and upper back, but the vast majority of body fat is white fat, and that’s the type that accumulates in excess.
How Your Body Stores Extra Calories
When you consistently eat more energy than you burn, the surplus gets stored as fat. But the process is more nuanced than “extra food turns into fat.” In most real-world diets, the body preferentially burns carbohydrates for fuel. When carbohydrate intake is high, your body shifts to burning those carbs instead of the fat you ate, so dietary fat gets shuttled directly into storage. Direct conversion of carbohydrates into fat (a process called de novo lipogenesis) is actually quite minor under normal conditions, producing only about 3 to 8 grams of new fat per day even during overfeeding. The body stores roughly 60 to 75 grams of dietary fat per day during a caloric surplus, almost all of it from fat that simply wasn’t burned.
The rare exception is extreme carbohydrate overfeeding. In one documented tradition in Cameroon, adolescent boys consuming over 7,000 calories per day almost entirely from carbohydrates gained 12 kilograms of body fat in ten weeks. Under those extreme conditions, the body does ramp up its ability to manufacture fat from carbs. But in everyday life, excess body fat comes primarily from dietary fat that was never used for energy.
Where Excess Fat Accumulates
Not all fat is stored in the same place, and location matters enormously for health.
Subcutaneous fat sits just under your skin, all around your body. It’s the fat you can pinch. On its own, subcutaneous fat is relatively benign. It provides insulation and cushioning, and while having a lot of it signals that visceral fat may also be elevated, it doesn’t directly interfere with organ function.
Visceral fat is stored deep in the abdomen, surrounding your liver, kidneys, and intestines. This fat is far more metabolically active and far more dangerous. It physically crowds organs, puts pressure on them, and disrupts their normal function. Visceral fat is strongly linked to high blood pressure, high cholesterol, and high blood sugar.
Ectopic fat is what happens when even visceral storage fills up. In a state of persistent caloric surplus, fatty acids begin depositing in organs that aren’t designed to store them: the liver (fatty liver disease), the muscles, the tissue surrounding the heart and coronary arteries, and even the kidneys. Fat accumulating in the liver and muscles directly interferes with how your body processes sugar and insulin. Fat deposits around the heart can impair cardiac function, and fat in the kidney’s drainage system may compress blood vessels, affecting kidney performance.
How Excess Fat Changes Your Hormones
Fat cells produce a hormone called leptin, which is the body’s primary appetite regulator. Leptin levels rise in direct proportion to how much fat you carry. In theory, more leptin should signal your brain to reduce hunger. But in people with excess fat, the brain often stops responding to leptin effectively, a condition sometimes called leptin resistance. The result is a frustrating cycle: more fat produces more leptin, but the appetite-suppressing signal doesn’t get through.
Leptin also plays a role in inflammation. It activates certain immune cells and promotes an inflammatory state in blood vessel walls. This chronic, low-grade inflammation is one of the key ways excess fat contributes to heart disease and other conditions. Fat tissue also releases other inflammatory molecules that further amplify this response, creating a persistent state of immune activation throughout the body.
Another hormone produced primarily by fat cells, adiponectin, generally works in the opposite direction, reducing inflammation and improving insulin sensitivity. Paradoxically, adiponectin levels tend to drop as fat mass increases, removing a protective brake at the same time that pro-inflammatory signals are ramping up.
Metabolic Syndrome and Disease Risk
Excess fat rarely causes problems in isolation. It typically drives a cluster of metabolic changes that, taken together, dramatically increase the risk of heart disease, stroke, and type 2 diabetes. This cluster is called metabolic syndrome, and it’s diagnosed when three of the following five markers are abnormal: waist circumference, blood triglycerides, HDL cholesterol, blood pressure, and fasting blood sugar.
Waist circumference is included because abdominal fat is the strongest predictor of insulin resistance that can be measured with a tape measure. The standard thresholds are 102 centimeters (about 40 inches) for men and 88 centimeters (about 35 inches) for women, though lower cutoffs apply for certain ethnic groups, particularly people of Asian descent, who tend to develop metabolic complications at smaller waist sizes. A 2025 study in the British Journal of Sports Medicine found that women exceeding the 88-centimeter threshold had a 17% elevated risk of cancer, and that exceeding these waist measurements increased cancer risk even among people who were physically active.
Measuring Excess Fat
BMI (body mass index) is the most commonly used screening tool. The World Health Organization defines a BMI of 25 or above as overweight and 30 or above as obese. BMI is useful at a population level but has well-known blind spots for individuals. It doesn’t distinguish between muscle and fat, and it can’t tell you where your fat is stored.
Body fat percentage gives a more direct picture. Women need more essential fat than men due to biological differences related to hormone regulation and reproductive function. For women, body fat below 14% is considered low, while levels above roughly 27% in your twenties (rising to about 35% by your sixties) enter the dangerously high range. For men, below 8% is low, and above about 23% in your twenties (rising to about 28% by your sixties) is dangerously high. These thresholds increase with age because some fat gain is a normal part of aging.
Waist circumference is the simplest way to gauge whether your fat distribution puts you at metabolic risk. It requires nothing more than a tape measure, and it captures the visceral fat that BMI misses. For a practical assessment, measuring your waist at the level of your navel while standing gives a reasonable estimate of whether you fall above or below the risk thresholds.