What is visceral fat, and why is it riskier?

The texts shown during the animation, to read without it.

1. A cross-section of the belly at the navel
   Picture the belly sliced at the level of the navel and seen from the feet, as on a CT image. The front is at the top, the back at the bottom. Two very different fats are found there, separated by the belly muscles.
2. Under the skin: the fat you can pinch
   Just under the skin lies subcutaneous fat, the kind you can pinch between two fingers. It covers the whole body, from the belly to the thighs, and holds most of our reserves: about 85% of the body's fat (Foster 2012).
3. Behind the muscles: visceral fat
   Visceral fat begins under the belly muscles. It forms an apron in front of the intestines, the omentum, and fills the mesentery, the thin sheet that holds the intestine and carries its vessels. It cannot be pinched: the muscles are in front. In men, it makes up about 10 to 20% of the body's fat (Wajchenberg 2000, Foster 2012). Before menopause, for the same amount of fat, women have about half as much.
4. Around the kidneys: a fat of its own
   The kidneys sit against the back, behind the belly cavity. Their fat is often counted with visceral fat on images. But its blood usually leaves for the kidney vein and the vena cava, so toward the heart without passing through the liver, unlike blood from the omentum and the mesentery (Jensen 2008).
5. Belly A: mostly under the skin
   This belly is wide, but almost everything is under the skin: it is soft and easy to pinch. The organs, on the other hand, stay surrounded by little fat.
6. Belly B: same size, mostly inside
   This belly is about the same size as A, but the fat is behind the muscles. It is firmer, tight, and hard to pinch. A tape measure gives the same waist circumference: only a CT scan or an MRI sees the difference.
7. Thin outside, fat inside
   A flat belly can hide visceral fat. Researchers call this "thin outside, fat inside" (TOFI). In a study that put 477 volunteers through whole-body MRI, the researchers describe this group as having a higher metabolic risk (Thomas 2012).
8. How is it really seen?
   On a CT scan or an MRI, the area of visceral fat is measured on a single cross-section, often between the L4 and L5 vertebrae, in square centimeters. Waist circumference, on the other hand, adds up both belly fats without separating them: it is an estimate, useful because it only takes a tape measure.

1. A fat cell: a droplet of reserve
   Each fat cell is almost entirely filled with a droplet of reserve. On its surface, receptors set the release: beta receptors speed it up, alpha-2 receptors and insulin receptors slow it down. The visceral cell responds more strongly to the accelerators and less to the insulin brake. The alpha-2 brake acts about equally on both sides (Wajchenberg 2000 review).
2. After a meal: insulin closes the tap
   After a meal, insulin rises. It slows the release of fats and helps the cells fill up. But visceral fat responds less to this brake: even after eating, it keeps letting out more fatty acids.
3. Exercise or fasting: adrenaline opens the tap
   During exercise or a fast, adrenaline and noradrenaline bind to the beta receptors. On the visceral side, the cell responds more strongly: fatty acids come out much faster. That is why this fat often melts first when the body draws on its reserves.
4. Stress: cortisol made on the spot
   Belly fat does not just receive cortisol from the adrenal glands: thanks to the 11β-HSD1 enzyme, it turns cortisone from the blood into cortisol, especially in the omentum. In cells from 16 patients grown in the lab, this conversion was seen in the omentum and not under the skin; the researchers spoke of a "Cushing's disease of the omentum" (Bujalska 1997). Other studies find a smaller gap.
5. The takeaway: a fat that moves fast
   Visceral fat fills and empties faster than the fat under the skin. It is smaller, but very active. And what it releases does not go just anywhere: its blood goes straight to the liver. That is the subject of the next tab.

1. The usual route: blood returns to the heart
   Blood that has passed through the fat under the skin, the legs or the arms returns to the heart through the veins, here the vena cava. The heart sends it to the lungs, takes it back, then pushes it into the aorta toward the whole body. Only a small share takes the liver's artery. Fatty acids released under the skin make this long loop before reaching the liver.
2. The intestine has its own route: the portal vein
   Blood from the intestine does not go straight back to the heart: it first passes through the portal vein, which takes it to the liver. Blood from the visceral fat of the omentum and the mesentery takes the same route (Jensen 2008). Its fatty acids therefore go straight to the liver. The portal vein also collects blood from the stomach, the spleen, the gallbladder and the pancreas (Eipel 2010 review). Fat around the kidneys, on the other hand, sends its blood to the vena cava (Jensen 2008).
3. The liver receives two bloods
   The liver weighs about 2.5% of the body, but receives nearly 25% of the blood pumped by the heart. About three quarters arrive through the portal vein, the rest through the liver's artery; each brings it roughly half of its oxygen (Eipel 2010 review). Blood in the portal vein has first passed through the intestine: some of the fatty acids from the rest of the body therefore also arrive this way.
4. What comes from visceral fat arrives without a detour
   Fatty acids from the fat under the skin pass through the heart and mix with all the blood in the body: only part of them reach the liver. Those from visceral fat go there directly, and the liver takes up a good share of them on the way (Jensen 2008). That is why a blood test from the arm does not show what the liver really receives (Nielsen 2004).
5. What the liver does with these fatty acids
   The liver stores some of the fatty acids as fat droplets: in 9 people with fatty liver, 59% of the liver's fat came from fatty acids in the blood, 26% was made on the spot and 15% came straight from the meal (Donnelly 2005). It also sends some back into the blood, packed into VLDL. And when it receives too many, insulin holds back the sugar it releases less well (Jensen 2008).
6. An idea to qualify
   For a long time it was thought that visceral fat harms the liver mainly through this direct route: this is the "portal theory," proposed as early as 1990 (Björntorp 1990; Item 2012 review). Measurements paint a more nuanced picture: even in people who have the most, the largest share of the liver's fatty acids still comes from the rest of the body, mostly from upper-body fat outside the belly (Nielsen 2004). The direct route matters, without explaining everything.

1. Healthy visceral fat
   Under the microscope, fat is made of round cells packed together, with small vessels between them that bring oxygen. A few immune cells, the macrophages, quietly clean up. The cells send out adiponectin, a hormone that helps the body use sugar well.
2. The cells swell
   As reserves build up, each cell fills and grows. The small vessels, however, do not multiply as fast: they end up farther from the center of the cells.
3. The vessels cannot keep up: oxygen runs short
   The cells farthest from a vessel run short of oxygen (in gray). They start sending alarm signals. In humans, the studies that measured oxygen did so in the fat under the skin of the belly, not in visceral fat (Goossens 2015 review), and they contradict each other: less oxygen in people with obesity for one (Pasarica 2009), more for the other (Goossens 2011).
4. Cells die, macrophages surround them
   Some cells that grow too large die. Macrophages arrive and form a crown around each one to clear it away: under the microscope, these are called "crown-like structures." More than 90% of the macrophages in fat sit around dead cells (Cinti 2005, mice and humans).
5. Macrophages switch to inflammatory mode
   While cleaning up, macrophages change mode: from calm, they become inflammatory (in red). They send out IL-6 and TNF-alpha. TNF-alpha acts mainly on the spot and makes neighboring cells less sensitive to insulin. In the fat under the skin of the belly, they make up less than 10% of cells in thin people and up to nearly 40% in people with obesity (Weisberg 2003).
6. IL-6 goes to the liver, which makes CRP
   In people with obesity, fat from the omentum releases 2 to 3 times more IL-6 than fat under the skin (Fried 1998). And its IL-6 goes straight to the liver through the portal vein. The liver responds by making CRP, the protein measured by the "inflammation" blood test. In 25 people with severe obesity who had surgery, there was about 50% more IL-6 in the portal vein than in the wrist artery, and this level tracked CRP (Fontana 2007).
7. Less adiponectin, the protective hormone
   The more visceral fat there is, the less adiponectin the blood contains (in 190 men, 7.0 versus 11.1 µg/ml above or below 130 cm², Côté 2005), even though this hormone helps the muscles and the liver use sugar well. Not all of fat's hormones come from it: leptin, the satiety hormone, is made mostly by the fat under the skin (Wajchenberg 2000).
8. A low-grade inflammation
   Nothing to do with an infection: this inflammation is weak but constant, hence its name, "low-grade" inflammation. It is not automatic: in a thin person, visceral fat is not very inflammatory. It rises with the amount of fat and the size of the cells.

Other texts from the animation

• In 68 women and men measured with catheters in the liver veins, visceral fat supplied from less than 10% to nearly 50% of the fatty acids reaching the liver (Nielsen 2004). With little visceral fat, its share is around 5 to 10%: almost everything comes from the rest of the body.
• The more visceral fat rises, the more its share climbs: in some people who have a lot of it, it goes above 30%. For the same amount of visceral fat, the share rose faster in women than in men (Nielsen 2004).
• In the people who had the most, nearly half of the fatty acids reaching the liver came from it. The article gives this range, not an average value per group: the three buttons follow it.
• What share comes from visceral fat?