Why is the stomach flat when lying down but protrudes when standing up?

Generally, a flat abdomen when lying down that appears enlarged when standing up may be caused by abdominal fat distribution, abdominal muscle laxity, gastrointestinal gas, ascites, or abdominal wall hernia. It is recommended to seek timely medical attention, identify the underlying cause, and undergo symptomatic treatment under a doctor's guidance. Detailed explanations are as follows:

1. Abdominal Fat Distribution: Excess subcutaneous abdominal fat that is loosely distributed flattens when lying down due to gravity, but sags and accumulates when standing, causing abdominal enlargement. The abdomen feels soft upon touch, without other discomfort. Control intake of high-calorie foods, increase aerobic exercise, and incorporate strength training such as sit-ups and planks to reduce abdominal fat and improve abdominal appearance.

2. Abdominal Muscle Laxity: Long-term lack of exercise or aging weakens abdominal muscle strength and elasticity. When standing, the muscles cannot effectively tighten the abdomen, resulting in outward protrusion; lying down relaxes the muscles, making the abdomen appear flatter. Consistently perform core-strengthening exercises like crunches and glute bridges to enhance abdominal muscle strength. Abdominal binders can also be used to assist in tightening the abdomen and improving muscle laxity.

3. Gastrointestinal Gas: Accumulation of excessive gas in the gastrointestinal tract appears flat when lying down due to even gas distribution, but when standing, gravity causes gas to gather downward, leading to lower abdominal distension and bloating, accompanied by belching. Reduce intake of gas-producing foods such as beans and onions. Follow medical advice to use medications like domperidone tablets, lactase tablets, or simethicone emulsion to enhance gastrointestinal motility, expel gas, and relieve bloating.

4. Ascites: A small amount of fluid in the abdominal cavity distributes evenly when lying down, causing minimal abdominal change. However, when standing, gravity causes fluid to accumulate in the pelvic and lower abdominal regions, enlarging the abdomen and causing a feeling of abdominal heaviness. This is commonly seen in early stages of diseases such as cirrhosis and heart failure. Treat the underlying condition—for example, cirrhosis patients should follow medical advice to take spironolactone tablets, furosemide tablets, torasemide tablets, etc., to reduce fluid accumulation. Limit salt intake, and if fluid accumulation is significant, perform paracentesis for drainage.

5. Abdominal Wall Hernia: Weak spots in the abdominal wall allow herniated contents to retract into the abdominal cavity when lying down, making the abdomen appear flat. When standing, increased intra-abdominal pressure causes the hernia to protrude, enlarging the abdomen locally. A palpable lump may be felt, which can be pushed back in with slight pressure, accompanied by mild heaviness. Avoid strenuous exercise and heavy physical labor to minimize intra-abdominal pressure increases. For larger hernias, undergo abdominal hernia repair surgery to reinforce weak areas through suturing or mesh implantation, preventing hernia protrusion.

Maintain regular eating habits in daily life, avoid overeating, chew food slowly to reduce swallowed air that causes bloating. Develop regular bowel habits to prevent constipation-induced increases in intra-abdominal pressure. Maintain proper posture while standing and sitting, keeping the chest up and abdomen in, to reduce the effects of abdominal fat accumulation and muscle laxity. Long-term adherence can improve noticeable abdominal size changes.