1. Poop Is About 80% Water, So What Makes Up the Other 20%?
According to sources, about 80% of the components of poop are water [1]. When a person is constipated, this drops to around 70%, and during diarrhea, it can exceed 90%. Because the percentage of water varies by source, "about 80%" is a rough guide. The remaining solid matter, which is about 20%, is made of food residue, shed intestinal lining, and gut bacteria [1]. The intestinal lining is a thin membrane covering the inside of the gut, and parts of it peel off and mix into the poop. Since the ratios of these three parts differ across sources, this article does not fix specific numbers for them. This raises a question: how much space do bacteria take up within the solid part?
2. How Much of the Solid Part of Poop Is Bacteria?
A study published in the science journal *Nature* in 1980 reported that bacteria make up the major part of the solid matter in human poop [2]. For people eating a Western-style diet, one compilation states that about 55% of the dry weight is bacteria [3]. Other reports give ranges of 25–54% or 44%, showing variation between studies [11]. This percentage refers to dry weight after removing water. It does not mean that half of the total wet poop is bacteria. Both living and dead bacteria are included. Regarding numbers, the Health and Longevity Net introduces the idea that there are about 1 trillion gut bacteria in 1 gram of poop [1]. On the other hand, the BioNumbers compilation lists a range of 1011 to 1012 per gram, which is 100 billion to 1 trillion [3]. Since "1 trillion" is at the upper end of the range, it is safer to read these numbers as a range. With so many bacteria, are they just flowing through? Let us look at the color of poop for a clue.
3. Where in the Digestive Tract Does Liquid Food Turn into Solid Poop?
According to the National Cancer Center, the total water entering the digestive tract from food and digestive juices is about 9 liters per day [4]. Of this, about 8 liters are absorbed in the small intestine. The remaining about 1 liter that flows into the large intestine is also mostly absorbed. Very little water remains in the poop. Because sources vary on the volume of digestive juices (3–6 liters), these numbers are rough guides. The daily amount of poop for an adult is estimated at about 100–250 g, with an average of about 150 g for groups eating high-fiber diets [10]. Even though about 9 liters of water enter, the total poop weighs this little, so you can imagine that most of the water is returned to the body. The time it takes for food to become poop varies. Some sources say 24–48 hours, while others say 1–3 days, with large individual differences [4]. The longer it stays in the large intestine, the more water is absorbed, and the poop becomes harder [5]. This explains why water decreases during constipation. If shape is determined by water amount, is the brown color related to water?
4. Why Poop Is Brown: Bile Pigments and Gut Bacteria
Bile contains a pigment called bilirubin. According to encyclopedic explanations, bilirubin changes into other substances through the action of gut bacteria. One of these, a brown pigment called stercobilin, gives poop its color [6]. This means the brown color is not from the food's color but is the result of a joint effort between pigments made by the body and bacteria. The details of the enzymes and bacteria types involved in this conversion are still being researched. Conversely, in diseases like obstructive jaundice where bile does not reach the intestine, bilirubin does not arrive, and poop may become a pale, clay-like color [7]. If you notice a different color persisting, it is recommended to consult a medical institution. Next, let us look at smell. Is that strong odor also the work of bacteria?
5. Do the Smell and Amount of Poop Change with What You Eat?
One source of odor is indole and skatole, which are produced when gut bacteria break down the amino acid tryptophan [8]. It is known that skatole increases when more protein is consumed (mostly reported in animal experiments). However, these are not the only causes of smell. Dietary fiber is not digested in the small intestine and reaches the large intestine. The Ministry of Health, Labour and Welfare’s e-Health Net explains that insoluble fiber, which does not dissolve in water, increases stool bulk and helps intestinal movement, while soluble fiber, which dissolves, helps regulate the gut environment [9]. The target amount for adults is set by the Dietary Reference Intakes (2025 edition) based on age. For example, women aged 18–64 need at least 18 g per day, and men aged 30–64 need at least 22 g. A 1980 study showed that different fibers work differently [2]. Cabbage fiber is almost fully broken down by gut bacteria, helping them grow. Wheat bran fiber is hard to break down, holds water, and swells the poop. These foods may influence the high number of bacteria seen earlier. How can we check this in daily life?
6. How to Check Fiber Amounts on Food Labels
Nutrition labels on the back of bread, cereal, and vegetable packages often list the amount of dietary fiber. Read the grams and compare two foods. The adult targets are at least 20 g for men and 18 g for women. Here, we read numbers just to compare. Not all foods have labels, so it is fine to only look at those that do. You can find the abstract of the 1980 *Nature* study and BioNumbers figures in the source list. It is also good to think about why the bacteria percentage moves between 25–54% in different studies. If the color or shape of your poop is different from usual for a long time, do not decide on your own. Tell an adult and consult a medical institution.