The Spleen energy is Earth (soil) within 5 organs' energy (5 organs = Liver, Heart, Spleen, Lung and Kidney).
The energy is the middle, between Yin and Yang.
The organ which has the same Earth energy and works under the control of the Spleen is the Stomach.
So if the Spleen becomes weak, the Stomach also becomes weak or overworks, resulting in the imbalance of the Stomach energy.
The main job of the Spleen and the Stomach is digesting food, producing Ki and Blood and sending nutrition all over the body.

The Spleen and Stomach (SP and ST) control the function of digestion.
When the SP and ST become weak, we have no appetite and may have diarrhea or constipation.
Our sense of taste diminishes, food loses its delicious quality and we can't eat very much.
On the other hand, sometimes when SP becomes weak and ST is out of the control of the SP, we can eat unlimited amount of food because we digest the food while eating.

Also all forms of diarrhea and constipation are not caused by the weakness of SP and ST, so we need to distinguish.
Diarrhea caused by the weakness of SP and ST is a condition in which there is undigested food in the stool. In this case the person often feels better after having a bowel movement. This kind of diarrhea is mainly caused by food poisoning.

Constipation caused by the weakness of SP and ST results from indigestion, meaning the food and drink in the ST have fermented. The abdomen feels bloated and there is a lot of gas. When the abdomen feels bad because of the weakness of SP and ST, it is good to drink hot water with kudzu vine.