Why can humans not eat wood?

Wood, as in lumber is not easily consumed, and the human body does not digest cellulose (the main constituent of wood) easily. However, there are several products made from trees (the source of wood) that people do consume. Among these are:

Gum extracts

Cinnamon

Maple syrup

Cellulose

• food thickener
• milk shakes
• ice cream
• cake frosting
• pancake syrup
• cough syrup
• anti-caking ingredient in grated Parmesan cheese
• toothpaste
• some products made with flour such as bread and pastries