Re: Raw potatoes: Do they provide energy when eaten?

Hi Jeff,

You've got it half right. There are several types of carboydrates in the plants we eat, but for the purpose of answering your question, we can just focus on the starch and the cellulose components. This answer applies to most vegetables, but I'll just talk about potatoes for simplicity.

Now, when we eat a potato, we want to digest the starches. These are basically glucose polymers that branch and coil, and which are used by the plant as a sort of long term energy storage. Our digestive enzymes (amylases) convert starches back to glucose for nutrition. We also eat vegetables for the fiber. Fiber is basically cellulose, which we cannot digest (we don't have the proper enzymes). Like starches, cellulose is a polymer of glucose, but it is very linear and instead of branching or coiling, cellulose polymers make hydrogen bonds and form very strong fibers that form the walls of plant cells and give plants their structure.

Now when we cook a potato, the goal is to break those hydrogen bonds between the cellulose polymers so that the cell walls can be broken open, allowing our digestive enzymes have access to the starch molecules. Thats why a raw potato (or any raw vegetable really) is hard -- strong cellulose fibers. If you were to eat a raw potato, the cellulose fibers in the cell walls would be intact, and the starches would be inaccessible to your amylase enzymes. This is the reason why many raw vegetables that we do eat (like carrots for example) pass through the digestive tract undigested and in pieces the same size that they were when we finished chewing them.

So, part of your understanding is correct. The cellulose carbohydrates in a raw potato are indigestible and protect the starches contained in the cells from being digested. That being said, I'm not sure about the energy being expended in the digestion of starches, but I do know that no energy is expended in trying to digest cellulose. The mechanical breaking open of cells when you chew your raw potato does likely free some amylase, but you would really have to chew that raw potato into a paste to try to get the same effect that you would from cooking it. That doesn't sound like fun to me.

A simple experiment might be to chew a piece of raw potato into a paste (if you don't want to chew a chunk of raw potato, I suppose you could run it through a food mill or food processor) and then hold it in your mouth for a while. If you notice a sweet taste, then that means that the amylase in your saliva is breaking the starches into glucose. You might want to compare that to similarly sized potato chunks that were cooked for longer periods of time.

I hope you find this answer useful.