Alcohol can provide enough energy. Why isn't classified as a food then?

Because of its harmful effects. Although alcohol provides great amounts of energy, it provides no vitamins, minerals and fiber. Vitamins, minerals and fiber are essential to maintain normal health. So, a person who takes alcohol and skips meals becomes deficient in vitamins, minerals and fibers. This leads to many untoward effects on health.

The most common vitamin deficiency encountered in alcoholics is vitamin B deficiency. This vitamin is not stored in the body in sufficient quantities, and must be provided in daily diet. Alcoholics rapidly develop vitamin B deficiency, manifested as neurological and psychiatric problems.

All the vitamin deficiencies are very common in the alcoholics, particularly those who fail to take their normal diet regularly.