The effect of sugar on energy levels can be quite significant. Initially, consuming sugar leads to a quick spike in blood glucose, resulting in a brief boost of energy. However, this is often followed by a rapid crash, causing fatigue and feelings of lethargy. Frequent consumption of high-sugar foods may also lead to insulin resistance and other metabolic issues over time. To maintain stable energy levels, opting for foods with a low glycemic index, which release energy more gradually, can be beneficial.