During a recent survey about dietary habits, it was found that individuals who consume smoothies daily tend to report feeling more energetic throughout the day. Researchers have concluded that drinking smoothies causes an increase in energy levels. However, it is important to note that those who consume smoothies daily might already lead a healthier lifestyle, which includes regular exercise and balanced diets.
What can be inferred to best justify the researchers' conclusion that drinking smoothies causes an increase in energy levels?