There are six basic categories of natural whole foods: vegetables (including legumes); fruits; nuts, seeds, and healthy oils; unprocessed meat and seafood; whole grains; and dairy. The overwhelming majority of elite endurance athletes regularly consume all six of these “high-quality” food types. The reason they do so is that a balanced, varied, and inclusive diet is needed to supply the body with everything it needs nutritionally to handle the stress of hard training and to derive the maximum benefit from workouts.
In addition to the six high-quality food types, there are four “low-quality” food types: refined grains, sweets, processed meats, and fried foods. Most elite endurance athletes allow themselves to eat small amounts of each of these food types. Indulging in a treat here and there does them no harm and is even beneficial in the sense that it makes their overall diet more enjoyable and sustainable.