Possible Clues:

• Low-diet filler?
• ___ up (have a lot of pasta, say)
• Pasta, e.g., to a marathoner
• Potatoes, to a marathoner
• Atkins dieter's bane, briefly
• Keto diet no-no
• Atkins taboo
• Pasta nutrient, briefly
• Paleo diet restriction, informally
• Fuel server, for short