Possible Clues:

• Bagel or baked potato, to a dieter
• Concern for some losers
• Diet no-no,for some
• Dieter's no-no, briefly
• Prepare for a marathon, with 'up'
• ___-loading (pre-marathon diet regimen)
• Low- -- meal
• Keto no-no
• Bit of pasta, informally
• Runner's fuel