Did they say:

  reduce their daily calorie intake to 1,500 - 1,800 calories
  or
  reduce their daily calorie intake by 1,500 - 1,800 calories

These are very different answers, unless you’re consuming ~3,300 calories per day. These kinds of ‘subtle’ phrasing issue often results in AI mistake as both words are commonly used in advice but the context is really important.