A diet consistently high in sugar and other refined sweets will first lead to hypertrophy of the beta cells ( to increase insulin production) and eventual burnout, with diabetes being a possible result.
In adult onset diabetes a fast for 4-7 days followed by the before mentioned diet will often partially or totally bring down elevated blood sugar. This should be done only under a doctor’s supervision.
Which basically says if you have a heart condition and people pray for you and you know about that; then you are more likely to suffer further complications than if they don't or you don't know about it...
Well... stress will result in increased cortisol levels, and thus blood glucose, so if you find prayer relaxing it might actually help. The author, presumably, assumed that everyone found prayer supremely relaxing.