In adults with severe obesity, a 150-day protein-sparing modified fast diet delivered via nasogastric tube is associated with higher HbA1c levels (6.2% vs. 5.4%) compared to oral delivery, despite lower fasting insulin levels.

When protein is delivered continuously through a tube into the intestine, it keeps amino acids high in the blood, which tells muscles to keep building protein without needing insulin. This same process also tells the liver to make less sugar, so blood sugar stays lower after meals. But because the liver keeps making some sugar and the muscles are using less of it due to reduced insulin signaling, sugar builds up slowly in the blood over weeks, raising the long-term marker HbA1c even though insulin levels are low.