1 Leptin is a powerful and influential hormone produced by your fat cells. 2  It plays a key role in regulating energy intake and energy expenditure.
(Upgrades are not available on fares in the L, T, and U range, however, which are the most deeply discounted.) To calculate the price of an upgrade, Delta charges a dollar amount per 500 miles. $50: Up to 500 miles. $75: 501-1000 miles. $100: 1001-1500 miles.
For travel outside the United States, Canada, Puerto Rico, and the U.S. Virgin Islands, the change fee for Delta-marketed flights is typically $200 to $500, but can vary based on location and type of fare. Changes are usually permitted only to the return portion of an international itinerary.