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.
Use either SkyMiles or a credit card to purchase an upgrade on the day of your travel if seats in the other cabin are available. Alternatively, if you have SkyMiles elite status, Delta automatically places you on a list for available upgrades the day of a flight and notifies you in the event that you receive one.