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.
Upgrading to business on a Delta flight from the United States to Europe, for example, will cost you 15,000 to 25,000 miles on top of your economy fareâoften with no guaranteed seat; the lowest-tier business award seats go for just 62,500 miles.