The promise of losing five pounds in a week for a Valentine's Day event belongs to a genre of fitness advice that is popular for its specificity and deadline, and that health professionals almost universally advise against. Understanding why requires understanding what a week-long "weight loss" actually consists of.

Five pounds in seven days is physiologically achievable, but almost none of it will be fat. Fat tissue requires a caloric deficit of approximately 3,500 calories per pound — so losing five actual pounds of fat in a week would require a deficit of 17,500 calories, which is biologically impossible without severe starvation.

What crash dieting and aggressive water restriction achieve is the loss of glycogen stores (each gram of glycogen stores approximately three grams of water), digestive contents, and water weight generally. This shows up on the scale convincingly, can make clothes fit differently, and reverses almost entirely within a few days of returning to normal eating and drinking.

More concerning are the downstream effects: metabolic adaptation to dramatic restriction, the hunger and cognitive impairment that accompany severe deficit, the statistical likelihood of rebounding past the starting weight.

The practical advice for looking and feeling your best before a specific event is more mundane: reduce sodium (which reduces water retention), get adequate sleep (which affects face appearance significantly), increase vegetable intake, drink plenty of water, and wear clothes that fit well. None of this generates five pounds of scale movement.

The five-pound-in-a-week framing appeals to the human desire for a quantified result with a deadline. The body, unfortunately, runs on different timescales than Valentine's Day.