From https://highline.huffingtonpost.com/articles/en/everything-you-know-about-obesity-is-wrong/ :

Since 1959, research has shown that 95 to 98 percent of attempts to lose weight fail and that two-thirds of dieters gain back more than they lost. The reasons are biological and irreversible. As early as 1969, research showed that losing just 3 percent of your body weight resulted in a 17 percent slowdown in your metabolism—a body-wide starvation response that blasts you with hunger hormones and drops your internal temperature until you rise back to your highest weight. Keeping weight off means fighting your body’s energy-regulation system and battling hunger all day, every day, for the rest of your life.

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A 2016 study that followed participants for an average of 19 years found that unfit skinny people were twice as likely to get diabetes as fit fat people. Habits, no matter your size, are what really matter. Dozens of indicators, from vegetable consumption to regular exercise to grip strength, provide a better snapshot of someone’s health than looking at her from across a room.

The responses you get here will be subject to survivorship bias,

a form of sampling bias that can lead to overly optimistic beliefs because multiple failures are overlooked, ... [It can] lead to the false belief that the successes in a group have some special property, rather than just coincidence