A mother of four and passionate weightlifter has shared an amazing photo of her physical transformation on social media - revealing that she has only lost one kilo (2 pounds) even though her body looks radically different.

Adrienne Osuna, 31, who blogs about her healthy habits, went through years of yo-yo dieting from middle school to adulthood, losing weight before gaining it again and struggling with binging.

But after four pregnancies, during which she managed to run two half-marathons, the Santa Maria, California mum quit dieting four good and started lifting heavy weights - something she says has transformed her metabolism, her shape and her approach to health and fitness.

The photo of Adrienne's progress, which shows her posing in a pink tank top at a size 16 and at a size 10, first appeared on her Instagram account seven weeks ago and resurfaced Tuesday on social media.

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Many praised the married mum for showing that the number on the scale does not adequately reflect fitness.

Muscle is denser than fat, which means a pound of muscle takes up less room than a pound of body fat. This is why the mother of four boys can look so different in each photo and yet weigh the same.

One woman said in a comment on Adrienne's Instagram that the photo had helped her not to feel discouraged on her own fitness journey.

"Seeing how numbers don't show progress but your body instead, has me motivated now more than ever!" she wrote.

"I rely so heavily on the scale numbers, but can now see it's all in hard work!"

Other comments, however, weren't as positive.

"When I posted this picture last time the amount of comments I got calling me a liar, saying it was photoshopped, or that it's two separate people, was crazy and made me not want to post another like it!" Adrienne said on Instagram next to the transformation snap.

But she received a string of direct messages from other women telling her the photo had helped them stop obsessing over the scale and had empowered them to continue on their fitness journey at times when they were tempted to give up.

These messages, Adrienne said, convinced her to keep sharing pictures of her transformation.

The mother, who is also a photographer and wants to become a nurse, now lifts weight four times a week, with two sessions focused on her upper body and two dedicated to her legs.

On top of that, she goes two or three cardio workouts per week and practices yoga.

Adrienne has urged other women not to be intimidated by the weight room and to try out lifting.

"Will you be the only girl in there? Maybe. I often am. Does it matter? Nope," she wrote on her blog. "Everyone in there is there for the same reason. No one is better than anyone else, and everyone is willing to help."

She also told aspiring lifters not to be discouraged if they can't keep up with their fitness routines perfectly - and, of course, if the scale is not budging.

"It's not all or nothing. If you miss one day, go the next. If your diet isn't perfect that's okay, keep going," she wrote.

"Don't obsess over the scale. You are focusing on your body building muscle and losing fat. The scale may not change!"