If you've noticed that, for some reason, you're no longer able to to crush an entire pizza and walk it off like it never happened, well, first, we offer our condolences.

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And second: no, it's not just you.

Melissa Halas-Liang, media representative for the California Academy of Nutrition and Dietetics, told ATTN: via email why this happens, and when you can start to expect it.

"Muscle mass and strength starts to decline at age 30 for men and women, with an increasing decline as we get older."

"So this is why it seems like we can't eat as much and have to work out harder," Halas-Liang explained. "Because in a way our metabolism isn't as good due to less muscle mass. Basil energy expenditure (the amount of energy you need to breathe and carry out basic functions) decreases by as much as 10 percent by age 50, and 20-25 percent by age 70."

And it only gets harder for women: "Women have further hormonal changes with menopause, which can make it even more challenging to maintain weight. Keep in mind people also tend to exercise less as they age, so it’s just not hormones. You've got to move a little more and eat a little less which each passing decade after 30."

Despite how much you work out, men still have an edge.

"Men naturally have more muscle mass than women, so they are more metabolically active. So it’s easier for men to lose weight as they naturally have a higher metabolism due to more muscle mass. As people age, we lose muscle mass, which is why weight resistance training is necessary. 

But all hope is not lost.

As  Halas-Liang  noted, "The best way to boost metabolism is through increasing muscle mass. It’s like creating a burning furnace for your body, which keeps on heating well after your work-out is over. "  

In addition to regular exercise that includes "muscle strength training exercises 2 or more days a week that work all major muscle groups," Halas-Liang also cautioned to "stay clear of the diet cycle" because "when you lose weight, it consists of both fat and muscle. However, when a crash diet ends and you gain weight back, you're probably gaining back mostly fat."

So just because things are changing doesn't mean that you're doomed. The bottom line? "You've got to move a little more and eat a little less which each passing decade after 30."