Acidic foods like meat, wheat, and dairy are no-nos on the trendy alkaline diet, which claims to even pH balance for optimum health

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Elle Macpherson has said she checks her urine's pH balance with a tester she keeps in her purse, and Kelly Ripa recently gushed about the alkaline diet cleanse that "changed (her) life." But what is an "alkaline diet," and should you be on one?

First, a brief chemistry lesson: pH balance is a measure of acidity. Anything below a pH of seven is considered "acidic", and anything above seven is "alkaline" or base. Water, for example, has a pH of seven and is neither acidic nor alkaline. To sustain human life, your blood needs to remain in a slightly alkaline state, research shows.

Proponents of alkaline diets say the stuff you eat can lower your body's acid levels, which in turn could help or hurt your health. "The thought is that some foods-like meat, wheat, refined sugar, and some processed foods-cause your body to over-produce acid, which can supposedly lead to health implications such as osteoporosis or other chronic conditions," says Joy Dubost, Ph.D., R.D., a food scientist and nutrition expert. Some also claim alkaline diets combat cancer. (And that's not something to laugh at! Check out these Scary Medical Diagnoses Young Women Don't Expect.)

But there's no solid evidence to back up those claims, Dubost says.

While it's true that the modern, meat-heavy American diet contains unhealthy foods with a high "acid load," that doesn't have much of an impact on your body's pH levels, adds Allison Childress, R.D., a nutrition sciences instructor at Texas Tech University.

"All food is acidic in the stomach and alkaline in the intestine," Childress explains. And while your urine's pH levels can vary, Childress says it's not clear how much your diet has to do with that.

Even if what you eat does change your urine's acid levels, "your diet does not affect your blood pH at all," Childress says. Both Dubost and national health authorities agree with her. "Altering the cell environment of the human body to create a less-acidic, less-cancer-friendly environment is virtually impossible," according to resources from the American Institute for Cancer Research. Research on avoiding dietary acid for healthier bones has also failed to turn up proof of pH-related benefits.

So long story short, claims about alkaline diets changing your body's pH levels are likely bogus, and at best unsubstantiated.

But-and this is a big but-alkaline diets may still be good for you.

"An alkaline diet can be very healthful as it contains lots of fruits, nuts, legumes, and veggies," Childress says. Dubost backs her up, and adds, "Every diet should have these components, even though they will not directly affect the body's pH level."

Like lots of other fad diets, alkaline programs get you to make healthy changes by feeding you spurious justifications. If you're eating tons of meat, processed foods, and refined grains, ditching those in favor of more fruits and vegetables is beneficial in all sorts of ways. It just has nothing to do with changing your body's pH levels, Childress says.

Her only reservation: Meats, eggs, grains, and other foods on the alkaline diet's no-list contain amino acids, essential vitamins, and other stuff your body needs. If you adopt a hard-core alkaline diet, you may end up hurting your health by depriving your body of these nutrients, Childress says.

Like vegans and others who remove whole food groups from their diets, those who go all-out when it comes to alkaline diets need to make sure they're getting plenty of protein, iron, and other necessary nutrients from other foods, Childress says. Luckily, there's no urine testing required. (Speaking of pee, however, rumor has it that Urine Could Be the Solution to Bad Skin Conditions.)