It's no secret that "McDonald's" and "healthy" aren't exactly synonymous. But if you find yourself at the fast-food chain, it's important to know that not all menu items are created equal. Although you won't find a healthy, nutritious meal or snack at McDonald's, registered dietitians and nutritionists agree that there's one food to avoid at all costs: the McFlurry. And between the McFlurry options, the M&M flavor is the worst offender.According to the nutrition information from the McDonald's website, a regular-sized M&M McFlurry clocks in at 640 calories. It also contains 21 grams of fat, which is 31% of the recommended daily value. And the amount of saturated and trans fat in the McFlurry takes up a whopping 67% of the recommended daily value.For more, don't miss these 15 Classic American Desserts That Are No Longer Made.The M&M McFlurry is high in sugarThe M&M McFlurry also contains a staggering amount of sugar. Bansari Acharya, MA, RDN, of FoodLove.com, explains that combining M&M's with a soft-serve vanilla base from McDonald's creates a serving of a sweet treat that contains more than 80 grams of sugar. "Thatalmost doubles the daily recommended amount of added sugars recommended by the FDA by just one dessert, as they recommend no more than 50 grams of added sugars a day," says Acharya.RELATED: Sign up for our newsletter to get daily recipes and food news in your inbox!The McFlurry is full of empty caloriesIt's not just the calorie count that makes the M&M McFlurry an order to avoid. As Acharya points out, it doesn't even give you the satiety of a full meal.On a similar note, Megan Wong, an RD working with AlgaeCal, says that by consuming an M&M McFlurry, "the average woman is using up around 30% of her day's [recommended] calorie intake." For context, Wong notes that the McFlurry contains more calories than you'd find in a Quarter Pounder with both cheese and bacon.RELATED: Your ultimate restaurant and supermarket survival guide is here!In addition to the McFlurry's calorie count and high amounts of sugar, fat, and saturated fat, Vanessa Rissetto, RD, CDN, nutritionist and co-founder of Culina Health, cited the ingredient carrageenan as yet another reason to avoid the shake at all costs. "Carrageenan is used for thickening and stabilizing but also has some negative effects on the body," says Rissetto, who also named the McFlurry as the worst food to order at McDonald's. "Food-grade carrageenan is extracted from red seaweed and processed with alkaline substances." She explains that sometimes with processing, carrageenan can release an inflammatory substance that can carry significant health warnings."[The McFlurry] basically has nothing redeemable," says Rissetto. Case closed.And for more, don't miss these 52 Life-Changing Kitchen Hacks That'll Make You Enjoy Cooking Again.