Sipping down spinach and kale is great and all, but there are some things to consider before swapping full meals for bottled juices and smoothies

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Somewhere along the way, health food took liquid form and anything green was stamped with a seal of approval. Smoothie and juice bars became like Starbucks-popping up on every corner, our go-to home come 3 p.m. But while a fresh-pressed green juice is an easy go-to for a quick (and of course, healthy) meal replacement, can you overdo it?

To answer that, you first have to know what you're sipping-and that there is a difference between a juice and a smoothie: A smoothie uses the entire fruit or vegetable, whereas juicing extracts and separates the pulp and fiber from the juice, explains Angela Onsgard, R.D., dietitian a Miraval Resort & Spa in Tucson, AZ. Smoothies have two key advantages: Fiber keeps you full for longer, plus you can blend satiating ingredients you can't necessarily juice like avocados and peanut butter. This means a smoothie makes a much more substantial and filling meal than a juice. However, a juice requires a lot more produce to yield an 8-ounce glass, so you're packing in more nutrients than in a smoothie. (Are Green Juices as Healthy as Green Powders?)

The Sugar Trap

The biggest problem with both smoothies and juices? They tastiest ones are loaded with sugary fruits. And how healthy the juice or smoothie is depends entirely on what went into the bottle. "I have seen up to 90 grams of sugar in one drink, all from 'natural' foods," says Manuel Villacorta, R.D., author of Whole Body Reboot and the Peruvian Superfood Diet. The problem with these sky-high levels? "Without a doubt, this amount of sugar will shoot up your insulin," he adds.

When you eat sugar (or any carbohydrate), your pancreas releases the hormone insulin to help transport the fuel into your tissues to use for energy later. "One of the biggest complaints I hear about juicing or drinking a smoothie first thing in the morning is that someone's energy levels crash and they feel hungry shortly afterwards," says Onsgard. When you eat a lot of sugar at once, your blood sugar spikes. But what goes up must come down, so the initial rush of the sweet stuff leaves you feeling hungry, fatigued, irritable, and craving more carbohydrates to balance your levels back out, she explains.

A Naked Green Machine, for example, contains 28 grams of sugar per 8-ounce serving (with two servings per bottle), whereas a Green Guardian smoothie from Juice Generation contains only 11 grams per 20-ounce serving. That's a massive difference in how your blood sugar levels will be affected.

This isn't necessarily comparing apples to apples, though, because while each of these drinks are both part of each company's "green" line, Naked's concoction has five kinds of fruit compared to Juice Generation's one. The key is really in the ingredients: Tropical fruits like mango and papaya contain higher levels of sugar, and, according to Onsgard, tend to have a higher glycemic load (a measurement of how a food will affect your blood sugar levels). Even certain vegetables, namely ones from the root family like carrots and beets, have high enough sugar counts to send your levels soaring.

A Better Recipe

Luckily, adding greens like kale, spinach, and cucumber to a fruit smoothie or juice not only adds flavor and contributes to your daily serving of veggies, it can help balance the sugar content too. A veggie-dominant smoothie also packs a heavy nutritional punch, since leafy greens are high in essential vitamins like Vitamin K (crucial for building strong bones and helping to prevent heart disease) and Vitamin A (important for vision and immune health). Those sugary fruits and vegetables we mentioned before, though, do contain powerful nutrients, so don't skip them all together-just don't overload on the sweet stuff, and balance it with greens. (Don't miss these 14 Super Smoothie Boosters.)

You can also avoid the crash by opting for low-sugar fruits like blueberries, strawberries, or apples, and adding a protein source like Greek yogurt or soft tofu, a healthy fat like cashew butter, or a fiber-rich carbohydrate like steel cut oats, recommends Onsgard.

Also important to remember: A healthy smoothie isn't just limited to being green. "The minute someone starts thinking about juicing, they think green. Green is great! But it limits the person from an array of other health benefits," says Villacorta. Fruits and veggies range every color of the rainbow, and they get their color from phytonutrients. By sticking to green, you're missing out on the benefits of, say, anthocyanin-a phytonutrient found in blue and purple foods like blueberries and eggplant, he says. Not to mention beta-carotene, a red-orange pigment found in colorful veggies like carrots.

Balance Your Diet

We all know about juice cleanses. And while they get a bad rap, a few days of juicing for weight loss isn't necessarily unhealthy, VIllacorta says. Just make sure you're choosing low-sugar, high-vegetable drinks and only juicing straight for three days max since there is no such thing as a well-balanced juice when it comes to protein and fat, he adds. As for everyday life, substituting a smoothie for a meal and a juice for a snack is totally okay, both nutritionists agree. But don't go too overboard-you need to be eating solid meals too, Villacorta advises. If you want to have a balanced drink for breakfast, then the rest of your food that day should be real, nutritious grub, like lean protein, whole grains, and fats.

And your smoothie needs to be enough calories to consider it a meal (or few enough calories for it to qualify as a snack!), Onsgard says. Her foolproof formula? One cup fruit and 1/2 to 2 cups leafy greens, depending on taste preference. And when you are going green, branch out from just spinach to score more nutrients (Onsgard likes kale, chard, or mint). We like these 10 Super Greens to Add to Smoothies and Juices. She also recommends adding 1/4 cup cooked grain or 2 tablespoons steel cut oats to help stimulate the production of leptin (what makes you feel full) and to ensure your meal replacement has enough protein, fat, and fiber. Since juices are typically fewer calories and don't have enough fiber to keep you full, stick to these as snacks to hold you over in between meals.

No time to stop at the grocery store to snag the right ingredients to blend at home? No problem. When opting for store-bought juices, look for organic and a higher veggie content (Onsgard recommends Mother Earth by Juice Press, Green Juice by Blue Print, and Green Love by Organic Avenue). When looking for smoothies on-the-go, ask for ones with whole ingredients and avoid those that contain juices (opt for actual apples instead of apple juice) or added sweeteners.