These 15 science-backed approaches to healthy eating have nothing to do with calories

Author Michael Pollan may have condensed the best nutrition wisdom into one line when he wrote: "Eat food, not too much, mostly plants."

Dozens of scientific studies have tied diets high in vegetables — especially greens — to better health outcomes, including weight loss and a decreased risk of a handful of chronic diseases.

Veggies like watercress, spinach, chives, and collard greens all rank highly on the Center for Disease Control and Prevention's list of "powerhouse foods." So find a few you like and start adding them to your plate.

But don't worry: Most research does not suggest a need to slash meat, dairy, or fish from your diet. In fact, the best results typically seem linked with diets that combine high amounts of vegetables with healthy sources of protein, which can include seafood, eggs, and meat. Eating plans like these include the popular Mediterranean diet and MIND diet.

Sweetened beverages like soda and juice can make up a surprising portion of the calories you consume each day, yet they don't fill you up the same way solid food does.

As part of an eight-year study that included nearly 50,000 women, Harvard researchers tracked what happened when people either slashed their intake of sweetened drinks or started consuming more of them. Not surprisingly, the participants who raised their sugary-drink intake gained weight and increased their risk of developing Type 2 diabetes. In fact, the more sweet drinks women consumed, the more weight they gained and the more their disease risk went up.

Those who curbed their intake, on the other hand, did not see those negative results.

So the next time you're looking for something other than water to drink, try seltzer or unsweetened tea. Even diet soda is probably a better choice. Every time you pick one of these over a sweetened beverage, you'll also be cutting anywhere from 150 to 400 calories.

Protein is a key ingredient that helps fuel our muscles and keep us feeling full. It also slows the breakdown of carbs into sugar, thereby acting as a sort of buffer against sharp dips and spikes in insulin levels. For these reasons, it's a good idea to make sure you're getting enough protein in every meal.

Many Americans whose diets are based around meat actually get too much protein. But there's some evidence that people who try to switch to a more plant-based diet can have a hard time getting enough.

To make sure your protein intake isn't slouching, add items like eggs, beans, tofu, lentils, fish, and dairy products to vegetable- and whole-grain-based meals.

If weight loss is part of your healthy-eating goals (though it certainly doesn't have to be!) you should know that exercise is not a shortcut to weight loss. This is true for two reasons: first, when we amp up our activity levels, our hunger levels tend to increase as well. Second, it's far easier to eat lots of calories in a single sitting than it is to burn them off in one gym session.

That said, regular movement of any kind is a key component of a healthy lifestyle — and it's especially important if you're looking to slim down and keep weight off for the long haul.

If you normally drive to work, try walking, biking, or taking public transit when possible. If you're used to taking the elevator, hit the stairs next time. And make regular gym sessions part of your routine — but keep in mind that your appetite may increase a bit.

Even if you're eating whole grains instead of refined ones, you should keep in mind that some researchers believe they all end up getting processed the same way. That means cutting back on any kind of carbohydrate is likely a smart move. Try swapping flour-based noodles with spiralized carrot or zucchini noodles, for example.

Several studies suggest that curbing your carb intake is an easy way to help stabilize blood-sugar levels as well. Having steady blood-sugar levels — also known as tight glycemic control — has been linked with beneficial health outcomes including weight loss, better energy levels throughout the day, and a reduced risk of chronic disease.

"Tight glycemic control is necessary to maintain health and to prevent disease," Ellen Blaak, a professor of fat metabolism and physiology at Maastricht University, wrote in a review of studies published in the journal Obesity Reviews. Her study found that poorly controlled blood-sugar levels were linked with higher likelihood of obesity, Type 2 diabetes, and heart disease.

Low-fat products sound great. Reduce your fat intake, get slim, right?

The majority of scientific research suggests it actually doesn't work this way.

One reason for this is that many products labeled "low fat," "light," or "reduced fat" (a common label on foods like yogurt, ice cream, and peanut butter) are highly processed and engineered to taste like their original full-fat predecessors. To accomplish this, food manufacturers typically add extra sugar — and sugar, unlike fat, has been strongly implicated as a leading factor contributing to obesity and weight gain.

One reason many dieters curb their fat intake — besides the lingering influence of the low-fat dieting trend of the 1990s — is that it's an easy way to cut calories. Fat is high in calories. Trim the fat, trim the calories.

But research is starting to show that eating fat does not necessarily lead us to put on pounds. Instead, it may help people lose weight, perhaps by making us feel full and curbing our sugar consumption. This appears to be especially true for fats from sources like nuts, olive oil, avocados, and fish.

"There is one thing we know about fats. Fat consumption does not cause weight gain. To the contrary, it might actually help us shed a few pounds," Aaron Carroll, a professor of pediatrics at the Indiana University School of Medicine, wrote in his book, "The Bad Food Bible: How and Why to Eat Sinfully."

Here's what that means for people who are counting calories: Fatty foods are higher in calories than their low-fat equivalents, so account for that and cut back on carbs and sugar instead.

For sustainable weight loss, dietitians, exercise scientists, and nutritionists all recommend aiming to lose only one to three pounds per week — at the most.

Slimming down slowly instead of all at once gives you enough time to create healthy new eating and exercise patterns that you can maintain for life, according to Andy Bellatti, a registered dietitian and the cofounder of Dietitians for Professional Integrity.

"You've got to give yourself two, three, four years of consistent behavioral changes. That is hard work. You're building new habits. And that takes time," Bellatti told Business Insider last year.

A growing body of evidence suggests that if there is a single villain in our diets when it comes to weight gain, it's sugar.

The authors of a review of 50 studies on diet and weight gain published in the journal Food and Nutrition Research found that, on average, the more refined carbohydrates (such as sugar) that someone ate, the more weight they tended to gain over the study period.

Similarly, the researchers behind a large review of 68 studies published in the British Medical Journal found that the more sugar someone consumed, the more they weighed.

So cut back on sweets and start paying attention to the sugar content on the labels of processed foods — especially in sauces, salad dressings, and dairy products.

One of the least healthy components of most American diets appears to be refined carbohydrates, a category that includes white bread and white rice. Refined carbs can also be found in lots of other processed foods — they appear on nutrition labels as "refined flour" or just "flour."

A 2012 study published in the journal Food and Nutrition Research found strong links between diets high in refined carbohydrates and weight gain. One reason for this may be that refined grains are processed quickly and turned into sugar in the body.

Whole grains, on the other hand, get digested slowly and fill you up for hours. The key difference is that whole grains still have their nutritious, fiber-rich outer shells, such as the germ and bran. Those parts get stripped off of refined carbs in a factory before you eat them.

Roxanne B. Sukol, medical director of the Cleveland Clinic's Wellness Enterprise, said people should think of refined carbohydrates simply as "stripped carbs" and avoid them whenever possible.

If you choose to incorporate regular workouts into your plan, research suggests that an early-morning workout on an empty stomach helps speed weight loss and boost energy levels by priming the body for an all-day fat burn.

Exercising first thing in the morning may push the body to tap into its fat reserves for fuel instead of simply "burning off" the most recent snack or meal.

Plus, working out early could mean you get more sunlight, which is key to properly setting your body's internal circadian rhythm. In one study, people who basked in bright sunlight within two hours after waking were thinner and better able to manage their weight than those who didn't get any natural light, regardless of what they ate throughout the day.

But the best fitness plan is one you can stick to consistently. If your morning motivation is too low to form a habit, working out after your workday is probably a better choice.

Unlike plain old fat, trans fat is created in an industrial process that adds hydrogen to liquid vegetable oils to make them more solid.

Trans fat has been strongly linked to heart disease, since consuming it appears to raise levels of so-called bad cholesterol and lower levels of good cholesterol.

The US Food and Drug Administration announced a plan to ban trans fats in 2015. According to a statement released by the FDA that year, "there is no safe level of consumption of artificial trans fat."

Although trans fats are now banned for most food in the US, they're still present in a number of processed foods in other countries, including many premade or packaged cakes, cookies, chips, and pastries. Some breads also contain them, along with some of the oils used to fry french fries and other fast foods. To identify trans fat on a nutrition label, look for "partially hydrogenated oils" on the ingredient list.

In May, the World Health Organization called on governments worldwide to ban industrially produced trans fats within five years.

If you tend to stick to a healthy eating plan most of the time but you're still having trouble losing weight, it might be worth thinking about the places or events that encourage you to veer away from nutritious choices.

Places like airports, drug stores, and even home-goods stores all sell food, but it's usually not very healthy. Instead of shopping until you feel famished then buying whatever unhealthy items are available near the checkout stand, plan ahead and pack a nutritious snack.

Sliced apples and peanut butter, carrots and hummus, or Greek yogurt and nuts are all inexpensive and convenient options.

If you're looking to lose weight and other diets have failed you, you might want to try an eating plan known as intermittent fasting — after checking with your doctor, of course.

There are several versions of this diet, but one of the most popular involves fasting for 16 hours and eating for 8. Most people opt for an eating window of 12 p.m. to 8 p.m , meaning that you essentially skip breakfast but eat whatever you want within the eight-hour "feeding" window.

Large studies have found intermittent fasting to be just as reliable for weight loss as traditional diets. And a few studies in animals suggest it could have other benefits too, such as reducing the risk for certain cancers and even prolonging life. But those studies need to be repeated in humans before any real conclusions can be drawn.

The baseline portion sizes of our snacks and meals have ballooned over the past 40 years. The average size of many of our foods — including fast food, sit-down meals, and even items from the grocery store — has grown by as much as 138% since the 1970s, according to data from the American Journal of Public Health, the Journal of Nutrition, and the Journal of the American Medical Association

Even the plates and cups we serve meals on have gotten noticeably bigger.

So be mindful of portion sizes. If you're eating out, consider taking anywhere from a third to half of your meal to go.