Diet and Weight Loss: The Best Ways to Eat

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Low carb, or low fat? Should you go Atkins, Zone or Paleo? Or does it even matter which diet you choose when you want to lose weight? Most weight loss experts say that shedding pounds comes down to a simple formula: calories in versus calories out. In other words, if you burn more calories than you take in, you'll lose weight.

However, the question of exactly how to cut calories — in a healthy, sustainable way — has often perplexed dieters. To find the best diets for weight loss, Live Science conducted a months-long search for information. We spoke with many weight loss experts and dove deep into the most well-regarded studies on the topic done to date. We wanted to know what these studies found and, ultimately, determine the best approaches to healthy eating for weight loss.

We found that the calorie equation reigns supreme as the most important aspect of losing weight, but also that there's still plenty of room to choose a diet that fits your personal preferences. For example, Dr. Frank Hu, a professor of nutrition and epidemiology at the Harvard University T.H. Chan School of Public Health, told Live Science that "the No. 1 factor is still a calorie deficit, so the question is, what kind of styles or what kind of foods can help people achieve a calorie deficit, and what can sustain the calorie deficit?" [What Are Calories?]

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"The best food someone on a diet should eat? The same foods they should eat when they're not on a diet, but just less of them," said Dr. Frank Sacks, a professor of cardiovascular disease prevention, also at Harvard's School of Public Health. All of the experts we spoke with agreed that those foods should include the staples of a "healthy" diet — fruits, vegetables, whole grains and healthy fats. These foods are important not only for achieving or maintaining a healthy body weight but also for good health in general.

In this article, we'll highlight some of the most popular diets people turn to in order to lose weight, and explain what the science really says about how well they work. But before we delve into the diets, it's important to break down the macronutrients of the foods we eat — carbohydrates, fats and protein — and the roles they play in the body. Of course, individual food can contain more than one more macronutrient.

Sugars (found in fruit, vegetables and dairy) and starches (found in grains, vegetables and beans) ultimately suffer the same fate: They're broken down into glucose and are used by the body for energy. But your body can use only so much energy at once, so not all of the glucose you eat is immediately used for fuel. Some of the extra glucose can be stored in your liver or muscles and be used later.

The difference between sugars and starches — which are sometimes referred to as "simple" and "complex" carbohydrates, respectively — is the complexity of their structure. Sugarscontain only one or two molecules, so it's very easy for the body to digest sugars and absorb them into the blood. Starches, in contrast, contain many simple carbohydrate molecules linked together. Because of their size and complexity, starches take longer to be digested into single molecules.

Fiber is an entirely different ball game. Fiber is found alongside sugars and starches in fruits, vegetables, whole grains and legumes. Like starch, it's a complex carbohydrate, meaning it contains many carbohydrate molecules linked together. However, your body can't digest fiber, so this carbohydrate doesn't get absorbed by the gut. It never enters the bloodstream, and it is never broken down for energy. The stuff passes through your body relatively untouched.

The benefit of eating starches, and especially fiber, rather than sugars is that they can help you feel fuller for a longer time because they are either broken down slowly or aren't broken down at all. And the idea is, if you're feeling full, you'll eat less. Of course, keep in mind that complex carbs aren't a free pass to eat as much as you like — extra calories consumed will still be stored as fat.

Protein-rich foods are important for everyone, not just bodybuilders. Protein serves as the building blocks for our bodies, from the tiniest structures inside our cells, to the largest parts of our anatomy. Unlike carbs or fat, excess protein isn't readily stored by the body, so it's essential to eat enough of this macronutrient every day. Of course, just because your body doesn't store the protein doesn't mean you have free rein to eat as much as you'd like without gaining weight. The body can convert excess protein into glucose, or store it as fat. [What is Protein?]

When you eat protein, the large protein molecules are broken down into their basic components, which are known as amino acids. There are 20 amino acids that are important in the body, and some can be converted from one type of amino acid to another as needed. However, there are also several amino acids that cannot be produced in the body by converting other amino acids, meaning you must get these amino acids from your diet. These are known as essential amino acids.

Both animal-based proteins (such as those found in meat, fish, dairy and eggs) and plant-based proteins (found in soy, legumes, nuts and some grains) contain essential amino acids. However, whereas all animal-based proteins contain all of the essential amino acids your body needs, plant-based proteins generally contain a smaller set of amino acids. That means that if you eat a vegan diet, you need to eat a variety of plant types to get all of your essential amino acids.

Enter clinical trials, which allow researchers to directly compare the effects of different diets. And even among these trials, some are better designed than others. Researchers consider many factors when looking at the quality of a trial, including the size of the trial (the more participants, the better) and the length of the study period (the longer, the better), when deciding how much stock to put into the results.

The largest clinical trial that compared different diets was the Women's Health Initiative (WHI) Dietary Modification Trial, which included more than 48,000 postmenopausal women and had a follow-up period of seven years. However, the trial was not designed to look at weight loss. Rather, the goal of the trial was to see how fat in the diet affected the women's risk of cancer and heart disease. (Because the trial was only in women, it's unclear if the results also apply to men.)

The WHI Dietary Modification Trial was designed in 1990, when researchers were asking many questions about whether high-fat diets or low-fat diets were better for people's health, said Barbara Howard, a senior scientist at the Georgetown University Medical Center in Washington D.C. In a study published in the Journal of the American Medical Association (JAMA) in 2006, Howard took data from the original WHI Dietary Modification Trial and looked to see whether the two diets had different effects on weight loss.

In the WHI, 40 percent of the participants were encouraged to reduce their fat intake so that only 20 percent of their total daily calories came from fat. (At the study's start, all of the participants had reported that at least 32 percent of their daily calories had come from fat.) The women were encouraged to increase their daily servings of fruits, vegetables and whole grains, and attended group sessions where they got advice about cutting fat. However, because the trial wasn't designed with weight loss in mind, these women were not encouraged to cut calories.

The researchers concluded that there was a "clear relationship" between the change in the women's fat intake and their weight, they wrote in their study. The questionnaires showed that the women in the low-fat-diet group increased their daily intake of fiber, fruits, vegetables and whole grains, and decreased their daily intake of total fat, saturated fat and unsaturated fat. [Which States Are Eating Their Fruits and Veggies?]

But although that large, high-quality study found a relationship between a low-fat diet and weight loss, other studies conducted since have found that a low-fat diet is no more effective than other types of diets in helping people lose weight. It's important to note that the women in the study who switched to a low-fat diet didn't replace the fat in their diet with white bread and other refined carbohydrates, Howard said. In other observational studies, researchers have shown that when "high-carb" means sugar and refined carbohydrates, people don't lose weight, she noted.

The trial included 322 people, ages 40 to 65, all of whom were overweight and worked at the same research center. The participants were randomly assigned to different diets — a low-fat diet, a Mediterranean diet or a low-carb diet — for a two-year period. The researchers also conducted a follow-up four years after that, so the total study period was six years. The participants in the low-fat diet group and the Mediterranean-diet group were instructed to cut their calories, while those on the low-carb diet were given no calorie restrictions.

As in the WHI study, the participants received diet guidance in small group sessions. (One key difference between the two studies, however, was that in the DIRECT study, everyone was assigned to a group session.) In addition, because all of the participants worked at the same place, the researchers provided lunch each day. Each diet group was given a lunch that fit their diet, according to the report of the study, published in 2008 in The New England Journal of Medicine.

But unlike in the WHI study, the low-fat diet in the DIRECT study didn't result in the most weight loss. Rather, weight loss was the highest in the low-carb group — the participants in this group lost an average of 12.1 lbs. (5.5 kg) after two years. The participants in the Mediterranean-diet group lost, on average, 10.1 lbs. (4.6 kg), and the participants in the low-fat group lost, on average, 7.3 lbs. (3.3 kg) after two years. [Low-Carb Diet: Facts, Benefits & Risks]

Additionally, there was a difference in which diets were the most effective for each sex. The women in the study who were on the Mediterranean diet lost more weight than the women on the low-fat diet. In comparison, the men on the low-carb diet lost more weight than the men on the low-fat diet, the researchers found. (However, only 45 women completed the study, compared with 277 men.) In other words, both women and men lost more weight on a diet other than the low-fat diet.

The DIRECT study showed that one diet doesn't fit all, said Iris Shai, a professor of nutrition and epidemiology at Ben-Gurion University of the Negev in Israel and the lead author of the study. In other words, the researchers learned that there are some alternatives to a low-fat diet that also work for weight loss, Shai told Live Science. These alternatives (the low-carb diet and the Mediterranean diet) also have more health advantages in the long run, such as improved cholesterol and blood sugar levels, respectively, she said.

In the follow-up four years later, the researchers found that all of the participants had regained some of the weight. Over the entire six-year period, the average total weight loss was 6.8 lbs. (3.1 kg) for the Mediterranean-diet group and 3.7 lbs. (1.7 kg) for the low-carbohydrate group. The average weight loss of 1.3 lbs. (0.6 kg) for the low-fat group was not statistically significant, meaning that the finding could have been due to chance, the researchers wrote.

"I'm a fan of using a high-quality, good-pattern diet with lower amounts of food, and one way of doing that is with portion control," Bray said. Plenty of evidence suggests the Mediterranean diet is highly beneficial for people's health and, combined with portion control, is a good strategy for weight loss, Bray told Live Science. The DASH diet, which was developed to lower blood pressure, also has shown weight loss benefits, he said. The DASH diet is a low-sodium diet that's rich in fruits, vegetables, low-fat dairy, nuts, beans and seeds.

The people in the study were assigned to counseling sessions to learn about their diets, and there were group sessions held throughout the course of the study. The researchers provided the participants with daily meal plans, and the plans were largely similar across the four diets, with only small tweaks — for example, to include a bit of extra olive oil, or a bit less meat. All of the diets in the study were healthy — they all contained healthy fats, whole grains, fruits and vegetables, said Sacks, who was the lead author of the study. And all of the diets contained an equal number of calories — fewer than the number of calories the people in the study were normally eating, he said. [2016 Best Food Scales]

The main takeaway of the study is that all of the diets produced the same amount of weight loss, Sacks said. "There was no advantage of one diet type over another," he said. The researchers observed that, at six months, the participants in each diet had lost, on average, 13.2 lbs. (6 kg). After the one-year mark, on average, the participants began to regain some of their weight. By the end of the study, the average weight loss for all of the diets was 8.8 lbs. (4 kg).

Bray, who ran the Baton Rouge arm of the study, noted that there was certainly a range for the weight loss observed among the people on each diet. However, the range was similar in all of the groups, he said. Some people on each diet lost as much as 33.1 lbs. (15 kg), and some people on each diet gained a bit of weight, Bray told Live Science. But even the proportions of people losing a lot of weight or gaining a little weight were similar in all four diets, he said.

Of course, it's important to keep in mind that, while these three trials have revealed some of the best evidence yet about weight loss strategies, countless smaller or shorter clinical trials have focused on diet and weight loss. To truly get a sense of the state of the field of research, scientists look at many studies together, in a type of study called a meta-analysis. This type of research can be especially valuable when looking at a number of smaller clinical trials because it reveals what the studies show when they are combined as a whole.

"Adherence is crucial," Johnston told Live Science. Because different people have an easier time sticking to different diets, the ideal diet is the one that the individual can adhere to best, so that they can stay on the diet as long as possible, the researchers wrote in their study. And in smaller studies, healthy eating habits have been linked with not only weight loss but also improved health, such as lower blood pressure, lower cholesterol levels, lower blood sugar levels and even improved cognitive function.

In another meta-analysis, published in October 2015 in the journal The Lancet, researchers concluded that low-fat diets were no better than other types of diets evaulated for long-term weight loss. The researchers looked at 53 studies that had weight loss as a goal, as well as other studies for which weight loss was not the primary goal (think back to the Women's Health Initiative study, for example).

When comparing weight loss trials specifically to one another, the researchers found that low-carbohydrate diets resulted in greater weight loss than low-fat diets. For both the weight loss trials and the other trials, the researchers found that higher-fat diets resulted in greater weight loss than low-fat diets. And low-fat diets only resulted in greater weight loss when compared with a person's usual diet, according to the study.

And the amount of weight loss induced by any of the diets was not very impressive, Hu added. Most people regained the weight they lost in six months to a year, he said. Studies have shown that many people tend to regain lost weight over time, and experts think that more research is needed on how to keep weight off successfully. "A low-carb diet may be a little bit more effective than a low-fat diet, but in the long run, both diets are not significantly different from each other in terms of weight loss," Hu told Live Science.

And quality counts — people who eat a lot of refined starches and added sugars are more likely to feel hungry and regain their weight because these foods aren't satiating, Hu said. "On the other hand, if you eat a healthy diet with high-quality foods like fruits and vegetables, high-fiber whole grains and [healthy fats such as] avocados and nuts," you may be able to lose more weight and also sustain your weight loss with that kind of diet, he said. [Best Diet and Nutrition Apps]

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Sara is a staff writer for Live Science, covering health. She grew up outside of Philadelphia and studied biology at Hamilton College in upstate New York. When she's not writing, she can be found at the library, checking out a big stack of books.