A Harvard doctor says it's harder than ever to lose weight right now - but there are 5 ways to do it well

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We live in an overwhelming smorgasbord of colorful food choices.

• It may be harder than ever to maintain a healthy weight, according to a top Harvard researcher who's studied people's diets for decades.
• The best way to avoid gaining weight in the long run may be to pick a healthy diet you can stick to and eat a little less.
• You may need to re-think your relationship with fat and ramp up intake of plant-based foods like vegetables, nuts, and seeds, while consuming less meat and sugar.

Nutritionists agree that it is getting harder and harder for people to maintain a healthy weight - and that's not all your fault.

"There is so much great-tasting food, and it's abundant and in your face all the time," Dr. Meir Stampfer, an epidemiologist and nutrition expert at the Harvard T.H. Chan School of Public Health, wrote in a recent blog post. "To me it's kind of a miracle that people aren't even heavier than they are."

Stampfer, who has pioneered many long-term top-notch health studies, said the easiest way to get people to lose weight is to simply limit how much they eat every day.

"But for free-living people that's really hard," he said.

Average portions in the US have ballooned as much as 138% over the past five decades, and sugar is hiding in everything we eat, from salads to plain bagels and almost every low-fat product out there.

Sara Seidelmann, a cardiologist and nutrition researcher at Brigham and Women's Hospital and Harvard Medical School, sees the issue in a similar way.

"There's absolutely nothing more important for our health than what we eat each and every day," she recently told Business Insider.

Here are some of the best tips for how to slim down for the long term, from Stampfer and Seidelmann:

Larry Crowe/AP

Hearty enough for a main dish or served as a side, this bean salad is packed with fiber, protein, and other plant-based nutrients.

Healthy eating isn't necessarily low-carb

Seidelmann recently published a study involving more than 447,000 people around the world. The results revealed that people who eat too many or too few carbs don't live as long as those in the middle who eat a moderate amount.

Her team's data suggests people should focus on putting whole, healthful foods on their plate, like vegetables, whole grains, nuts, and beans.

Even though some veggies and beans might be considered "high-carb," eating them is associated with a longer life than low-carb diets that push people to eat large quantities of meat and animal products.

Focus on choosing healthy fats

"Eating fat doesn't make you fat," Stampfer said. That sound advice has been backed up by study after study after study.

"Eating healthy fats helps people control their weight better than diets than exclude them," he added.

Fatty foods have more energy gram per gram than carbs or proteins, and they can also help keep you full and satisfied until your next meal.

Some of the best plant-based sources of healthy fats include olive oil, avocados, walnuts, and chia seeds. Even oatmeal has a potent dose of fat, making it a great way to fuel up in the morning.

Eat 'just a little bit' less

Although incorporating movement into your day can yield immense benefits for your brain and body, nutritionists agree that the most surefire way to control your weight is to properly gauge (and perhaps reduce) how much food you're putting in your mouth.

Eating less and forgoing food for an occasional fast may even help you live longer, studies suggest. Some Silicon Valley biohackers have even decided to skip one meal a day, a version of the "intermittent fasting" craze that eliminates about a third of a day's calories.

But we're not suggesting anyone has to starve themselves. Just remember that a standard serving of whole-grain bread is one slice, a slice of meat should fit in an imaginary checkbook, and your cut of cheese should be about the size of four dice.

As Stampfer put it, "adopt a healthy diet, and eat just a little bit less."

Don't discount strength training

Your brain and your heart are some of the biggest calorie-burning machines in your resting body. But muscles can help keep your metabolism going all day, which means that incorporating some strength training into your routine can be a great way to maintain a healthy weight. But the benefits don't end there.

"Muscle building can not only bring up your body's metabolic rate, but also brings its own distinct health benefits that are often not as well appreciated as those associated with aerobic activity," Stampfer said.

Those benefits include improving mental health, fighting off depression, and even reversing some of the physical effects of aging. The American College of Sports Medicine suggests regular strength training two or three times per week.

You don't need a wide or colorful variety of foods - just find the healthy ones you like

Many principles of healthy eating that you might have learned as a kid are being debunked.

One such idea is that everyone should try to eat a varied, colorful "pyramid" of foods. Instead, the American Heart Association now suggests focusing on getting enough plants, protein, and healthy fats like nuts into your diet and not worrying as much about a diverse diet.

Recent studies suggest that people with the most varied, colorful diets also tend to eat more food of all kinds, including processed foods. That can wind up meaning they have less healthy, whole foods on their plates and bigger waistlines as a result.

"It's O.K. if your diet is not very diverse if you're focusing on healthy foods and trying to minimize consumption of unhealthy foods," University of Texas epidemiologist Marcia Otto recently told the New York Times.