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Taylor Stinson's sheet pan Greek chicken meal prep bowls are a low-carb lunch or dinner idea with a lemon-oregano marinade, and they're ready in 30 minutes.Taylor Stinson/The Canadian Press

Many studies have demonstrated the effectiveness of a low-carbohydrate diet for short-term weight loss. Few studies, though, have investigated how well such a diet keeps weight off long-term.

Now, a new study led by researchers from the Harvard T.H. Chan School of Public Health provides a detailed answer.

It turns out, not all low-carbohydrate diets are created equal when it comes to keeping the pounds off.

The latest research

The study, published Dec. 27 in the journal JAMA Network Open, analyzed data from 123,332 healthy adults, with an average age of 45, taking part in one of three large continuing U.S. studies – the Nurses’ Health Study, the Nurses’ Health Study II and the Health Professionals Follow-Up Study.

Participants were followed from as early as 1986 up to 2018. Diet information and body weights were collected every four years.

Based on participants’ dietary intakes, the researchers created five categories of a low-carbohydrate diet, each one emphasizing different compositions and quality of macronutrients (protein, fat, carbohydrate).

The researchers then scored how well each participant adhered to one of the five low-carb diet categories.

A “total low-carbohydrate diet” referred to simply an overall lower carbohydrate intake, regardless of macronutrient quality.

An “animal-based low-carbohydrate diet” emphasized animal proteins and fats, while a “vegetable-based low-carbohydrate diet” gave prominence to plant-based proteins and fats.

A “healthy low-carbohydrate diet” focused on plant-based proteins, healthy fats and whole grains. An “unhealthy low-carbohydrate diet,” on the other hand, emphasized animal-based proteins, unhealthy fats and refined carbohydrates.

All low-carb diets contained 38 to 40 per cent of daily calories from fat.

Healthy vs. unhealthy low-carb diets: four-year weight change

The study found that the healthy low-carbohydrate diet made up of high-quality proteins, fats and carbohydrates was tied to a slower rate of long-term weight gain than the other four low-carb diets.

Among participants who followed the unhealthy low-carbohydrate diet, those who tightened their adherence to such a plan over four years experienced even greater weight gain.

Compared with people who were least adherent to this eating pattern, those who were most adherent gained, on average, an additional 5.1 pounds over four years.

By contrast, healthy low-carbohydrate dieters who increased their adherence scores the most over four years, gained, on average, 4.9 fewer pounds compared with those whose adherence scores decreased the most.

The results were most pronounced among participants who were under age 55, overweight or obese and/or less physically active.

These findings underscore the importance of considering macronutrient quality within a low-carbohydrate diet for weight control.

Strengths and limitations

The researchers examined the link between low-carbohydrate diets and long-term weight change from a broad scope, one that considered macronutrient quality as well as quantity.

Other notable strengths were the large sample sizes and long follow-up periods of the three studies.

Limitations include the observational nature of the research, which can’t prove with certainty that following a healthy low-carbohydrate diet mitigates long-term weight gain.

Consistent with past research

This isn’t the first study to suggest that a high-quality diet makes a difference for managing weight.

An analysis of the DIETFITS trial published last year determined that people who had both high diet quality and high diet adherence were the most successful at losing excess weight, whether they followed a low-carbohydrate or low-fat diet.

And findings from the PREDIMED trial have consistently shown that a Mediterranean diet (40-per-cent carbohydrates), rich in extra virgin olive oil, fruits, vegetables, whole grains, beans and lentils, leads to greater weight loss compared with a low-fat diet (43-per-cent carbohydrates).

A high-quality diet provides more fibre that can promote satiety and reduce overall calorie intake.

Studies have also found that consuming healthy plant oils from nuts, seeds and olive oil is tied to better weight outcomes.

On the other hand, diets higher in saturated fat from animal foods have been associated with increased inflammation, insulin resistance and a higher risk of weight gain.

A diet high in refined grains and added sugars can cause rapid fluctuations in blood sugar, which could lead to hunger and overeating.

Key ingredients in a healthy low-carb diet

Highlight plant proteins such as beans, lentils, bean pastas, edamame, tofu and nuts.

Lean animal proteins include chicken, turkey, seafood, plain low-fat Greek yogurt, cottage cheese and egg whites. Limit red and processed meats.

Eat small portions of whole grains such as brown and red rice, quinoa, farro, millet and oats.

Eat plenty of non-starchy vegetables (for example, leafy greens, broccoli, cauliflower, bell peppers). Potatoes, green peas, corn, parsnips and winter squash are starchy.

Include some whole fruit, too. Berries, pomegranate seeds, cantaloupe, kiwi and citrus fruit are lower in carbohydrates.

Choose unsaturated plant oils. Extra virgin olive oil, avocado oil, peanut oil, nuts and seeds are good choices.

Leslie Beck, a Toronto-based private practice dietitian, is director of food and nutrition at Medcan. Follow her on X @LeslieBeckRD

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