Diets that focus on restricting carbohydrates have been touted as the key to losing weight for decades, but it turns out that not all low-carb diets are slimming — and some could even lead to weight gain.
Harvard researchers examined more than 123,300 people from three different health databases.
Participants ate one of five types of low-carb diets and their weight loss and body mass index (BMI) were tracked every four years for 20 years.
The researchers found that despite an overall low-carb diet, weight loss over the study period was not significant unless participants consumed less animal protein – and some people even gained weight.
People who followed a low-carb diet but continued to eat unhealthy carbohydrates and animal protein actually gained weight during the study.
The average weight gain for people following this unhealthy diet was 2 pounds over a four-year period, although one group of participants experienced a weight gain of almost 5 pounds.
Only participants who followed a low-carb diet high in plant-based foods and whole-grain carbohydrates like oatmeal and whole-grain bread lost weight—and kept it off.
Harvard researchers analyzed people who followed five different diets with varying levels of plant protein intake, meat consumption, fats and carbohydrates. These included: a total low carbohydrate diet (TLCD); a low-carb animal diet (ALCD); a low-carbohydrate plant-based diet (VLCD); a healthy low-carbohydrate diet (HLCD); and an unhealthy low-carb diet (ULCD)
Study participants who ate what researchers called a “healthy low-carb diet (HLCD)” diet, consisting of fewer refined carbohydrates – such as in white rice, soda and white pasta – and more plant-based protein – such as lentils, tofu, Peanuts and chickpeas – and healthy fats – like those in fish, avocado and eggs – lost the most weight and maintained their weight loss over the study period.
The five diets included: a regular low-carbohydrate diet, a low-carbohydrate animal-based diet; a plant-based low-carb diet and a so-called “healthy low-carb diet” with fewer refined carbohydrates, plant protein and low-calorie plant fats such as olive oil.
These participants' results were also compared to those who ate an unhealthy low-carb diet – containing refined carbohydrates, animal protein and unhealthy animal fat.
The databases the researchers relied on were the Nurses' Health Study from 1986 to 2010; Nursing Study II from 1991 to 2015; and health professionals follow-up study from 1986 to 2018.
The amount of weight participants gained or lost after each diet varied depending on their BMI, which was measured at the start of the study.
Overall, people who followed the unhealthy low-carb diet experienced a weight gain of 0.86 pounds – the largest weight gain among the five diets.
People classified as overweight (BMI or 25 to 30) on this diet gained an average of one pound over the study period, and those with a BMI of 30 or more, who are in the obese range, gained almost two pounds.
Study participants of all BMIs who consumed a general low-carb diet or a vegetarian low-carb diet experienced no significant weight gain or loss.
The healthy low-carb diet had the greatest impact when it came to sustainable weight loss.
People with a healthy BMI lost about 0.5 pounds. Overweight people lost almost 1 pound and those with obesity lost almost 2 pounds.
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As part of the study, researchers also ranked the five diets according to Harvard's Alternative Healthy Eating Index (AHEI). This system assigns scores to foods and nutrients that predict chronic disease. The aim of the index is to rate foods based on their impact on reducing chronic diseases.
The study found that the “healthier” low-carb diets had better AHEI scores than the others, potentially suggesting that the healthier diet could help with chronic health problems.
This healthier diet consisted of whole grains, non-starchy vegetables, fruits, and low consumption of processed meats, sugary drinks, and desserts.
Carbohydrates are a type of macronutrient found in many foods and drinks. Most occur naturally in plant foods such as grains, but food manufacturers may also add carbohydrates to processed foods in the form of starches or added sugars.
Carbohydrates provide important nutrients and are one of the main sources of calories that humans consume.
There are two types of carbohydrates: simple and complex.
Simple carbohydrates are digested quickly and deliver instant bursts of energy into the bloodstream. Simple carbohydrates can be natural or added and are found in foods such as candy, white rice, white pasta, and sodas.
Added carbohydrates don't keep you full for long because they lack vitamins, minerals and fiber.
However, whole grain or “complex” carbohydrates contain vitamins, minerals and fiber and take longer to pass through the digestive system, making the temptation to snack on less.
These are found in foods such as whole grain pasta, oats, apples and brown rice.
Carbohydrates play many important roles in the body, including healthy digestion and a source of energy. An excess of simple carbohydrates or added sugars can negatively affect blood sugar levels and the pancreas and increase the risk of cardiovascular disease.
The study was published in the journal JAMA.