What Science Says About the Best Way to Lose Weight

A registered dietitian weighs in on which diets work for weight loss

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What Science Says About the Best Way to Lose Weight
Two bowls of salad with barley, cherry tomatoes, lettuces, sliced apples with a small bowl of dressing and pine nuts on the side

You're not alone if you’ve been trying to lose a few (or more) stubborn pounds. An estimated 45 million Americans go on a weight-loss diet each year, typically after the decadent holiday season. 

Losing weight can be difficult, and knowing which diet works for you can be harder. But don’t toss your scale! I’m a registered dietitian and I combed through tons of published research to uncover whether diets actually work — and which one is the best for losing weight. This science-backed review looks at every popular trend, from calorie restriction to low fat to low carb (most diets fall into one of these categories, no matter the name of the diet), and reveals the winners and losers. 

The Best Way to Lose Weight

Weight loss diets come and go and get recycled into “new-and-improved” methods every year. While there isn’t one best way to lose weight, data shows that some weight-loss diets have better results than others. 

Taking the weight off is only half the battle. The real challenge comes from adopting new habits for life. What may work for you?

Low-fat Diets

Fat was demonized in the ’80s and ’90s because of its calorie connection. By weight, fat provides 9 calories per gram while carbs and protein provide 4 calories each per gram. If the “calories in, calories out” theory of weight loss were true, restricting fat alone should aid in weight loss.

This research begs to differ: A 2020 review of 38 studies in nearly 6,500 adults compared a low-fat diet to a low-carb diet for weight loss. The results favored a low-carb diet for weight loss, but low-carbers only lost 2.86 pounds more over six to 12 months than the low-fat dieters.

Another study comparing low-fat to low-carb diets based on genotype (genetic variations in how carbs and fat are metabolized) or insulin secretion was done in 2018. Previous research hinted that people with high blood insulin secretion may lose more weight on low-carbohydrate (versus low-fat) diets.

In the genotype study, 609 non-diabetic, overweight adults aged 18 to 50 were told to follow either a healthy low-carb (HLC) or healthy low-fat (HLF) diet for one year. Forty percent of subjects showed a genetic pattern that should have responded to a low-fat diet while 30% of subjects had the low-carb genetic pattern. Results showed no difference in HLC over the HLF diet when it came to weight reduction. Genotype and insulin secretion did not impact the success of either diet.

Here’s a summary of the research we reviewed:

  • Low-carb dieters lost almost three pounds more than those on a low-fat diet over the course of six to 12 months.

  • Genetic factors and insulin secretion do not predict weight loss success in either low-fat or low-carb diets.

The verdict: Some research shows that dieters lost slightly more weight — we’re talking three pounds — on low-carb vs. low-fat diets. Keep in mind that weight loss was tracked over six to 12 months without information about how long dieters maintained the weight loss after the study. 

Low-carb Diets

While the above study did not show a difference in genetic predisposition with a low-carb versus low-fat diet, other studies do suggest that low-carb may be better for individuals with high blood sugar. A 2017 review comparing three different diets compared a low-fat, high-carb diet (20% fat, 66% carb), medium-fat, medium-carb (30% fat, 56% carb), and high-fat, low-carb diet (40% fat, 46% carb).

In people with normal blood sugar, a low-fat, high-carb diet resulted in slightly more weight loss than the other diets — about one pound more over six months.

In pre-diabetic subjects, the type of carbohydrate is important; higher-fiber carbs, including whole grains and beans help control blood sugar. Individuals with diabetes fared best on low-carb, high-fat diets for weight loss and blood sugar (glucose) control.

Researchers from Tulane University found that participants on a low-carb diet reduced blood sugar by 17% and lost 13 more pounds in six months than those following a traditional diet. The low-carb group received diet counseling while the control group did not, which could have impacted results.

The big takeaway from this research: Don’t go it alone! The use of a remote nutritional professional may be beneficial for improved weight loss results. A dietitian or nutritionist can provide feedback, support, and meal ideas to make your weight loss journey less stressful.

Here’s a summary of the research we reviewed: 

  • Individuals with pre-diabetes or diabetes may lose more weight on a lower-carb diet.

  • Reducing added sugar and including high-fiber foods in your diet supports weight loss.

The verdict: Low-carb diets help those with difficulty managing their blood sugar to lose weight and reduce high blood sugar. One study that showed impressive weight loss and blood sugar reduction in low-carb dieters revealed the participants received nutrition counseling to help them lose weight. 

Calorie-restricted Diets

Restricting calories is still a valid way to lose weight. Calorie restriction may be moderate (25–35% reduction; for example, from 1,500 to 1,200 calories per day) or severe (50% reduction; for example, from 800–1,200 calories per day) for weight loss.

One study in 101 post-menopausal women found that those in the severe calorie-restricted group lost more weight (14.52 pounds versus 7 pounds) but also lost more lean body mass (2.64 pounds) compared to those on moderate-calorie reduction. 

Sarcopenia is a fancy way to say we lose lean body mass (muscle) as we age. Along with this loss comes a less strength and increased risk for falls, disability, and death. 

Two concerns with severe calorie restriction:  

  • the risk of weight regain 

  • the risk of bone loss

Weight may be easily regained with severe calorie restriction because your BMR (basal metabolic rate) decreased when you lost the weight. Also, this type of diet is usually short term and, once you stop the diet, you could resume the previous habits that resulted in weight gain.

A very low-calorie diet may be used to aid in initial weight loss in obese patients awaiting bariatric (weight loss) surgery who are advised to lose weight pre-op to lower blood pressure or other comorbities such as sleep apnea. Hormonal changes, nutritional factors (such as protein, calcium, and vitamin D intake) can impact bone health during weight loss when calories are restricted a lot. These types of diets are best monitored by a medical professional.

Intermittent fasting (only eating during certain hours of the day, or on certain days of the week) and continuous calorie-restricted diets show similar weight loss results. A review evaluating intermittent fasting and continuous calorie restriction showed no significant difference in BMI overall, but intermittent fasting resulted in more weight loss at the start of the diet. 

Here’s a summary of the research we reviewed:

  • Severe calorie restriction may result in faster weight loss, but carries the risk of weight regain and bone loss. 

  • Intermittent fasting and continuous calorie restriction result in similar weight loss.

The verdict: You may lose weight fast when you follow severe calorie restriction, but this plan may put you at risk for bone loss and gaining the weight back once you start eating more calories again. You may lose more weight when you start an intermittent fasting plan compared to (moderate) calorie cutting, but overall results evened out at the end of the two diets.

Protein and Weight Loss

What’s on your plate can make a difference in your weight. In a study of 105 adults, participants were given a standard-protein diet (.8 gm/kg) or a high-protein diet (1.34 gm/kg) to follow for six months. While both groups reduced their waist size, people stuck to the high-protein diet better — and this resulted in more weight loss overall.

The type of protein matters. Compared to animal protein, some research suggests whey protein (made from dairy) and plant protein (from beans and legumes) support weight loss better. Scientists believe branched-chain amino acids including methionine, tryptophan, and the metabolite glutamate may favor muscle maintenance and help you feel more satisfied.

High protein is different from high fat (keto diets). One of the first studies on the Atkin’s diet showed promise in overweight and obese women following a high-protein diet versus a calorie-controlled diet. In the six-month study, those on the high-protein diet lost twice as much weight as those in the low-fat, calorie-controlled group.

More recent research shows the same results. Individuals following a high-protein, low-carb diet lost more weight than those on a low-fat diet. However, LDL (bad cholesterol) increased in the high-protein group. If you’re at risk for heart disease, be cautious and talk to your doctor before trying a high-protein, low-carb diet.

Here’s a summary of the research we reviewed:

  • High protein diets do result in weight loss, but may raise LDL cholesterol and the risk for heart disease.

  • Plant-based protein from beans and legumes supports weight loss and provides dietary fiber.

The verdict: Eating more protein appears to help dieters lose more weight than low-fat diets, but those who have a higher risk for heart disease should talk to their doctors first. Research also found that whey protein (made from dairy) and plant-based protein support weight loss better than other animal protein.  

The Bottom Line

As you can see, there are multiple ways to lose weight. Low-carb, higher protein diets do result in more weight loss in most populations, but carry some risks such as increased cholesterol. Very low-calorie diets may result in fast weight loss, but could impact your bone health and muscles. Including plant-based protein may mitigate some of these risks. 

The best way to lose weight could be less about the diet you choose and more about how motivated you are to maintain the weight you lose. If you can’t keep the pounds off, does it matter how quickly you lost weight, or whether you survived low carb? What’s more, if some aspect of your health was sacrificed during the diet (such as your bone density or your mental health after severe calorie restriction), you have to decide if trading one potential risk (extra weight) for another was worthwhile. 

There are ways to lose weight without restriction and tracking every little bite. You might be surprised how gradually transitioning to a healthy lifestyle — where you eat a balanced diet of healthy whole foods, drink plenty of water, get enough quality sleep, and include more physical activity in your days — can help you find a healthy weight again. If you feel stuck, consider working with a registered dietitian or personal trainer through the Kickoff app.