If you’re trying to lose weight you may be thinking a low-carb diet is the way to go — the fewer carbohydrates, the better.
The findings suggest that cutting carbs, particularly highly processed and refined carbs, may help you lose weight and improve some measures of heart health — but what you eat instead of those carbs matters. Replacing healthy carbs (such as vegetables, fruits, whole grains, and nuts) with foods high in saturated fat could increase certain heart disease risk factors.
Given the mixed results, the jury is still out on whether carb-limiting diets are good for heart health in the long run, the study authors wrote.
Researchers Crunched the Numbers on a Wide Variety of Low-Carb Diets
To explore how different types of carb-restricted diets impacted weight and heart disease risk factors, investigators analyzed more than 170 randomized controlled trials from 27 countries involving more than 11,000 adults, most of whom were overweight or had obesity.
The researchers compared low- and moderate-carb diets in how they lowered or raised:
- Cholesterol
- Triglycerides (fat your body stores for later use)
- Blood pressure
- Inflammation
- Blood vessel function
- Body composition, including weight, body fat, waist size, and muscle mass
People on Carb-Cutting Diets Improved Their Heart Health Measures — With 1 Exception
Across all carbohydrate-restricted diets, the data revealed overall reductions in weight, BMI, waist size, and fat mass, including belly fat, which raises heart disease risk more than fat stored in other parts of the body.
Carb-cutting diets, on average, also produced the following heart-health benefits:
- Lower triglyceride levels (about 15 milligrams per deciliter [g/dL] lower)
- Higher HDL “good” cholesterol (about 3 mg/dL higher)
- Slight reductions in blood pressure
- Lowered markers of inflammation
- Lower estimated 10-year risk scores of events like heart attack and stroke
These effects generally appeared within weeks to months.
The downside is that, on average, study participants who were following any kind of low-carb diet saw their LDL “bad” cholesterol go up (about 4 to 5 mg/dL), and they lost a small amount of muscle mass.
Why would that happen? “Low-carb diets are higher in fat. When the percentage of calories from fat increases, it’s not surprising to see an increase in LDL cholesterol, especially if the type of fat is mostly saturated, like with butter, cheese, and fatty meats,” says Elisabetta Politi, CDCES, RD, a certified diabetes care and education specialist at the Duke Lifestyle and Weight Management Center in Durham, North Carolina, who was not involved in the study.
Even so, the other heart-health benefits could justify a low-carb diet for many people, says Politi — if it’s done with nutritional guidance to help ensure that the regimen includes the right kinds of healthy carbs and limits saturated fats.
Comparing Different Low-Carb Diets
Investigators drilled down further and looked at the benefits and drawbacks of the different types of carb-cutting diets. They found:
Keto diets produced the biggest reductions in body weight, BMI, and body fat, and led to major improvements in triglycerides and HDL cholesterol. But in the negative column, keto dieters had notable increases in LDL cholesterol.
Ketogenic diets are the strictest carb-slashing diet, allowing roughly 20 to 50 grams (g) of carbs per day (typically under 10 percent of daily calories). This approach pushes the body into ketosis, where fat becomes the primary fuel.
In real-world eating, keto diets emphasize eggs, oils, cheeses, meats, fish, avocado, nuts, seeds, and nonstarchy vegetables, while largely excluding grains, beans, starchy vegetables, most fruits, sweets, and processed snacks.
Low-carb diets improved triglycerides, HDL, blood pressure, inflammation, and weight, with LDL cholesterol rising more modestly than on a keto diet.
The low-carb diets in this analysis allowed about 50 to 130 g of carbs per day (10 to 26 percent of daily calories).
Moderate-carb diets offered balanced benefits: They improved triglycerides, HDL cholesterol, blood pressure, and inflammation without consistently increasing LDL cholesterol.
For this analysis, moderate-carb diets allowed 26 to 45 percent of daily calories from carbs. A Mediterranean diet (widely considered one of the healthiest ways to eat) would fall into this category.
Which Kind of Low-Carb Diet Is Best for Heart Health?
Looking beyond the numbers, the authors offered the following observations.
Replacing carbs with both fat and protein delivered the strongest overall benefit for heart health and metabolism. People who cut back on carbs and replaced them with fats (of any kind) saw their HDL “good” cholesterol go up and their triglycerides go down. Those who replaced carbs with protein lowered total cholesterol levels. People who incorporated some of both saw improvements in blood pressure, inflammation, endothelial function, and multiple body composition measures.
Endothelial function refers to the healthy activity of the endothelium, a thin layer of cells lining the inside of blood vessels that regulates blood flow, prevents clotting, and controls inflammation. If it’s not functioning well (called endothelial dysfunction) it may increase the risk of heart attack and stroke.
This suggests that combination replacement (a mix of protein and fats on low-carb diets) appears optimal for improving cardiovascular health, weight, and muscle mass, the authors wrote.
An example of this is the Mediterranean diet, though the same balanced nutrient breakdown could be applied to other styles of cuisine, says Politi.
Women and people with type 2 diabetes benefited the most from low-carb diets. Women showed larger improvements in heart health measurements than men when they followed a low-carb diet.
People with type 2 diabetes had greater triglyceride and HDL improvements — without the LDL or total cholesterol increases seen in other groups — and they experienced the biggest reductions in weight and waist size.
Even people with overweight or obesity experienced LDL increases on low-carb diets. Contrary to past findings, LDL rose in participants with higher BMI, possibly because high saturated fat intake on low-carb diets may increase unhealthy cholesterol in the blood.
Potential drawbacks. These included a small amount of muscle loss, increases in LDL and total cholesterol, and other reported issues such as possible nutritional deficiencies, gut microbiota changes, gastrointestinal symptoms, hyperuricemia, and (in some research) bone-health concerns.
Hyperuricemia is when there is too much uric acid in the blood. That can lead to kidney stones, kidney damage, and gout.
The authors recommended that people only do very low-carb diets, especially keto diets, under the supervision of a healthcare professional.
Some Experts Find the Analysis Lacks Key Information
While some aspects of the analysis were “thorough and comprehensive,” there are major limitations that make it hard to use these findings to give people guidance on what type of carb-cutting diet is best for heart health, says Christopher Gardner, PhD, a researcher and professor at Stanford Health in California.
One reason is that investigators did not assess food quality. The authors acknowledge this, says Dr. Gardner, who wasn’t involved in the study.
It’s critical to assess and document the nutritional quality of the different carbs, proteins, and fats people were eating, he says, because a food’s nutritional quality depends on more than its ratio of carbs to fat to protein.
For example, he says:
- Lentils and lollipops are both low fat.
- Avocado and bacon are both low carb.
- Chickpeas and ground beef from factory farmed cattle are both high protein.
The above pairs are clearly very different nutritionally, says Gardner.
Additionally, researchers didn’t evaluate how well people stuck to their assigned diet, and that could make a big difference, says Gardner.
Improving your diet can certainly help you manage your weight and lower your heart disease risk — but if you can’t stick to the diet, the benefits don’t last, says Gardner.
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