If you’re trying to decide between a low-fat, high-carb diet or a low-carb, high-fat diet, it’s easy to get confused since the science seems conflicting. Proponents of both diets are often dogmatic in their approaches to nutrition.
During the second half of the 20th century, saturated fat was considered a dietary villain. Now, more studies suggest that carbohydrates are a significant factor behind the steady increase in obesity, not to mention the rise in diabetes and other chronic health issues.
But obesity and disease are complex. No single variable can account for them. Is it possible that most pop science articles — and even many studies — miss crucial details when making their claims?
Just because a diet works for weight loss doesn’t necessarily mean it improves overall health, enhances brain function, or prevents disease.
Read on to learn how to navigate the macronutrient maze and gain a clear idea of what type of diet will work for your body and goals.
Before you accept a statement on the effects of high-fat diets, make sure you understand how “high-fat diet” is being defined. It turns out that this term is thrown around with different and unrelated definitions.
High-Fat Diets in Studies
Researchers have studied so-called “high-fat diets” since the 1940s (or even earlier). And the meaning of the term varies greatly.
For example, a single study from 1967 defined “high-fat” intake as anywhere from 35-60% of daily calories[*]. Some of the participants in the study were given nothing but corn oil as a fat source, which is hardly a realistic scenario (or a healthy source of fat).
Animal studies use the term just as loosely as human studies.
A 2011 review of the “effects of high-fat diet in mice” stretched the definition to include diets containing 32-60% of calories as dietary fat[*]. These high-fat mouse diets had different ingredients. Some of them contained soybean oil, while others used Crisco or other hydrogenated oils. Again, not the best sources of fat.
Even recent human studies can’t seem to decide on a specific definition of high-fat.
A human study published in 2017 called a diet containing 20% protein, 35% carbs, and 45% fat “high-fat.” A different 2015 human study defined high-fat as 55% fat and 30% carbohydrates[*][*]. In the latter study, participants ate two sausage, egg, and cheese biscuits for breakfast.
The bottom line is that researchers tend to label any diet with more than 25% of calories from fat as a high-fat diet. They also don’t specify how many carbs the diet has.
This has led the public, journalists, and even researchers to focus on fat as the main cause of adverse effects in studies. But the ratio of carbs to fats is important too[*].
Quality of fat also matters a great deal. Crisco is quite different from wild-caught salmon.
High-Fat Diets in Real Life
When people talk about a high-fat diet, they usually mean a low-carb, high-fat diet. A reasonable definition of a low-carb, high-fat diet is one that has fewer than 100 grams of carbs per day.
Some scientists still use the terms “high-fat diet” and “Western diet” interchangeably. But the Western diet or standard American diet is full of processed foods that are high in both fat and sugar. That’s not the same thing as a healthy low-carb, high-fat diet[*][*].
Since this article isn’t about the Western diet, the terms “high-fat diet” and “low-carb, high-fat diet” will be used interchangeably.
What About the Keto Diet?
The ketogenic diet is one that keeps you in ketosis by limiting carbohydrates.
The most extreme version of the keto diet — mostly used for treating epilepsy — consists of 90% calories from (preferably healthy) fats[*]. However, you don’t have to eat that way to be in ketosis or reap other benefits.
If you want to use the keto diet to lose weight, feel better, and improve your health, you can stick to these macros:
- 20-25% of your calories from protein
- 5-10% of your calories from carbs
- 70-80% of your calories from fats
As with high-fat diets, different researchers mean different things when they label a diet “high-carbohydrate.” In some cases, scientists use the term without providing a definition at all[*].
Because of the traditional dogma against fat, most studies pushing the benefits of high-carb diets refer to these diets as low-fat diets.
Most low-fat diets include at least 50% of calories from carbs and 15-20% or fewer calories from fat[*]. Also, most studies of carbohydrate-rich diets for weight loss involve restricting calories.
Some high-carb diets include processed foods, grains, and sugar, while others limit carb sources to whole foods or whole grains.
This article will use the terms “high-carb diet” and “high-carb, low-fat diet” interchangeably.
Remember that no diet is perfect for everyone. When you’re considering the pros and cons of a high-fat, low-carb diet, you must factor in your own goals and health history to decide what’s right for you.
Most benefits of high-fat, low-carb diets come from increasing fat oxidation, lowering inflammation, reducing appetite, and improving insulin sensitivity[*].
When you restrict carbs and focus on eating healthy fats and protein, your body gets better at using fat for energy[*]. Fat burning also increases the levels of ketones in your body. In addition to helping you lose weight, ketones are anti-inflammatory and health-promoting[*][*][*][*].
Despite the common misconception that fat makes you fat, or that it increases your risk of heart disease, the risks associated with low-carb, high-fat diets have nothing to do with weight gain or heart issues.
If you’re overweight, insulin resistant, or have type 2 diabetes, eating more fat may temporarily raise inflammation in your body.
Elevated free fatty acids (FFAs) can turn on inflammatory pathways in people who struggle with obesity or insulin resistance[*]. However, if you stay on a low-carb, high-fat diet, you may be able to reverse insulin resistance[*]. Some people on a keto diet can even get off medication for type 2 diabetes[*].
That said, temporarily eating a low-carb, high-fat diet and then eating lots of sugar can worsen the negative effects of carbs[*]. If you start a keto diet or other low-carb, high-fat diet, commit to it.
If you want to get healthier by restricting carbs, it pays to exercise and avoid overeating.
High-carb, low-fat diets are less popular than ever. By now, many people have realized that eating more carbs and fewer dietary fats is linked to higher rates of obesity, heart disease, diabetes, and cancer.
But that doesn’t mean that you can’t lose weight on a high-carb diet, or that every person who eats a low-fat diet destroys their health. Low-fat diets work for some people.
Not everyone feels good when they restrict carbohydrates. If you’re always stressed or have problems with fatigue, eating more carbs may be necessary. They can help your body deal with stress and give you the energy you need.
If you’re very active or under a lot of stress, restricting carbs and cutting calories at the same time can increase the negative effects of stress. On the other hand, eating enough carbs allows your body to produce more thyroid hormone and balance your cortisol levels, which can help you cope with energy demands[*][*].
In the long run, if you’re stressed out or tired all the time, it’s wise to fix the underlying cause. But in the meantime, be cautious about making major changes to your diet.
Some carbs are unhealthier than others. Fructose, the kind in table sugar, high-fructose corn syrup, and most fruits, can promote the gain of visceral fat, the type around your abdominal organs[*].
If you’re a woman, eating high-glycemic carbs, too many carbs, and too little fiber can also increase your risk of polycystic ovarian syndrome (PCOS)[*].
There’s no definitive answer for which diet is best across the board. Each comes with its own set of pros and cons. Having the facts can help you determine whether a high-fat diet or high-carb diet is best for you.
For Weight Loss
If you want to lose weight, a high-fat, low-carb diet works at least as well as a high-carb diet — except it’s less likely that you’ll have to count calories if you go low-carb.
A review of multiple studies, including about 450 subjects, concluded that following a high-fat, low-carbohydrate diet without calorie restriction was as effective as, or more effective than, low-fat diets that restricted calories[*].
A separate 24-week study of 120 overweight subjects comparing low-carb, high-fat versus low-fat, high-carb diets found that the high-fat diet resulted in more weight loss and a lower dropout rate from the study[*].
A two-week crossover study from 2015 comparing high-carb and high-fat diets concluded that higher-carb diets result in more fat loss. But it’s worth noting that the study had many flaws[*].
For one thing, the “restricted carbohydrate” group ate about 140 grams of carbs per day. This doesn’t meet the strict definition of a low-carb diet. And it’s certainly not keto.
These study participants were also sedentary and locked in an indoor room for the entire study. The researchers didn’t allow exercise and the subjects weren’t able to move much. Over time, this decreased their metabolic rate (the number of calories they burn).
Basically, in terms of weight loss, both high-fat and high-carb diets can work. The question is which one works better for you and feels more sustainable.
For Better Body Composition
If your goal is to slim down, there’s a big difference between weight loss and fat loss. Low-carb, high-fat diets like the keto diet promote greater fat-burning. This helps you lose fat instead of muscle, especially if you’re physically active.
The ketones your body produces on a ketogenic diet can help you retain lean muscle mass as you shed body fat[*].
In a study of female endurance athletes who followed a ketogenic diet, their increased ability to burn fat resulted in about nine pounds of fat loss and lower body fat percentage without losing muscle mass[*].
A study of male endurance athletes who followed either a high-carb diet or low-carb ketogenic diet found that the keto group lost more fat and less muscle mass[*].
For Appetite and Satiety
If you’ve ever attempted a weight loss diet and failed, appetite and satiety may have been to blame. Feelings of hunger and fullness can make or break a diet plan. In this respect, low-carb, high-fat diets are a clear winner over diets that are high in carbs.
High-carb diets, especially those low in protein, can increase your appetite and cause you to gain body fat[*]. On the other hand, low-carb, high-fat diets are so filling that most people can lose weight without counting calories[*].
That’s partly due to Peptide YY — a hormone made in your small intestine. When Peptide YY levels increase, it reduces your appetite. And when you have less of an appetite, you tend to eat less.
A randomized study of 148 overweight adults comparing a low-carb diet and a low-fat diet found that the low-fat diet reduced peptide YY more than the low-carb diet[*].
For Athletic Performance
The keto diet and other high-fat, low-carb diets can increase your athletic performance, reduce inflammation in your body, and improve your recovery.
Most of the benefits of high-fat, low-carb, or keto diets on performance come from improved fat oxidation.
A study of endurance athletes found that the keto diet improved their recovery, reduced their inflammation, and enhanced their sense of well-being[*].
However, you don’t need to eat high-carb 24/7. You can eat a low-carb, high-fat diet most days, then use a cyclical keto diet or targeted keto diet to fuel up on hard training days and before competitions[*].
For Balancing Hormones
Both men and women need to eat enough fat and cholesterol to produce enough hormones. One problem with eating a high-carb, low-fat diet is that it can cause hormonal imbalances, especially in sex hormones like estrogen and testosterone.
In fact, reducing dietary fat can lower women’s estrogen by as much as 7.5%[*]. In men, decreasing total fat and saturated fat intake reduces concentrations of three hormones: androstenedione, testosterone, and free testosterone[*][*][*].
Low testosterone can lead to problems with muscle mass, motivation, sex drive, and fertility in men and women.
Eating a keto diet may occasionally lead to reduced thyroid hormone production in some people. If you have hypothyroidism symptoms (or you have a diagnosis), speak to your doctor before trying the keto diet[*].
Carbohydrate-rich diets or diets high in both fat and sugar can lead to insulin resistance, which causes other hormone imbalances as well as fertility issues[*].
For Blood Sugar, Insulin Sensitivity, and Diabetes
Managing your blood sugar and improving your insulin sensitivity are essential in preventing type 2 diabetes.
If you already have diabetes, eating a high-fat, low-carb diet can still improve your health by lowering blood glucose and reducing insulin resistance.
A comparison of a low-carb diet with a low-fat diet in diabetes patients found that the diets achieved comparable reductions in weight and glycated hemoglobin (HbA1c), which is a measure of your blood sugar over time.
But the low-carb diet was more beneficial for reducing blood sugar and diabetes medication requirements[*].
The keto diet can activate the AMPK pathway, which improves your insulin sensitivity and lowers your blood glucose. A 10-week study of people with metabolic syndrome found that pairing a ketogenic diet with high-intensity interval training (HIIT) reduced their HbA1c and fasting insulin and improved other markers of health[*].
A separate 32-week study of diabetic patients found that compared to a conventional “low-fat diabetes diet,” a very-low-carbohydrate ketogenic diet resulted in more weight loss, better triglyceride levels, fewer study dropouts, and better glycemic control[*].
In one trial, after just two weeks of a low-carb diet, a small group of adults with type 2 diabetes reduced their food intake, lost weight, achieved better blood sugar levels, improved their cholesterol and triglycerides, and increased their insulin sensitivity[*].
For Inflammation and Heart Health
Contrary to popular belief, eating saturated fat is not associated with heart disease[*]. However, your carbohydrate intake, especially the amount of processed grains you eat, may increase your risk of heart problems[*].
A 12-month study of 148 healthy men and women found that a high-fat, low-carb diet helped participants lose more weight than a low-fat diet[*]. It also led to more significant improvements in heart health.
For Cognition, Brain Health, and Dementia Prevention
Insulin resistance, oxidative stress, and inflammation can cause brain fog, dementia, and other neurodegenerative disorders.
Researchers consider the ketogenic diet promising for various areas of brain health, like brain trauma, Alzheimer’s disease, Parkinson’s disease, ALS, brain tumors, migraine, and autism spectrum disorders[*].
In mice, a 4-month ketogenic diet even reduced levels of beta-amyloid, a marker for Alzheimer’s disease[*].
The benefits of low-carb, high-fat diets on brain health may be due in part to increased levels of beneficial gut bacteria[*].
For Cancer Prevention
Because most types of cancer rely on the metabolism of sugar, very-low-carb diets that are high in fat may lower your risk of cancer compared to high-carb diets[*].
Animal and human studies show that ketogenic diets not only have anti-tumor effects, they’re also safe and well-tolerated by cancer patients[*].
For patients with squamous cell cancers of the head and neck, higher total carb intake (especially in the form of simple carbohydrates) can increase all-cause mortality compared to lower carb intake.
On the other hand, higher fat intake may reduce the risk of the cancer coming back. Higher fat intake may also increase the chance of survival[*].
Patients can use the keto diet along with other treatments like chemotherapy, radiation, or prescription drugs[*].
Is a High-Carb Diet Better for You?
Before you accept the next diet headline, article, or study as fact, make sure to check the definition of high-fat and low-carb. And remember that the quality of fats, not just the quantity, matters.
Unless you’re severely fatigued or under a lot of stress, you can use a high-fat, low-carb diet to lose weight, reduce your risk of disease, and improve your quality of life. High-carbohydrate diets simply don’t offer the same range of health benefits as low-carb diets.
While some (but not all) evidence shows that high-carb diets can be as effective as high-fat, low-carb diets for weight loss, a low-carb or keto diet is usually more filling and can work well without the need for calorie counting.
Regardless of which diet you choose, exercising regularly, steering clear of overeating, and managing your stress levels can improve your results.
Losing weight or improving your health is a process that takes time. But if you make a commitment, stay patient, and listen to your body, you’ll likely see excellent results.