If you’re trying to decide between a high-carb, low-fat diet and a low-carb, high-fat diet, it’s easy to get confused. The science can appear conflicting, and proponents of both diets are often dogmatic in their approaches to nutrition.
During the second half of the 20th century, saturated fat was the dietary villain of choice. 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 health issues in the developed world.
But obesity and disease are complex, and 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 pronouncements?
Just because a diet works for weight loss doesn’t necessarily mean it improves longevity, enhances brain function, or prevents disease, for example.
Read on to learn how to navigate the macronutrient maze and emerge with a clear idea of what 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
For example, a single study from 1967 labeled a dietary fat intake anywhere from 35% to 60% of daily calories as “high-fat”[*]. And 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 are as guilty as human studies of using the term loosely.
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 differed as to their ingredients, sometimes containing soybean oil, other times using Crisco or other hydrogenated oils.
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,” while a 2015 human study labeled a diet with 55% fat and 30% carbohydrate “high-fat”[*][*]. In the 2015 study, participants ate two sausage, egg, and cheese biscuits for breakfast.
Essentially, any diet with more than 25% of calories from fat is likely to be labeled a high-fat diet by researchers.
The problem with this paradigm is that study titles and abstracts often label a diet “high-fat” without specifying the carbohydrate content.
This has led the public, journalists, and even researchers to focus on fat as the sole cause of adverse effects in studies, rather than the ratio of carbs and fats and other essential factors[*].
Quality of fat also matters a great deal; Crisco is very different than wild-caught salmon when it comes to your health.
High-Fat Diets in Real Life
When people discuss a high-fat diet these days, they generally mean a low-carb, high-fat diet. A reasonable definition of a low-carb, high-fat diet is a diet that includes fewer than 100 grams of carbs per day.
The Western diet or standard American diet of processed foods are high in both fat and sugar, but don’t confuse that with a healthy low-carb, high-fat diet[*].
Some scientists still use the terms “high-fat diet” and “Western diet” interchangeably, but a Western diet is completely different than a low-carb, high-fat diet[*].
This article isn’t about the Western diet, so the terms “high-fat diet” and “low-carb, high-fat diet” will be used interchangeably.
What About the Keto Diet?
The ketogenic diet is a diet that keeps you in ketosis by restricting carbohydrates.
The most extreme version of the keto diet — mostly used for treating epilepsy — derives 90% of calories from (hopefully healthy) fats[*]. However, you don’t have to eat that way to be in ketosis or get other benefits from your keto diet.
If you want to use the keto diet to lose weight, feel better, and improve your health, you can eat between 20-25% of your calories from protein, 5-10% of your calories from carbs, and 70-80% of your calories from fats.
As with high-fat diets, different researchers mean different things when they label a diet “high-carbohydrate.” And in some cases, scientists use the term without providing a definition at all[*].
Because of the traditional dogma against fat as a macronutrient, most studies touting 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[*].
And most studies of high-carb diets for weight loss involve calorie restriction.
High-carb diets vary significantly in their food selection. Some high-carb diets include processed foods, grains, and sugar, while others limit carb sources to whole foods or whole foods and 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, it’s essential that you factor in your goals and health history.
Most of the 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 instead, your body gets better at using fat as a fuel source[*]. Fat oxidation also increases the levels of circulating ketones in your body, which has anti-inflammatory and health-promoting effects[*][*][*].
Low-carb, high-fat diets like the keto diet may work better for decreasing inflammation and improving your health when paired with exercise, calorie restriction, or fasting[*].
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 cardiovascular issues.
If you are overweight, insulin resistant, or have type 2 diabetes, eating more fat may temporarily raise inflammation in your body.
Elevated free fatty acids (FFAs) can activate inflammatory pathways in people who struggle with obesity or insulin resistance[*]. However, if you stay on a low-carb, high-fat diet, you can easily reverse insulin resistance[*]. People on a keto diet can even get off medication for type II diabetes[*].
Temporarily eating a low-carb, high-fat diet and then eating lots of sugar may worsen the adverse effects of carb consumption[*]. 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 dietary guidelines that emphasize eating more carbs and restricting dietary fats coincide with increased rates of obesity, heart disease, diabetes, and cancer.
But that doesn’t mean that no one ever loses weight using a high-carb diet, or that every single 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 experience chronic stress or fatigue problems, eating more carbs may be necessary to enable your body to deal with stress and provide you with energy.
If you’re very active or under a lot of stress, simultaneously restricting carbs and cutting calories can increase the impact of stress, while eating enough carbs allows your body to produce more thyroid hormone and balance your cortisol levels to 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 more unhealthy than others. Fructose, found in table sugar, high-fructose corn syrup, and most fruits, can cause you to gain visceral fat around your abdominal organs[*].
Eating high-glycemic carbs, too many carbs, and too little fiber can even increase your risk of polycystic ovarian syndrome (PCOS) if you are a woman[*].
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 prescribing 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 vs. low-fat, high-carb diets found that the high-fat diet resulted in more weight loss and a lower drop-out rate from the study[*]
A two-week crossover study from 2015 comparing high-carb and high-fat diets concluded that high-carb diets result in more fat loss, but the study had several flaws[*].
For one thing, the “restricted carbohydrate” group ate about 140 grams of carbs per day, which 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 duration of the study. The researchers didn’t allow exercise during the study, and the subjects were unable to move much, which decreased their metabolic rate.
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, which helps ensure that you lose fat instead of muscle, especially if you’re physically active.
The ketones your body produces during a ketogenic diet can help you retain lean muscle mass as you shed body fat[*].
In a study of female endurance athletes who adopted 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 self-selected for 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 high-carb diets.
High-carb diets, especially those low in protein, can increase your appetite and cause you to gain body fat[*]. In contrast, low-carb, high-fat diets are so filling that most people can lose weight without counting calories[*].
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[*].
Peptide YY is a peptide hormone made in your small intestine that reduces your appetite and limits food intake, and it’s one reason why high-fat, low-carb diets help you feel more full.
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, and use a cyclical keto diet or targeted keto diet to fuel up on hard training days and before competitions[*].
For Balancing Hormones
For men and women, hormone production requires adequate dietary fat and cholesterol intake. One problem with eating a high-carb, low-fat diet is that it can lead to hormonal imbalances, especially in sex hormones like estrogen and testosterone.
Reducing dietary fat can lower women’s estrogen by 7.5%[*].
In men, decreasing dietary fat and reducing saturated fat intake reduces concentrations of 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, so if you already experience symptoms of hypothyroidism, speak to your doctor if you’re interested in trying the keto diet[*].
High-carb 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 considerations if you want to prevent 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).
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 risk of cardiovascular disease[*]. However, dietary carbohydrate intake, particularly processed grains, 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 resulted in more weight loss and greater improvements in cardiovascular health than a low-fat diet[*].
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 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 glycolytic (sugar-based) metabolism, very-low-carb diets high in fat may lower your risk of cancer compared to high-carb diets[*].
Animal and human studies demonstrate 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 carbohydrate intake, total sugar, glycemic load, and simple carbohydrate intake significantly increase all-cause mortality compared to lower carb intake. Additionally, higher fat intake reduces cancer recurrence and improves patient survival[*].
Patients can use the keto diet along with other treatments like chemotherapy, radiation, or drugs[*].
The Takeaway: Which Diet is Best?
Before you accept the next diet headline, article, or study, make sure to check the definition of high-fat and low-carb that’s being used. And remember that the quality of fats, not just the quantity, counts toward health outcomes.
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 easier, more filling, and can work well with no need for calorie counting.
Regardless of which diet you choose, your results will be better if you exercise regularly, steer clear of overeating, and manage your stress levels.
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 can look forward to excellent results.