What Happens to Your Body When You Eat Too Much Fat, According to Dietitians
Fat is an interesting topic. You may not hear many people say that they want more fat, and yet our body needs fat to function. It's one of the three macronutrients, along with protein and carbohydrates, that we need in order to survive. Fat contains fatty acids that are called "essential" because the body can't make these nutrients and your health will suffer if you don't eat enough fat. On the flip side, eating too much fat can, well, lead to some unwanted consequences.
For good health, we need to perform a macronutrient balancing act, say dietitians. "As an all-foods-fit dietitian, I encourage my clients to eat a balance of fat, carbohydrate, and protein at every meal," says registered dietitian Jessica Jones, RD, a nutritional consultant for Premier Protein Cereal. "Nutrient-dense sources of dietary fat, such as nuts, seeds, fatty fish, and avocado, can have positive effects on overall health. Fat is essential in providing energy and aiding in the absorptions of [fat-soluble] vitamins."
However, just like too much of anything—work, exercise, people-pleasing, and other good things—even good fats can be bad for you in excess. Bow much fat is too much? Adults should limit their total calories from fats and oils to 20 to 35% of their daily caloric intake, according to the Dietary Guidelines for Americans. Saturated fats should only comprise about 10% or less of the daily calories consumed.
Here's what can happen to your body when you eat too much fat, according to dietitians. And for a diner's guide to avoiding too much fat, checkout The Fattiest Restaurant Meals in America.
Your cholesterol may rise
Certain types of fats are known to increase blood cholesterol levels. "Saturated fats, like the ones found in whole milk and red meat, should be enjoyed in small amounts because they can raise your LDL 'bad' cholesterol," says registered dietitian Amy S. Margulies, RD, owner of The Rebellious RD. Trans fats, or partially hydrogenated oils, found in fried foods, pizza dough, crackers, and baked goods, also cause your body to produce more low-density lipoprotein. This is the cholesterol that can build up on the walls of your blood vessels, causing them to narrow and become hard, compromising blood flow to parts of your body.
You may gain weight
Let's remove the misleading elephant in the room, first. Eating too many fat calories can make you gain weight. "Fat is calorie-dense at 9 calories per gram compared to carbohydrates and protein, both of which contain 4 calories per gram," says registered dietitian nutritionist Mary Wirtz, MS, RDN, a nutrition consultant at Mom Loves Best. "If an individual consumes too much fat, excessive calorie intake, and subsequent weight gain, may result."
Research also backs up this notion. A large 2020 Chinese study in Nutrients, for example, linked high-fat diets with elevated body weight, body mass index, and increased risk of overweight and obesity. But overeating carbohydrates can cause obesity, too. Carbohydrates, especially processed carbs that contain little or no fiber, elevate sugars that spike the hormone insulin, which triggers fat storage.
So, you can't assume that eating fat will make you fat. Calories from an egg fried in a healthy fat like olive oil are different from calories from a buttery croissant in how they impact your hormones, hunger, and metabolism. The type of fat, just like the type of carbohydrate eaten, matters. In a study in the journal Obesity based on data from the famed Nurses' Health Study, researchers found that weight gain was associated more strongly with the percentage of calories nurses got from animal fats, saturated, and trans fats than from total fat calories and not at all from increased intake of poly and monounsaturated fats. What's more, researchers found an association between dietary fat intake and greater weight gain among women who were already overweight.
You may increase your risk of heart disease
There's much controversy around saturated fats and heart disease, but the American Heart Association (AHA) maintains its longstanding recommendation to limit consumption of red meat, cheese, butter, tropical oils like palm and coconut oil, and other sources of saturated fats. "It's not that they're an enemy to your body, but they can raise your bad LDL cholesterol," says Margulies. "Often when someone eats too much saturated fat, it can lead to health problems, including a higher risk of heart disease."
You may risk having a stroke
A stroke is often called a "brain attack" because it's similar to a heart attack. In a heart attack, blood flow to your heart is blocked, but a stroke occurs when the supply of blood to the brain is blocked off. Your diet is one of the lifestyle factors that can affect your risk for having a heart attack or a stroke. Here again, the type of fat and how much you consume can play a role in whether you gain a protective advantage against stroke or increase your risk, according to a large study presented to the American Heart Association a few years ago.
The research, published in the AHA journal Circulation, analyzed data on 73,867 women and 43,269 men taken from the Nurses' Health Study, 1984-2016 and the Health Professionals Follow-up Study, 1986-2016. Scientists determined that the health professionals who consumed the highest amounts of animal fats (from non-dairy sources) had a 16% increased risk of stroke, while those whose intake of healthy vegetable fat had a 12% lower risk of stroke compared with people who consumed the lowest amount of vegetable fats.
You may develop nonalcoholic fatty liver disease
"Overeating saturated fats can cause liver damage because it can lead to the development of nonalcoholic fatty liver disease," says registered dietitian Katherine Gomez, RD, a medical and nutrition reviewer for Psyche Mag. NAFLD, or fatty liver for short, is like the liver inflammation and cirrhosis that can occur from heavy alcohol use, but in this case, it can happen to people who are teetotalers as well. A high-fat diet, characteristic of the Standard American Diet, can trigger this liver damage by causing too much fat to be stored in the liver, leading to inflammation, and possibly scarring, a condition known as nonalcoholic steatohepatitis.
You may develop diabetes
Diabetes is a devastating disease that can lead to blindness, limb amputation, and death. More than 34 million Americans have this metabolic disorder and many millions more are prediabetic and don't know they are at risk. While eating sugary, high-carb processed foods is often blamed for diabetes, a diet high in saturated fat can increase risk of developing type 2 diabetes as well, says registered dietitian Reda Elmardi, RD, CSCS, owner of The Gym Goat. In fact, even short-term binge eating of fat can lead to abnormally high levels of blood sugar that are the hallmark of diabetes.
One study in the journal Nutrients found that just one day of high-fat overeating caused insulin resistance, which impaired glucose metabolism in young, healthy adults. Insulin resistance is a component of "metabolic syndrome," a cluster of health conditions (including overweight and high blood pressure) that significantly increase the risk for type 2 diabetes. Imagine then what damage the long-term consumption of fatty foods can do.
You may find yourself running to the toilet
Beware if you don't normally eat a lot of fat and one day you tuck into a traditional barbecue meal of brisket, ribs, collard greens with bacon, and corn bread. All that high-fat fare may wreak havoc on your tender, inexperienced innards. "A big meal with too much fat can lead to digestive issues like bloating, nausea, and diarrhea," says Elmardi.
You may increase your risk of certain cancers
Dietitians we spoke with also point out that possible link between eating too much fat and certain types of cancers. According to the CDC, being overweight or having obesity is associated with increased risk for 13 types of cancers. As we mentioned earlier, a high-fat diet and an overweight body can trigger long-lasting inflammation and insulin resistance, which may increase your risk of cancer. A 2021 study in Trends in Cancer, for example, indicated that a high-fat diet enhanced tumor formation in the gut by proliferating certain intestinal stem cells, resulting in increased risk of colorectal cancer.
Your brainpower may weaken
Consuming a lot of fat increases your risk for developing chronic low-grade inflammation, which can cause a host of health problems, according to Gomez. Chronic low-grade inflammation is a condition where your immune system releases molecules and proteins that trigger inflammation (swelling) and can lead to cell death throughout the body. Studies have demonstrated that rodents fed a diet high in saturated fatty acids saw an increase in neuroinflammation (brain inflammation).
Human studies have shown the cognitive effects of brain inflammation. One report in the American Journal of Clinical Nutrition found that healthy people put on a diet consisting of 75% of calories from fat for five days developed symptoms of depression and impaired attention and memory retrieval. In another study, researchers concluded that midlife obesity is a risk factor for cognitive decline and is associated with the earlier onset of Alzheimer's disease and noted that diets high in fat increase obesity and neuroinflammation.
So, as you can see, eating too much fat may lead to some negative health outcomes. However, it most often depends on the type of fat, with saturated and trans leading to more complications than poly or monounsaturated fats.