What Impact Does Dietary Fat Have on Body Composition and Muscle Hypertrophy?

dietary fat


At one time, it was popular to follow a low-fat diet based on the belief that fat, being the most calorically dense macronutrient, was a major contributor to weight gain. Health-conscious people diligently read the labels on food products from yogurt to ice cream to make sure they got the low-fat version. Fast forward to the present and times have dramatically changed. Although food manufacturers still flash the words low fat on their food products, there’s growing awareness that processed carbs are more likely to be harmful to health than healthy sources of fat.  When manufacturers take the fat out of products, they usually replace it with sugar, fillers, and rapidly absorbed carbs.

The reality is that we need fat for good health. In fact, you need two essential fatty acids called alpha-linolenic acid and alpha-linolenic acid to make key fats called omega-6 and omega-3 fatty acids. These essential fats are important for healthy skin and joints and for producing brain chemicals that impact mood. They’re also intimately involved in cell function and inflammation. In fact, fats built from essential fatty acids are key components of the cell membrane. This membrane is the barrier that each cell uses to protect itself from the outside environment but still allow the transport of key nutrients into the cell. In the gut, you need fat to boost the absorption of fat-soluble vitamins, like vitamins A, E, C, and K.

Now you know that you must get fat in your diet for health purposes, you might also wonder what role dietary fat plays in strength performance and building muscle? We know that carbohydrates are a key source of energy for cells during exercise, particularly high-intensity exercise. However, fat is the primary fuel your body uses during low to moderate-intensity exercise where you aren’t huffing and puffing and dripping sweat on the floor. But even a lean person has enough body fat to fuel exercise for a sustained period.

But, what about resistance training to build muscle? What impact does dietary fat have on muscle gains? There are two ways in which dietary fat might impact resistance training performance. One, the fat you take in through diet could boost performance by supplying your muscle cells with the fuel they need to contract. Yet, fat plays only a minor role as a fuel source during intense strength training, as your muscle cells use anaerobic metabolism and glucose as a fuel source during intense lifts with the subsequent build-up of lactic acid in the muscle. Therefore, dietary fat isn’t a major fuel source during high-intensity exercise, including heavy resistance training.

The Hormonal Impact of Dietary Fat

The other way dietary fact could influence strength performance and muscle hypertrophy is by impacting the synthesis of key hormones involved in building muscle, primarily testosterone. Research shows that fat-restrictive diets modestly reduce testosterone production. However, it is not clear whether this leads to measurable differences in muscle hypertrophy. However, one study found that men who ate a low-fat, low-calorie diet lost significant amounts of lean body mass along with body fat. Therefore, the combination of low fat and low calorie may hinder muscle hypertrophy and lead to muscle loss. This is also a concern for people who go on a calorie-restricted diet.

Does Type of Fat Matter?

Another question is whether the type of fat you eat affects muscle hypertrophy gains. Fats can be divided into three main classes:

·        Saturated fat – mostly in animal products, meat and dairy, although coconut & palm oil contains it too.

·        Monounsaturated fat – abundant in olive oil, some nuts, avocado

·        Polyunsaturated fats – vegetable oils, nuts, flaxseed fish


The polyunsaturated fats are of two types: omega-6 and omega-3 fatty acids. The omega-3’s are most abundant in fatty fish and certain plant-based sources, like walnuts, flaxseed, hemp seeds, and sesame seeds. Western diets are skewed toward omega-6s due to the overconsumption of cheap, processed seed and vegetable oils, like soybean oil. A high ratio of omega-6 to omega-3 is linked, in some studies, with inflammation. Therefore, we need more omega-3s to compensate for the high omega-6 content of most diets.

What’s more, some research suggests that omega-3 fatty acids are more beneficial for muscle building than omega-6’s and saturated fats. Omega-3-rich diets, especially long-chain omega-3s in fish oil, are important for men and women over the age of 60. Studies show that muscles are less responsive to anabolic stimuli that cause muscle growth in seniors. Some research suggests that diets that contain more omega-3s help overcome anabolic resistance, a factor that interferes with muscle growth.

Lately, very-low-carbohydrate, high-fat diets have become popular. Often referred to as the ketogenic diet, this diet restricts carbs to no more than 50 grams daily and encourages the consumption of moderate protein and large quantities of fat. Studies suggest this type of diet can lead to rapid weight loss in the beginning, but, over time, weight loss on a low-fat, high-carbohydrate diet is roughly the same. The initial weight loss on a very-low-carb diet is partially water and glycogen stores.

What impact does a ketogenic diet have on muscle hypertrophy? Studies are conflicting, but the majority of research suggests that ketogenic diets offer a slight disadvantage for anaerobic and strength performance. Plus, people tend to lose more lean body mass when trying to lose weight on a ketogenic diet. It’s not clear whether this loss is mostly muscle tissue or whether water and glycogen are a prominent component of that loss.

So, there’s little evidence that a high-fat diet offers a muscle-building edge, but a diet low in fat may have a negative impact on muscle gains. This is because a very low-fat diet can reduce testosterone in and testosterone is an anabolic hormone. Women have 10 to 20 times lower levels of circulating testosterone relative to men, but testosterone still plays a role in hypertrophy gains in women.

The Bottom Line

There are no obvious benefits from a performance standpoint to eating a very high-fat diet, especially if it comes at the expense of healthy carb and protein sources. However, don’t eat a very low-fat diet either. Focus on healthy fat sources like omega-3s in fatty fish and plant-based foods like flaxseed, hemp, chia, and walnuts. Monounsaturated fats in olive oil and avocados are other sources of heart-healthy fat. Enjoy!



·        J Steroid Biochem. 1983 Mar;18(3):369-70.

·        J Clin Endocrinol Metab. 2005 Jun;90(6):3550-9. Epub 2005 Mar 1.

·        Healio.com. “Testosterone therapy plus a very low-calorie diet prevents lean mass loss in men”

·        Sports Med. 2004;34(5):317-27.

·        J Steroid Biochem. 1983 Mar;18(3):369-70.

·        Sci-Fi.net. “The Ketogenic Diet’s Impact on Body Fat, Muscle Mass, Strength, and Endurance”


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