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The In's and Out's of Dietary Fat

It goes without saying that by far the most commonly talked out trend in the world of nutrition is the switch from the low fat craze only a couple of decades ago to now praising fat as being the magic behind a healthy diet. You can't go anywhere without seeing avocado and bacon somewhere on the menu and droves of people are following high fat diets like ketogenic, Paleo, and more.

So why did we go from demonizing fat to glorifying it?

Is there validity in a high fat diet being superior to a low fat diet?

Before we can question high fat diets, let's first review over what dietary fat actually is:

Fat, or lipid, is a concentrated fuel source that is found in both animals and plants as triglycerides (three fatty acid chains attached to a glycerol backbone). (Nix, 2013) In general, animal fats contain saturated fatty acids (solid at room temperature) while plant-based fats, called oils, contain unsaturated fatty acids (liquid at room temperature). There is an exception to this where the tropical oils like palm oil and coconut oil, which are plant-based, actually contain saturated fatty acids and where fish contain unsaturated fatty acids. (Nix, 2013) Dietary fats can also be identified as visible fat (like the marbling in a steak, butter, oils) or invisible fats (like in cheese, nuts, seeds, olives, avocados, and lean cuts of meat). (Nix, 2013)

Saturated fats were previously considered hazardous to health (thus spurring the low fat craze) largely due to the fact that animal-based saturated fats contain cholesterol. Cholesterol is a naturally occurring compound that is primarily produced in the liver and is therefore present in animal food sources as well. (Nix, 2013) There were previous claims that high dietary cholesterol increased risk for cardiovascular disease primarily based on observational studies of dietary trends in various populations. (Hooper, et al. 2015) Unfortunately, as with any observational study based on dietary (and lifestyle) trends, you cannot simply deduce that one specific component was responsible for the entire trend as there would be multiple uncontrolled factors. In this case, it is likely the entire lifestyle that affords responsibility for increasing CVD risk includes the combination of high saturated fat intake, smoking, sedentary behavior, alcohol consumption, refined grains, etc. Randomized controlled trials (studies that aim to control for all other variables i.e. testing only one difference between groups) that looked at this relationship between saturated fat (and cholesterol) intake with cardiovascular disease showed a lack of evidence behind those previous observational claims. (Hooper, et al. 2015) In a review of several randomized controlled trials, there was a decrease in CVD risk when saturated fat was replaced with unsaturated fats, but not when replaced with carbohydrates or protein. (Hooper, et al. 2015) This is an important point to note where reducing TOTAL fat and saturated fat content alone (i.e. replacing saturated fat with carbohydrate or protein, thus reducing total fat intake) did not show any benefit in reducing risk of cardiovascular disease, however REPLACING saturated fat content with unsaturated fat did. (Hooper, et al. 2015)

So what are unsaturated fatty acids?

The unsaturated fatty acids can be classified as either monounsaturated fatty acids (found in avocado, olives, nuts like almonds, cashews, peanuts) or polyunsaturated fatty acids (also called PUFAs, found in fish, flax seed, walnuts, corn oil, soybean oil, etc). The PUFAs that are most commonly recognized are the essential fatty acids Omega-3 (found in fish, flax, walnuts) and Omega-6 (found in corn and seed oils) polyunsaturated fatty acids. (Nix, 2013) **Omega-3 and Omega-6 PUFAs are the only essential fatty acids which means that we must obtain these in the diet as the body cannot produce them.

And by replacing saturated fatty acids with unsaturated fatty acids, we also got this:

Trans fats are unsaturated fatty acids that have been hydrogenated (a change in the structure by introducing hydrogen) to create a saturated fatty acid. (Nix, 2013) There is a small amount of naturally occurring trans fatty acids in animal foods, but most of the trans fats in the food supply today are due to hydrogenated and partially hydrogenated plant oils like in margarine, vegetable shortening like Crisco and many shelf-stable snacks. Trans fatty acids are produced from plant oils but hydrogenated to mimic saturated fat, like maintaining a solid structure at room temperature. The idea behind the production of trans fatty acids came in response to the low fat craze, particularly when cholesterol was being associated exclusively with increasing risk for cardiovascular disease. Cholesterol occurs naturally in animal-based saturated fats, and thus the idea was to decrease cholesterol intake from animal-based saturated fats by instead hydrogenating plant oils (that are free of cholesterol) to mimic saturated fats, and use those in place of saturated fats (like using margarine instead of butter). While this idea was based on good intentions, the outcomes showed that trans fatty acid intake from partially hydrogenated oils actually INCREASED risk for cardiovascular disease further by increasing LDL concentrations and creating unfavorable ratios of total cholesterol to HDL cholesterol. (Lichtenstein 2014) Due to the consistent findings regarding this relationship, current dietary guidelines recommend avoiding trans fatty acids from partially hydrogenated oils. (Nix, 2013)

**This process of hydrogenation provides a more shelf-stable product which means that many of the processed food items that line grocery store shelves are likely to contain hydrogenated oils. Think about a no-stir peanut butter compared to a 'natural' peanut butter; the 'natural' peanut butter has oil separation in the jar because the oil from peanuts (a plant-based oil) contains unsaturated fatty acids, which means it is liquid at room temperature. Due to this oil separation, you need to stir the peanut butter yourself before using, and refrigerate after to prevent rancidity and further oil separation. However the no-stir peanut butter does not have this separation because those unsaturated fatty acids in the peanut oil have been partially hydrogenated into trans fatty acids so that they can mimic saturated fat, and therefore remain solid at room temperature.

So what kind of unsaturated fatty acids should I include?

The monounsaturated fatty acids (like in olives, avocado, and nuts) and essential fatty acids (Omega-3 and Omega-6) polyunsaturated fatty acids like those emphasized in the Mediterranean diet, have been associated with favorable effects, particularly for cancer and obesity. (Kwan, et al. 2017) The Mediterranean diet is characterized by a variety of vegetables, fruits, nuts, whole grains and fatty fish with low consumption of meat and dairy products. (Kwan, et al. 2017) Observational studies of Mediterranean populations have shown that, specifically in Crete, following this style of dietary intake (including olive oil as a main fat source, a variety of high fiber foods, and moderate amounts of red wine) is associated with the (comparatively) lowest morbidity and mortality rates from cancer. (Kwan, et al. 2017) Aside from the presence of antioxidants, fiber and micronutrients (vitamins and minerals) present in this diet, the oleic acid in olive oil (a monounsaturated fatty acid) can add additional anti-inflammatory benefits to Omega-3 fatty acids. (Kwan, et al. 2017)

Speaking of anti-inflammatory effects...

Omega-3 and Omega-6 fatty acids are actually precursors to eicosanoids, which are mediators of inflammation. (Kwan, et al. 2017) Omega-3 (found in fish, walnuts, and flaxseed) is specifically associated with decreasing inflammation and Omega-6 (found in seed oils like corn oil) associated with promoting inflammation, of which the latter is notably abundant in the current US food supply as linoleic acid. (Kwan, et al. 2017) Paleo-enthusiasts are familiar with the notion that a favorable ratio of Omega-6:Omega-3 is close to 3:1 but due to the heavy reliance on seed oils (corn oil, safflower, sunflower) in many food products, there is instead a skewed ratio closer to 20:1 in those eating a standard American diet. (Different sources will declare different ratios as being 'favorable' but the bottom line is: the more balanced this ratio is, the more favorable for overall health outcomes.)

Which brings us to one of the trending high-fat diets; the Paleo diet:

The first point to be made with regards to the Paleo diet is the fact that it is difficult to quantify or test, and so there is not a lot of solid evidence supporting it. The studies that have been conducted have been short duration with small sample sizes and where the associations were not strong, albeit the one study that is considered the 'strongest' showed no long term differences between the Paleo diet and the control diet. (Pitt 2016) In reality, the diet itself is extremely variable based on individual interpretations. Some of the hard-core Paleo enthusiasts may try to adopt a 'true' hunter-gatherer lifestyle by excluding any type of food that would not have existed during the Paleolithic era. Counter this exclusionary extreme with the trendy Paleo diet that includes bacon at every meal and 'Paleo-approved' meal bars and packaged goods. In either aspect, we may be getting further away from a balanced whole food dietary intake.

The primary vision behind the Paleo diet is noteworthy: increasing whole foods that are as close to their natural source as possible. Regardless of personal views, this idea is universally accepted as being a positive change in the diet (instead of having a juice box made with 'real fruit juice', just eat a real piece of fruit). Much of the focus is based on decreasing foods that have been processed or conventionally raised/grown, which would theoretically lead to an appropriate balance in the body (like the appropriate Omega 6:3 ratio). **I will address the big 'organic' question in another post

However the major concern that comes up with a Paleo diet is the extreme dietary restriction that comes from eliminating intake of whole grains, legumes, dairy, many fruits, caffeine, FODMAPs like nightshades (tomatoes, peppers, eggplant) and more. The argument in favor of these dietary eliminations stems from the idea that these foods can be associated with promoting inflammation and have been introduced to the human diet after the Paleolithic era with the theory that we are therefore unable to efficiently digest these foods based on our genetics.

**In my opinion, the premise that we are genetically identical to our Paleolithic ancestors and thus should only eat what they eat is a bit shortsighted. It should go without saying that being genetically identical does not take into account geographically-based evolutionary adaptations. There has been an extraordinary amount of evidence supporting the relationship with geographic location and subsequent genetic evolutionary adaptions.(Coop, et al. 2009) Differences in blood type, body structural characteristics, race, and notably an ability to efficiently use or tolerate certain foods can be associated to genetic differences arising from changes of geographic location throughout population histories. What does this mean? We are not all the same and we should not force ourselves to be the same as others, regardless of how similar our DNA is.

Another aspect of the Paleo diet that bears forewarning is a lack of quantifying or monitoring portions. The diet boasts free eating, or intuitive eating, where you are using internal hunger and satiety cues as portion control. Intuitive eating is a great practice of self-monitoring your own hunger and satiety signals so that you are eating when you are hungry and not eating when you are full. In theory, this is a healthy method of controlling dietary intake and is actually an innate quality as observed in young children. (Yes you heard that correct: we are born with an innate ability to self-regulate based on our energy and nutrient needs.) However, most adults have lost this ability to self-regulate and so to go straight into a high fat diet that is based on intuitive eating, there is high probability of overeating calories, leading to weight gain and other subsequent health consequences.

So what happens if you eat too much fat?

Dietary fat is a highly concentrated source of calories. It is also largely found in foods that are absent of fiber and water (lipids are hydrophobic) and so when you eat a food that is high in fat (like snacking on nuts), you are also NOT eating anything else that would 'fill you up'. While it does depend on the person, if you are 'snacking on nuts', you are most likely eating at least twice the serving size (1 serving=1/4 c of mixed nuts which is roughly 14g fat, 160 calories, whereas 1/2 c is roughly one handful, so by eating a handful of nuts, you are likely eating at least 2 servings, or 300 calories)

What happens with the excess calories from fat?

I had mentioned in the carbohydrate post that carbohydrates are broken down into monosaccharides (glucose) which are either used for energy or stored as glycogen in liver or skeletal muscle. Only if /when that glycogen storage capacity has been exceeded, then the excess glucose is converted into glycerol and combined with fatty acids in adipose tissue (fat cells). Dietary fats however are taken in as triglycerides, broken down in the GI tract, and packaged for travel (to first the liver, and then peripheral tissues that require fatty acids) or STORAGE in your adipose tissues (fat cells). (Gropper, et al. 2013) This means that when you eat dietary fat, it is not used for energy immediately, but rather sent to where it is required and otherwise stored in adipose tissue. Hence, an abundance (excess beyond your needs) will only increase storage in adipose cells (compared to carbohydrates that prefer glycogen as a storage form BEFORE being stored in adipose tissue).

*MCT (medium chain triglycerides) are an exception where they can be used as an immediate fuel source, but I will cover MCT in a future post

So why are high fat diets becoming popular now?

This may be due to a general attitude shift where now that fat has been widely accepted as an essential part of the diet, there may be a push to correct for that previous wrong-doing. There are many claims that high fat diets are superior in terms of health and weight loss compared to the previously pushed low fat diets; but this argument (in favor of high fat diets) is unconvincing due to a general lack of specific evidence naming fat content as the primary driver for health and weight loss. As stated previously, it is often the lifestyle that coincides with a particular diet that holds responsibility for associations with health changes, rather than one specific characteristic. **I will dive further into these diet trends like high fat diets for weight loss on the next post

And what about the newest high-fat diet trend: the ketogenic diet?

The ketogenic diet is another high-fat diet yet with more structure (compared to Paleo) as far as actually following (and quantifying adherence to) the diet. Ketones are products of uncontrolled fatty acid oxidation (during periods of starvation/impaired glucose utilization like diabetes) when blood glucose is low. Ketones are produced in order to provide a glucose-like substrate for the brain and other glucose-dependent cells to utilize while blood glucose is low. (Hence, this is a survival mechanism so that the brain and body can continue to function when glucose is not readily available) In a clinical setting, (urinary) ketosis, or the spilling of ketones over into the urine, is usually observed in cases of diabetes (an inability to utilize blood glucose) however being in a state of ketosis is the basis for which a ketogenic diet rests on. In regards to a ketogenic diet, the individual is eating so few carbohydrates (10% or less of total calories coming from carbohydrates) that their glucose availability decreases to the point that they force their body to utilize fatty acids for fuel. Many of us would like to picture this as fatty acids from (already stored) adipose tissue being used for energy which would lead to not only weight loss, but fat loss, right? Well here's the rub: by eating less carbohydrates and more fat (~70-75% of total daily calories from fat) the body is simply adapting to the energy that is being made available. In this case, there is less carbohydrate available but more DIETARY fat, so the body will utilize fat as a fuel source as the most efficient means of maintaining energy production (BASED ON WHAT IT IS BEING GIVEN). You will still be storing those dietary fats as triglycerides in adipose tissue and then hydrolyzing those fatty acids as needed. What does this mean? The body will adapt to using dietary fat as fuel, which will not necessarily equate to further loss of body fat (after initial weight loss from beginning the diet) but rather just a switch in energy utilization. i.e. NOT a long term fat loss plan, but just forcing your body to utilize what it is being given. *I will go into further detail on keto and weight loss on the next post

Here's another potential issue...

As has been widely observed and accepted in research, the ketogenic diet has been suggested to be a therapeutic diet for decreasing seizure frequency in cases of epilepsy.(Wijnen, et al. 2017) In some cases, it is proposed that the epileptic seizures are associated with hippocampal hyperexcitability (likely associated with impaired regulation of glucose utilization). (Wijnen, et al. 2017) This essentially means that there is an inability to regulate glucose utilization for energy (glucose is the primary fuel source for the brain). (Wijnen, et al. 2017) For cases like this, it is certainly noteworthy to instead use the only other alternative fuel that the brain can use, which are ketones. The brain CAN use ketones for energy, and it should in cases where starvation limits glucose availability (or where glucose utilization is impaired), thus providing an alternative source to maintain functioning. This is a survival mechanism though, it does not mean that the brain can function for extended periods of time (months or years) or thrive on ketones, particularly in a HEALTHY population (non-epileptic population). Unfortunately, there is still a lack of sufficient evidence to support brain ketone utilization in a healthy population for extended periods of time. **This is still an area of growing research

So how much dietary fat should I get in my diet?

Short answer: it depends.

The current RDA recommendations advise 20-35% of calories coming from fat with less than 10% from saturated fat. (Nix, 2013) If following a standard 2,000 calorie/day diet, that would equate to 44g-77g per day. As a reference, 2 Tbsp of peanut butter or 1 tbsp olive oil has about 14g fat. Your actual intake depends on your body composition, goals, health status, and activity level, but a good rule of thumb, in my opinion, would be the equivalent of your body weight in kilograms eaten as grams of fat per day. (This means that an adult who is 150lbs, or 68kg should try to get about 68g of fat per day) Again this is just a basic rule of thumb, but it typically falls near the higher end of that RDA range (the recommended 20-35% of calories from fat) which may suggest that (in my opinion) most of the population would likely fall closer to 30% of total daily calories coming from fat.

Take away from this what you will, but never stop learning.


Coop G, Pickrell J, Novembre J, et al. The Role of Geography in Human Adaptation. PLoS Genetics. 2009;

Gropper S, Smith J. Advanced Nutrition and Human Metabolism. 6th edition. Wadsworth-Cengage Learning. 2013.

Hooper L, Martin N, Abdelhamid A. Cochrane corner: what are the effects of reducing saturated fat intake on cardiovascular disease and mortality? British Cardiac Society: Heart. 2015; 101(24):

Kwan H, Chao X, Su T, et al. The anticancer and antiobesity effects of Mediterranean diet. Critical Reviews in Food Science and Nutrition. 2017; 57(1):

Lichtenstein A. Dietary Trans Fatty Acids and Cardiovascular Disease Risk: Past and Present. Current Atherosclerosis Reports. 2014; 16:433.

Nix S. Williams' Basic Nutrition and Diet Therapy. 14th edition. Elsevier Mosby. St. Louis, MO. 2013

Pitt C. Cutting through the Paleo hype: The evidence for the Paleolithic diet. Australian Family Physician. 2016; 45(1):35-38.

Wijnen B, Kinderen R, Lambrechts D, et al. Long-term clinical outcomes and economic evaluation of the ketogenic diet versus case as usual in children and adolescents with intractable epilepsy. Epilepsy Research. 2017; 132:91-99.

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