Our knowledge about food and nutrition continues to grow, reflecting an evolving body of evidence. We are far from reaching a definitive conclusion regarding the amount of fat intake that is optimal. However, there is more clarity as to the type of fat that is healthy.

For many years, an anti-fat bias was entrenched in our thinking. The low-fat message helped usher in the obesity epidemic as manufacturers replaced fats with processed carbohydrates in many foods. The consumption of a diet heavy in starch leads to surges in insulin, storage of body fat and blood glucose fluctuations, which compel further sugar cravings and hunger. Long-term, this dietary pattern can lead to obesity and its dangerous sequelae of diabetes, high blood pressure, cardiovascular disease, fatty liver and even cancer.

It is difficult to pinpoint when fat became the enemy on our plates.

In the 1940’s, American physiologist Ancel Keys began to study the effects of dietary fat intake on cardiovascular disease. He launched the Seven Countries Study, which pointed out the relationship between dietary patterns and prevalence of cardio-vascular disease (CVD) in Greece, Italy, Spain, South Africa, Japan, Finland and the US. Keys concluded that populations consuming large amounts of dietary fats had the highest cholesterol levels and the highest rates of CVD. Conversely, in cultures where diet was based on fresh fruits, vegetables, fish, whole grains and olive oil the heart attack rate was rare (Mediterranean countries).  Crete, although eating large amounts of fat had the lowest CVD rate of all.  Most of the fat consumed is polyunsaturated fat (like fish, olive oil, nuts and seeds). The story of dietary fat continues to evolve and is complex.

Understanding Fats

All fats are not created equal; there are good fats and bad fats. The truth is that good fats are not only beneficial to your health, but they are necessary and essential for life.

Healthy fats are vital for many body functions, including:

  1. Brain function: Fat provides the structural components for the cell membranes in the brain and myelin (fatty sheath surrounding axon of nerve cells). The brain’s composition is 60% fat.
  2. Cell membrane function: Fat is the major constituent of the membrane that surrounds each cell of the body.
  3. Energy production: Fat is the most efficient source of food and energy.
  4. Hormone production: Fat is part of the prostaglandins that regulate many bodily functions. These substances also regulate sex hormones and are important for fertility and reproduction.
  5. Nutrient absorption: Fat is necessary for our intestines to absorb the fat-soluble vitamins A, D, E and K, as well as minerals.
  6. Organ protection and function: Fat provides a cushion for our vital organs and it provides the essential fatty acids necessary for many organs to function optimally.
  7. Skin health: Healthy skin requires healthy fats, particularly essential fatty acids.
  8. Inflammation: Fats are needed to produce hormones that regulate inflammation.
  9. Blood Clotting: Fats are essential for clotting cascade.
  10. Maintain body temperature: Fats provide insulation, which helps to maintain body temperature.

Types of Fat

All fats have the same basic chemical structure, yet each varies by how many hydrogen atoms and double bonds it holds. The shape of the carbon chain helps determine the properties of the fat. Slight differences in structure can lead to crucial differences in function.

The three types of fats are:

  1. Saturated fat
  2. Unsaturated fat
    • Monounsaturated fat
    • Polyunsaturated fat
  3. Trans fat

All foods contain a mix of fat types, but one type usually predominates.

Saturated Fats

There are physically solid at room temperature.

Food sources include butter, lard, bacon grease, dairy fats, animal foods and coconut oil.

Saturated fats raise the total blood cholesterol by raising harmful LDL, but they also raise beneficial HDL. Researchers dispute how helpful this increase in HDL is.

Unsaturated Fats: Monounsaturated

Monounsaturated (MUFA)

Food sources include olive oil, avocado oil, nuts, seeds and olives.

MUFAs decrease LDL (bad cholesterol) and maintain HDL (good cholesterol), which benefits heart health.

Unsaturated Fats: Polyunsaturated (PUFA)

Food sources include fish oils and vegetable oils.

There are two primary types of polyunsaturated fatty acids:

  1. Omega-3
  2. Omega-6

These are the essential fatty acids (EFA) — meaning our bodies cannot make them and they must be consumed in the diet.

While both Omega-3 and Omega-6 fats are essential to life, a healthy diet should contain a balanced ratio of both. The optimal ratio has not yet been determined; however, it is believed a ratio of 1-4:1 Omega-6 to Omega-3 is ideal. The typical Western diet is 20:1. Our diet’s higher ratios of Omega-6 to Omega-3 promote chronic disease.

Omega-3 Fats

There are three types of Omega-3 Fats:

  1. Alpha-Linolenic acid (ALA)
  2. Eicosapentaenoic Acid (EPA)
  3. Docosahexaenoic Acid (DHA)

ALA is found in plant oils, walnuts, flax seeds and chia seeds. EPA and DHA are found primarily in fish and algae and are known as marine oils.

Although the body uses ALA for energy, it is also converted to EPA and DHA although this conversion is variable and inefficient.

Polyunsaturated fats trigger mechanisms in the liver for removing cholesterol from the body. The Omega-3 fats promote healthy heart rhythms, prevent heart attacks and lower triglyceride levels and increase HDL. They are also anti-inflammatory.

Omega-6 Fats

  1. Linoleic Acid (LA)

The Omega 6 fats are a family of both pro-inflammatory and anti-inflammatory polyunsaturated fatty acids.  Their biological effects are to initiate the inflammatory response and alert the immune system of cell damage.  They subsequently promote production of anti-inflammatory molecules during the resolution phase of inflammation.  A high intake of Omega 6 and low intake of Omega 3 fatty acids causes a shift to a more inflammatory state.  Inflammation is a protective mechanism but when prolonged and uncontrolled, it is associated with many diseases (CVD, cancer, arthritis, diabetes, etc.)  Omega-6 fats are primarily found in processed foods and in some vegetable oils (soybean, peanut, safflower, corn, sunflower).

Trans Fats

Trans fats are also known as partially hydrogenated oils and are found in solid margarines, vegetable shortening and commercially processed foods.

The food industry creates trans fats through a hydrogenation process, which turns oils into solids, to prevent foods from going rancid and prolong their shelf life. These fats do not meet the distinction of being “generally recognized as safe” for human consumption. They are highly toxic and are being phased out and banned in our food supply.

Trans fats

  • Raise LDL and inflammation
  • Lower HDL
  • Increase risk for cardiovascular disease and chronic diseases


Focus on eating food, rather than “nutrients.”  Some general rules to guide your consumption:

  1. All fats can make you fat if too many calories are consumed. They all contain 120 kcal per tablespoon.
  2. Healthy (good) fats contain essential fatty acids, no cholesterol and are low in saturated fat content.
  3. Fats that are in liquid form (oils) at room temperature are unsaturated and plant based.
  4. Replace saturated fat in the diet with polyunsaturated oils and whole grains.
  5. Limit saturated fat ingestion to 10% of your daily caloric intake.
  6. Avoid trans fats.

Eating whole foods that are unprocessed and in their natural form brings the most health benefits. Eat an abundance of plants with plenty of fruits, vegetables, whole grains, nuts, seeds and olive oil in the style of the Mediterranean diet.

Stay well, my friends,

Martha Mejia, MD


Ludwig, MD, PhD, Always Hungry, https://www.amazon.com/Always-Hungry-Conquer-Cravings-Permanently/dp/1409158853/ref=sr_1_2?s=books&ie=UTF8&qid=1500585365&sr=1-2&keywords=always+hungry+by+dr.+david+ludwig

Dr. David Brownstein and Sheryl Shenefelt, The Skinny on Fats, https://www.amazon.com/Skinny-David-Brownstein-Sheryl-Shenefelt/dp/0984086927

Siri-Tarino PW1Sun QHu FBKrauss R. Meta-analysis of prospective cohort studies evaluating the association of saturated fat with cardiovascular disease. American Journal of Clinical Nutrition. 2010 Mar:91(3):535-546. https://www.ncbi.nlm.nih.gov/pubmed/20071648

Howard, BV et al. “Low-fat dietary pattern and risk of cardiovascular disease: the Women’s Health Initiative Randomized Controlled Dietary Modification Trial. JAMA, 2006 Feb 8;295(6):655-66. https://www.ncbi.nlm.nih.gov/pubmed/16467234

Hamley, Steven. “The effect of replacing saturated fat with mostly n-6 polyunsaturated fat on coronary heart disease: a meta-analysis of randomised controlled trials.” Nutrition Journal. 2017 May 19;16 (1):30. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5437600/

American Heart Association- The Skinny on Fats: Saturated, Unsaturated and Trans Fats. http://www.heart.org/HEARTORG/Conditions/Cholesterol/PreventionTreatmentofHighCholesterol/The-Skinny-on-Fats_UCM_305628_Article.jsp#.WXEiAtMrK8p

Choose Unsaturated Fats for Heart Health, Harvard Public Health. https://soundcloud.com/harvardpublichealth

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