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The Skinny on Fats Martha Mejia, MD

Updated: Nov 14, 2019

Is butter really back? Can we eat bacon with abandon? These were the headlines in the popular media a few years ago—based upon a meta-analysis from 2014. These stories, with their click-bait titles, can be confusing and misleading. Let’s delve a bit deeper into the story about fat and clear up some of the confusion.

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 growing 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 cardiovascular 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 (Mediterranean countries), the heart attack rate was low. Still more interesting, he found that the people of Crete had the lowest CVD rate of all, despite a diet high in fat. The distinction was that their diet was high in polyunsaturated fats, 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:

  • 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.

  • Cell membrane function: Fat is the major constituent of the membrane that surrounds each cell of the body.

  • Energy production: Fat is the most efficient source of food and energy.

  • Hormone production: Fat is part of the prostaglandins that regulate many bodily functions. These substances also regulate sex hormones and are critical for fertility and reproduction.

  • Nutrient absorption: Fat is necessary for our intestines to absorb the fat-soluble vitamins A, D, E and K, as well as minerals.

  • 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.

  • Skin health: Healthy skin requires healthy fats, particularly essential fatty acids.

  • Inflammation: Fats are needed to produce hormones that regulate inflammation.

  • Blood Clotting: Fats are essential for clotting cascade.

  • 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 fats (typically solid at room temperature)

2.   Unsaturated fats

  • Monounsaturated: Food sources include olive oil, avocado oil, nuts, seeds and olives

  • Polyunsaturated: There are two primary types: Omega-3 (salmon, walnuts, flax seeds, etc.) and Omega-6 (processed foods and vegetable oils)

3.   Trans fats: Also known as partially hydrogenated oils. Found in commercially processed foods and solid margarines.

All foods contain a mix of fat types, but one type usually predominates. Click here to learn more about the various types of fats.


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


  • It is not the total fat that matters but rather the type of fat that one consumes. Not all fats are created equal.The healthiest fats to least healthy fats to eat are:  Seafood Omega-3 Fats—> Plant Omega-3 Fats —> Plant Omega-6 Fats—> Monounsaturated Fats —> Saturated Fats —> Trans Fats. Omega-3 fats, derived from marine sources such as fish and algae are the healthiest. The richest sources of Omega-3 fats are in fatty fish (think SMASH: salmon, mackerel, anchovies, sardines, herring). Next are the Omega-3 fats derived from plants: chia seeds, hemp seeds, algal oil, flax seeds, leafy greens, beans and cabbages. Then, plant Omega-6 fats found in corn, soy, safflower and sunflower oils. Next in line are monounsaturated fats such as nuts, avocados, olives and olive oil. Saturated fats, which are found mostly in animal foods such as cream, cheese, milk, butter and fatty meats, follow. If you choose to eat red meat, eat it in moderation and source it from grass-fed and grass-finished animals. At the bottom of the barrel are trans fats (also known as partially hydrogenated oils). They should be avoided because they are highly toxic. Trans fats are found in processed foods.

  • All fats can make you fat if too many calories are consumed. They all contain 120 kcal per tablespoon.

  • Eat fats mostly from plants and fewer fats from meat and dairy foods. Fats that are in liquid form (oils) at room temperature are unsaturated and plant-based.

  • Do not exclude fats when you are cooking because they are necessary to help transport needed vitamins and minerals to our cells and tissues. For example, when roasting vegetables, include a few tablespoons or more of olive oil to assure that the healthy nutrients are transported properly in the body. 

  • Keep intake of saturated fats to a minimum. Replacing saturated fats in the diet with polyunsaturated fats and whole grains will reduce your risk for heart disease. However, replacing saturated fats with refined carbohydrates (sugar and refined starches) and trans fats will increase your overall risk for heart disease. 

  • Low fat does NOT necessarily mean healthy. Often, if a food item is low in fat, it is also high in sugar, which helps make the food taste good.

  • Minimize the ingestion of fried foods, especially from fast food establishments. If you desire something fried, fry it yourself at home. Realize that when oils are heated to high temperatures their fat structure changes in unhealthy ways.

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


Martha Mejia, MD

Martha Mejia's love for learning about nutrition and bringing that information to the fore when treating patients has been a mainstay of her medical practice. It is her passion to disseminate information not only in a doctor-patient relationship but also in articles, group discussions and lectures. She is dedicated to providing comprehensive, up-to-date and holistic care, adding a more natural and nutritional approach to conventional medical treatments. Viewing and treating each person as an individual is her main priority.

Martha graduated from the Stanford University School of Medicine and completed a residency in Internal Medicine at Santa Clara Valley Medical Center followed by a Fellowship in Nephrology at Stanford. She has worked in urgent care and primary care clinics. Recently, she joined the Sequoia Medical Group after 20 years in internal medicine private practice.

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