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Why low fat eating can make you fat

by Dr Rangan Chatterjee   /  June 5, 2015

Everyday in my clinical practice, I see countless patients trying to lose weight, trying to lower cholesterol and generally trying to “be healthy” by following LOW FAT diets. Unfortunately, this rarely works. Here’s why:

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To fully understand this phenomena, we have to go back to the 1970s and Dr. Keys’ Seven Countries Study, which showed an association between saturated fat intake and cardiovascular disease. He showed that countries who consumed more saturated fat had higher rates of heart disease. This has led to the low fat dietary gospel that has been handed out by the medical establishment to this day despite a multitude of evidence to the contrary.

The primary problem with this study is that correlation does not mean causation – an association is very different from saying that eating saturated fat CAUSES heart disease. For example, every time you are at an airport, you will see planes taking off. This is clearly not the same thing as saying you being at the airport is causing the planes to take off!

In fact, countless studies and reviews have cast doubt on the suggestion that dietary fat intake is strongly correlated with heart disease. This British Medical Journal editorial from 2013 states the same opinion: “It is time to bust the myth of the role saturated fat plays in heart disease”.

I think one of the main reasons that the low fat diet promoted over the past 30-40 years has not made a significant dent in the prevalence of heart disease is because of what we have replaced saturated fat with: refined carbohydrates and sugar. The evidence that refined carbohydrates and added sugar can be hugely detrimental for your health and drive “modern epidemics” such as Type 2 Diabetes is compelling.


First of all, it is important to understand that fat promotes satiety and a feeling of fullness. Therefore, when following low fat diets people tend to replace fat with refined carbohydrates and other starches. This leads to blood sugar imbalances and insulin spikes throughout the day, all of which lead to increased weight gain, hunger cravings and increased caloric intake.

I see many patients who have their “healthy” bowl of sugar-filled cereal or their two slices of toast before they leave for work in the morning. Three hours later, they’re sitting at their desk, they feel hungry and in need of a quick fix. They are forced to grab a quick sugar loaded snack to give them energy and the cycle continues all day long (think about the 3pm slump!). This is a classic example of the insulin highs and the blood sugar lows that occur with the typical low fat diet.

This blood sugar roller coaster can also apply when eating so-called healthy “zero percent fat” carbohydrates such as white rice and potatoes. Because these are low fat foods, patients think that they can eat them in abundance, However, if eaten in excess they cause the same problems with blood sugar as mentioned above.


It’s simple: it is not fat making you fat – it is sugar! I would urge you to start looking at fat and sugar in a different way. Think about the quality of the food that you are eating rather than concentrating solely on the fat content (or the calorie content) – more of that in a later blog!

When it comes to eating fat, concentrate on natural, healthy forms of fat such as avocados, nuts (choose almonds, walnuts, brazil nuts and cashews), fatty fish (like salmon, tuna, sardines and mackerel), extra-virgin olive oil, plant-based saturated fat such as coconut oil, grass fed organic meats and eggs.

However, please make sure that you avoid ALL trans fats – no exceptions! These kinds of fats are found in fast foods, snack food and margarines and are most definitely associated with an increased risk of cardiovascular disease.

As a society, we spend a lot of wasted energy worrying about the amount of saturated fat in our diet. We should be more concerned about avoiding the real culprits for bad heath – refined sugars and trans fats.

— Dr Chatterjee


DISCLAIMER: The content in this blog is not intended to constitute or be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your doctor or other qualified health care provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read in this blog or on this website.

Dr. Rangan Chatterjee MbChB, BSc (Hons), MRCP, MRCGP

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