Fat isn't the devil we grew up believing it was
Fat has spent the last many decades as the dietary villain. Here's what it actually does and why you need it in your diet.
For most of the 1990s and early 2000s, fat was the enemy. Low-fat yoghurt, biscuits, everything came in low-fat version in favour of high sugar. An entire generation grew up believing that the fat on your plate immediately became the fat on your body.
The idea stuck long enough to shape how most of us think about food, and some of us are still operating on that logic two decades later.
Why was fat demonised in the late 1900s?
Concern developed back in the 1970s and 1980s over the relationship between saturated fat, blood cholesterol and cardiovascular disease, which led to early public-health guidance encouraging people to reduce total fat as well as saturated fat. The first US dietary guidelines, published in 1980, advised people to avoid too much fat, saturated fat and cholesterol. That gave birth to the low-fat food boom that told us that a food was automatically healthy if it was low in fat. Manufacturers could remove fat, add refined starches or sugar to restore the texture and still give the packet a small halo.
Removing all fat from your diet was never the same thing as improving the quality of the fat you ate. Unfortunately, nuance is difficult to print in large letters on a yoghurt lid.
What dietary fat does to your body
Fat is a component of every cell membrane in the body that insulates nerve fibres, allowing electrical signals to travel efficiently. Fat is required for the absorption of vitamins A, D, E, and K, they will pass straight through the body unused if there's no fat to carry them. The body also uses fat for manufacturing hormones, testosterone and oestrogen among them.
Chronically low dietary fat intake is associated with reduced testosterone levels; someone eating 15% of their calories from fat because they still think of fat as something evil may be unknowingly suppressing the hormonal environment their body needs to recover, adapt, and sleep properly. The fatigue and sluggishness get blamed on work stress or poor sleep, and the diet doesn't get questioned because it looks clean.
How much dietary fat should you eat?
European reference values place the general adult range for dietary fat at approximately 20 to 35 percent of total energy intake. Sports nutrition guidance similarly emphasises that fat intake should be sufficient to provide essential fatty acids, support the absorption of fat-soluble vitamins and help athletes meet their total energy needs.
What is the “fat-burning zone”?
At rest and during low-intensity activity like walking, easy exercise or just pottering around, fat is the body's primary fuel source. The aerobic system breaks down stored fat into fatty acids and converts them to energy. This process is slower than burning carbohydrate but vastly more efficient in terms of supply: even a lean person carries tens of thousands of calories in stored body fat, compared to roughly 2,000 calories of stored glycogen.
This is why low-intensity activity feels sustainable for hours: the body is drawing from a fuel source it has in near-unlimited quantity. As intensity increases, the energy system shifts toward glycogen because it can be converted to energy faster, which is why hard exercise depletes carbohydrate stores quickly and why pre-exercise carbohydrate are important for longer, harder sessions.
Does a “Low carb, high fat” diet work for runners?
High-fat diets, ketogenic approaches, and an entire industry of people putting butter in their coffee have all found enthusiastic audiences. The logic sounds compelling: eat more fat, train the body to burn fat more efficiently, become a metabolically superior version of yourself.
The reality is that fat oxidation is a slow process. At any intensity above moderate exercise, the body needs glycogen. Fat simply cannot be metabolised quickly enough to sustain high-intensity output, dramatically slashing your carbohydrate intake in favour of fat often translates to your pace feeling fine but your ability to go harder disappears. The gears above easy running stop being available.
Low carb, high fat diets work wonders for some people, but if you’re a runner it is worth considering adding at least some extra carbs before, during and after your runs depending on the length and intensity. You can probably get away with a 30 minute easy run without adding carbs, but throw in intervals or run for 60 minutes and your run will feel horrific.
Dietary fat is important, but your diet still has to add up
Fat is not the villain we were sold in the 1990s, nor is it the metabolic messiah promised by the more excitable corners of the internet.
Your body uses fat to build cell membranes, absorb vitamins, produce signalling molecules and supply energy. Eating too little can make it harder to meet your nutritional and energy needs. Eating a lot can work perfectly well, but only if protein, carbohydrate, fibre and total calories still make sense.
