Why Very Low-Fat Diets Can Mess With Your Hormones

Steroid hormones, the family that includes testosterone, oestrogen, and cortisol, are all built from cholesterol, which your body makes and also gets from food. Dietary fat feeds into that supply chain. When fat intake is very low, you are not just cutting calories, you are trimming the raw materials your endocrine system uses to build the signals that run your reproductive system, your muscle, and your mood.

Fat does a second job that people forget. Vitamins A, D, E, and K are fat-soluble, meaning they need dietary fat present in the gut to be absorbed properly. Vitamin D in particular behaves more like a hormone than a vitamin and is tied to testosterone and bone health. Eat a salad with zero fat and you absorb far less of what is in it. So a near-zero-fat approach can create shortfalls that have nothing to do with how many calories you ate.

The clearest evidence is in men. A 2021 systematic review and meta-analysis pooled six controlled intervention studies with 206 men and compared their hormone levels on low-fat versus higher-fat diets. When men switched to low-fat eating, total testosterone fell by roughly 10 to 15 percent on average. The effect was consistent across the studies, and it reversed when fat came back up.

A 10 to 15 percent dip will not tank a healthy young man overnight, and this is not a licence to drown everything in oil. But it matters if you already sit at the low end of normal, if you are older, or if you are dieting hard for a long stretch and cutting fat to make the calories fit. Those are exactly the situations where a very low-fat plan can push a borderline number the wrong way.

In women the story is less about fat grams in isolation and more about total energy. When you consistently eat fewer calories than your body needs for training and daily life, you reach a state researchers call low energy availability. The body responds by dialling down functions it treats as non-essential in a shortage, and reproduction is near the top of that list. Periods can become irregular, lighter, or stop altogether.

This sits at the centre of what the International Olympic Committee now calls Relative Energy Deficiency in Sport (REDs). Their 2023 consensus statement describes how too little energy over time harms not just menstrual and reproductive function but bone health, immunity, and metabolism, in both women and men. Very low-fat diets often push people into this state because fat is calorie-dense, so cutting it is an easy way to accidentally under-eat by a large margin.

Official guidance from the Dietary Reference Intakes sets an acceptable range for fat at 20 to 35 percent of total calories. That range is where you get enough fat for hormones and vitamin absorption without crowding out other nutrients. Sitting near or below the bottom of it for a short, structured diet phase is fine for many people. Living well under it for months is where problems build up.

A useful second guardrail is grams per kilogram of body weight. Keeping fat at or above roughly 0.5 grams per kilogram stops most people from dropping into the danger zone even when calories are low. For a 70 kilogram person that is about 35 grams of fat a day as a floor, which is not much. The mistake is not eating a lower-fat meal, it is treating fat as something to eliminate.

You can run a fat-loss diet and still protect your hormones. The trick is to defend a fat floor first, then fit protein and carbs around it, rather than slashing fat to nothing to make the numbers work. Fat is easy to add back in small, dense amounts, so a little goes a long way.

1. Set your floor before anything else. Lock in at least 0.5 grams of fat per kilogram, or about 20 percent of your calories, whichever is higher. Build the rest of the diet on top of that.
2. Anchor each day with one or two fat sources. A drizzle of olive oil, half an avocado, a handful of nuts, oily fish, whole eggs, or tofu cooked in oil. These add up fast without much volume.
3. Do not fear fat-soluble vitamins. Add a little fat to vegetable-heavy meals so you actually absorb the vitamins A, E, and K they contain.
4. Watch total energy, not just fat. If your period changes, your libido drops, or your recovery falls apart, you are probably eating too little overall. Eat more, especially if you train hard.

None of this means high-fat is automatically better. Very high-fat diets do not push testosterone endlessly upward, and piling on oil just adds calories. The goal is a sensible middle, enough fat to keep the machinery running, with the rest of your plate doing its job.