Fat as fuel

In a recent post I described the science behind a diet that is high in fat and low in carbohydrates (a ketogenic diet), and explained why the establishment’s high-carbohydrate low-fat dogma is neither good for us nor supported by science.

An example of a low-carbohydrate high-fat (LCHF) ketogenic diet would be one in which about 70% of energy needs (calories) came from fat, 20% from protein and 10% from carbohydrates. When such a diet is followed for 2-4 weeks, the body becomes trained to recognise fat as its main fuel source, rather than glucose (from carbohydrates). Thereafter, the body will deplete its fat stores for energy as needed. This is how a high-fat diet can, perhaps paradoxically, result in long-term weight loss.

The steps to adapt to LCHF

There are 3 steps to LCHF – keto-adaptation, weight loss and weight maintenance.

The initial adaptation step is when the body learns to switch from glucose to fat as its go-to fuel. This switch is known as keto-adaptation because the fats are broken down to release ketones that are used as fuel in place of glucose. Particularly by the brain – from now on the brain will run preferentially on ketones rather than on glucose (the brain cannot use fat as fuel because fatty acids cannot cross the blood-brain barrier). In a separate post, I will explain some of the benefits the brain might get from this ketone conversion.

adaptDuring the weight loss step, total calories are kept below energy demand. However, there is no sense of low energy levels or hunger. This is because the body makes up for the calorie shortfall by burning its fat stores, and because fats are satiating to eat. The body is getting all the calories it needs.

In the life-long maintenance step, calories are adjusted back up to meet energy demand and stabilise weight. Hunger signals will do this as fat reserves decline – total calories do not need to be counted.

The elegance of LCHF is that during the weight-loss step there is no hunger because there is no total calorie shortfall. There might even be an associated loss of appetite. As fat stores are depleted, the body increases appetite to acquire the calories it needs. All the dieter need do throughout is eat when hungry and monitor weight.

Be aware that bodyweight fluctuates by about ±1 kg due to random daily changes in water content. The body has a relaxed tolerance for water – only increasing micturition when water content has increased by ~1 litre (1kg) and only signalling thirst when it drops below 1 litre. So, while weight is best monitored daily, it is the underlying trend that matters – a suggestion is to average daily weight weekly and monitor that.

How does protein fit in?

Often, a high-fat diet is automatically thought of as high-protein. But not so for LCHF. It might be better to call it ‘high-fat, adequate-protein, low-carbohydrate’, because excess protein works against the diet. The body does not store protein, so what to do when there is an excess? One thing the body can do is convert the protein into glucose, thus undermining the adaptation phase that is getting the body off glucose and onto fat for its main fuel. Excess protein does not drive protein-synthesis, e.g. muscle building (only exercise can do that). There is nothing to be gained from high protein eating and it might have social consequences, as bodybuilders can experience.

403c8b81a137c4424e464840a7c520a0Bodybuilders often load up on protein and severely restrict carbohydrates in the lead up to a competition. Because their bodies are still glucose-dependent for energy, the overload of protein gets converted into glucose to meet their energy demands (so they might as well have eaten it in the first place). Glucose is made up of carbon, hydrogen and oxygen, whereas proteins also contain sulphur and nitrogen, so these are left over from the conversion. The sulphur is released in the form of sulphur dioxide gas (rotten-egg smell) and the nitrogen is converted to ammonia and excreted. These side effects give bodybuilders their reputation for a certain pre-competition miasma, and are signposts that protein-glucose conversion is underway.

Which just underscores our glucose dependence. Cells have the metabolic pathways to enable them to burn amino acids for fuel, but they cannot do so because they have adapted to glucose dependency. The only way to burn amino acids is to convert them to glucose first.

Are all fats equal?

With fats taking up such a large fraction of the diet, it is important to consider the type of fat. We store fats for fuel in about these proportions: mono-unsaturated (55%); saturated (30%) and poly-unsaturated (15%). It makes sense to eat fats in these proportions if we want to burn them as fuel. You can’t go wrong with animal fats – lard, tallow, chicken fat, duck fat. Animal fats contain fat-type ratios that are comparable to ours. Butter is good.

burn-fat-fast1Note that polyunsaturates are our least favoured, going against dietary advice to eat mainly polyunsaturated vegetable (plant) oils. We need some poly-unsaturated fats to get the omega-6 and omega-3 essential fatty acids, however we do not need much of these (about 1% of total fats). In a previous post I explained why refined vegetable (plant) oils should be avoided with extreme prejudice.

Extra virgin olive oil (EVOO) is an ideal addition for its monounsaturates. Combining an equal proportion of butter and EVOO is a good match for our body-fat composition. Coconut oil (cold-pressed) is a good source of saturated fats made up of medium-chain triglycerides (MCTs), which are more ketogenic than the long-chain triglycerides (LCTs) found in most fats. Other cold-pressed oils (avocado, some nut oils) are also recommended.


HFLC is not a zero carbohydrate diet. Even though carbohydrates themselves are not an essential nutrient and could be omitted. Rather, plants are Brussels sprouts - thumb IIeaten for their co-nutrients: vitamins, minerals, trace elements, anti-oxidants and other phytochemicals. The aim is to eat low-carbohydrate plants that have these nutrients. As a rule of thumb, nearly all vegetables that grow above the ground are low-carbohydrate. Exceptions are anything sweet, such as sweetcorn or sweet peas. Leafy green vegetables are an excellent choice, as is anything from the brassicas (cabbage, brussels sprouts, cauliflower, broccoli, kale etc).

Tubers, roots etc, that grow below the ground, such as potatoes and carrots, are high in starchy carbohydrates or sugars and should be minimised. An exception is the alliums (onions, garlic etc).

Sweet fruit should be eaten sparingly (there are no nutrients in fruit that cannot be found in vegetables), although berries (including strawberries) are at the lower end of the scale and fine in moderation. Nuts are fine (except for peanuts, which are legumes anyway). Mushrooms are excellent (as they are not a plant, but a member of the fungi kingdom).

Milk and milk products contain lactose (a sugar). However, they are high in valuable nutrients and should be a part of the diet. Look for anything full fat, double cream etc. Cheese is good because it is usually lower in lactose, and there is almost none in hard cheeses. Fermented milk products (yoghurt, real buttermilk) are a good choice because the lactose has been fermented into lactic acid.

For a more comprehensive list and description see here.


A 70:20:10 calorie ratio for fat:protein:carbohydrate equates to about a 55:30:15 ratio by weight, allowing for the higher caloric content of fat. A typical diet would contain ~50-100g of carbohydrate, aiming for the lower limit to start with. I provide these data for completeness, however, such calculations are challenging, tedious and imperfect. In practice, I don’t weigh or calculate food, rather I add fat generously to whatever I am cooking (that’s the fun part), cook with full-fat ingredients, have one large meal a day (to replenish fat stores), strictly limit starchy carbohydrates (and sugars) and eat when hungry. Regular eating schedules (such as breakfast, lunch and dinner with snacks in between) are only necessary with a high-carbohydrate diet because of the glucose cycles. With HFLC, you are carrying the fuel you need with you (in fat stores), and these can be replenished (in the maintenance phase of the diet) whenever it suits.

Other important health reasons to consider LCHF

This post has introduced LCHF from a weight-loss angle. However, people who do not need to lose weight should consider a switch to LCHF too. The potential benefits of a ketone rather than a glucose metabolism are numerous and relevant for most of the diseases of modern living that are on the rise, such as type 2 diabetes, heart disease, some cancers and perhaps even neurodegenerative diseases such as Alzheimer’s disease (which I have recently seen described as Type 3 diabetes). It will be chilling if decades of the high-carb low-fat ‘Dietary Guidelines for Americans’ turns out to underly the increase in these diseases. The research is taking off and the advantage of keto-adaptation is looking increasingly compelling to me.

Being keto-adapted is not a hardship – many people report a sense of euphoria because the body and the brain are finally getting what they need. HFLC eating is luxurious eating.

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For more information, visit my KETO-DIET Page