The low-fat message has been with us for decades. The basic argument was that saturated fat raises cholesterol and therefore reducing saturated fat should reduce cardiovascular disease.

The biology, however, is considerably more complicated than eat fat, get fat arteries. This remains an area of considerable debate, and national guidelines continue to recommend limiting saturated fat. But the effects of individual foods cannot necessarily be predicted simply from their saturated-fat content.

For somebody following a very-low-carbohydrate diet, this becomes particularly important. Take away most of the carbohydrate and the energy has to come from somewhere. Protein has important structural jobs to do. That leaves fat as a major energy source.

So a low-carb diet cannot sensibly become a low-carb, low-fat diet indefinitely. That leaves remarkably little to eat. The important questions become the quality of the food, the type of fat, what that fat is replacing and how the individual responds.

Low fat written on a packet is not, by itself, evidence that the food inside is healthy. Again: Turn it over.

The attraction of a ketogenic lifestyle for many people is that protein and fat are satiating.

Hunger often changes. And when hunger changes, people may naturally eat less without spending the entire day deliberately trying to eat less. That is very different from being permanently hungry while attempting to obey the instruction: “Eat less and move more”.

Of course energy matters. The body cannot manufacture body tissue without energy, and it cannot ignore the laws of thermodynamics. But the body isn’t a passive calorie container either. It is a biological system. Reduce energy intake and the body can respond. Hunger can increase. Spontaneous movement can decrease. Energy expenditure can change. Hormones change. The body adapts.

This is why simply telling somebody who has struggled with their weight for years to eat less and move more isn’t terribly enlightening. They already know. It is rather like advising somebody with insomnia to sleep more.

The interesting question is why they are hungry, what determines their fuel availability and what dietary pattern allows them to eat appropriately without continually fighting their own appetite. The universe may be a closed energy system – as far as we know; nobody has peeped round the back yet. The human body isn’t isolated from its environment. It takes energy in, expends energy and continually adapts what it does with that energy.

Calories matter. But the biology controlling those calories matters too.

What exactly are we left with?

Sometimes quite a lot of processing. Removing both naturally occurring fat and sugar from a food and then rebuilding the taste and texture with sweeteners, starches, gums, flavourings and other ingredients doesn’t automatically produce something healthier.

Don’t judge a food by what has been removed. Judge it by what remains. Or leave it on the shelf.

This is scientifically meaningless.

Everything around us is made of chemicals. Water is a chemical. Oxygen is a chemical. You are an extremely complicated collection of chemicals. Trees, food, duvets, chainsaws and T-shirts are all made of chemicals.

The closest you will get to genuinely chemical-free is a perfect vacuum, and I don’t recommend eating one.

So chemical-free tells us virtually nothing about whether a food is healthy.

Having said that, the manufacturer may genuinely be trying to say that the product contains fewer additives or is less processed. Fine. Ignore the slogan and check the ingredients.

“You need sugar for energy”…

We hear this surprisingly often. Yes, glucose is an important fuel. But it isn’t our only fuel. Humans are multi-fuelled. We can burn glucose. We can burn fatty acids. And when carbohydrate availability is sufficiently low, the liver can turn fat into ketones, which become another important fuel. Our glucose reserves are relatively small. Most is stored as glycogen in the liver and muscles.

Our fat reserves are enormous by comparison. Even a relatively lean person carries tens of thousands of calories of energy as body fat.

Think of it as the body’s large internal battery. Glycogen is the smaller, rapidly accessible battery. Which one we draw upon depends partly on what we eat, what we are doing and the hormonal environment.

Someone accustomed to a high-carbohydrate diet will rely heavily on carbohydrate during prolonged strenuous exercise and may benefit from replacing carbohydrate during long-duration events.

Someone who is well adapted to a low-carbohydrate diet can obtain a much greater proportion of their energy from fat.

The body changes according to the prevailing conditions. It shifts gears.

There is, of course, one enormously important exception for somebody with Type 1.

Hypoglycaemia.
If your blood glucose is dangerously low, fast-acting carbohydrate isn’t an optional energy food. It is treatment. That is a completely different situation.

So the blanket statement you need to eat sugar for energy isn’t correct. We need energy. Sugar is merely one way of supplying it.

Five a day has become so familiar that it sounds like a biological law.

It isn’t. There is nothing magical that happens when vegetable number five passes your lips. Fruit and vegetables are an enormously diverse group of foods. Spinach and mango have relatively little in common metabolically, yet they happily sit together under the same fruit and veg banner.