You Shall Not Take Your Body's Chemistry In Vain

The reason most diets and weight loss programmes succeed spectacularly for a fortnight and then fail is that they ignore your biochemistry. Let me give you the clearest example I know.

It is standard practice among promoters of weight loss cures and “detox” programmes to open with a very low calorie phase. The HCG diet, for instance, uses roughly 500 calories a day for the first three to five weeks. Most popular diets include some form of two-week kick-start during which you will lose 10 to 15 pounds. And I understand the appeal — a pound lost is a pound lost.

Or is it?

A tale of glycogen and water

Spoiler: after the next few paragraphs you are likely to become considerably more sceptical of any diet promising rapid, double-digit results.

First, glucose is the preferred fuel of the brain and most muscle. Second, the body stores several days’ worth of it as glycogen, deposited in muscle and liver whenever intake is adequate. When we sleep, fast, or eat significantly less than we need, glycogen is broken back down into glucose. This is why your blood sugar does not collapse overnight despite twelve hours without food.

Now suppose a 200 lb woman begins the HCG diet. Eating 500 calories a day leaves her more than 1,000 calories short of what she needs, so her glycogen is drawn down hard over the first two or three days and largely depleted.

Do the arithmetic and she can lose 12 to 15 lb in the first few days. Impressive — except that none of it is fat. What she lost was 3 lb of glycogen and 9 to 12 lb of the water that was stored with it.

The scale moved. Her body composition barely did. And the moment she eats normally again, the water comes straight back — which she will experience as failure.

Does it really matter?

You could argue that it does not — that if the early result motivates someone to continue, the mechanism is beside the point. I disagree, for three reasons.

It teaches the wrong lesson. A person who loses 14 lb in ten days learns that this is the achievable rate. When the true rate arrives — 1 to 2 lb a week — it feels like the diet stopped working, and that is precisely when people quit.

It costs muscle. Very low calorie intake without adequate protein and resistance training causes substantial loss of lean tissue alongside fat. Since muscle is metabolically active, losing it lowers the rate at which you burn energy, making the weight easier to regain.

And your body fights back harder than the arithmetic predicts. This is the part we understand far better than we did a decade ago.

What we have learned since: metabolic adaptation

The most instructive data came from an unlikely place. Researchers followed contestants from The Biggest Loser for six years after the competition. Participants had lost an average of 128 lb during the show. Six years later, most had regained a substantial portion — but the striking finding was metabolic.

Their resting metabolic rate remained suppressed by an average of roughly 500 calories a day below what their body size predicted, six years on. The adaptation did not fade with time; in several participants it deepened.¹ Their bodies were, in effect, defending the weight they had lost.

Careful metabolic ward work has since confirmed the broader principle: the body responds to energy restriction with coordinated changes in hormones and energy expenditure that persist long after the diet ends.² Leptin falls, ghrelin rises, and both stay shifted for a year or more.³

None of this means weight loss is futile. It means the aggressive approach is not merely ineffective — it is counterproductive, because the harder you push, the harder the counter-regulation. A slower deficit, adequate protein and resistance training preserve more lean mass and provoke less adaptation.

The practical version

• Expect the first week to lie to you. Some of that early drop is glycogen and water in both directions.
• Judge progress over a month, not a day. Weigh at the same time, under the same conditions, and look at the trend.
• Eat enough protein — roughly 1.2 to 1.6 g per kg of body weight when in a deficit — and lift something heavy twice a week. This is the best-established way to protect lean mass.
• Be suspicious of any programme promising more than about 2 lb a week beyond the first fortnight. It is either not telling you what you are losing, or it is not sustainable.

Respect the chemistry and it will work with you. Take it in vain and it will win, because it has several hundred thousand years of practice.