Crash diet: what losing weight too fast does to your blood values

A crash diet produces a lot of weight loss in a short time and, alongside it, a predictable list of problems: gallstones, muscle loss, deficiencies and fatigue. What most pages leave out is that exactly the same approach, under medical supervision, produced some of the best results in the diabetes literature.

The difference is not the speed. It is what gets measured along the way.

That makes the question more interesting than the standard warning suggests.

What is a crash diet exactly?

An eating pattern with a very low calorie count, far below what your body burns at rest, sustained over weeks. In the scientific literature it is called a very-low-calorie diet. What separates it from simply eating less is the size of the deficit, not the method.

What the Dutch Voedingscentrum calls a healthy eating pattern looks nothing like it.

Interestingly, many GLP-1 users end up in the same territory without meaning to. When your appetite disappears, your intake can fall to crash-diet levels while you are simply less hungry. Everything below applies just as much.

What happens inside your body?

Your body switches to reserves and starts economising at the same time. In the first days you lose mostly water and glycogen. Fat tissue follows, but with a large deficit muscle tissue is broken down for energy too. Your resting metabolism drops along with it.

That last row surprises most people, because the effect only shows months after the cause. Exactly how that delay runs is covered in weight loss and hair loss.

How large is the gallstone risk?

More than three times larger than with steadier weight loss. In a Swedish study of 6,640 matched participants, people on a very-low-calorie diet developed symptomatic gallstones at 152 cases per 10,000 person-years, against 44 on an ordinary low-calorie diet (Johansson et al., 2014).

Of those gallstone cases, 61% ended in surgery.

The mechanism is well understood: with little fat in your food your gallbladder contracts less often, so bile sits and thickens. The weight loss itself, incidentally, was only 3 kilos greater in that study, 11.1 against 8.1 kilos. A modest gain against a tripled risk.

Gallstones are also one of the leading causes of pancreatitis, which is the link to elevated lipase.

Which deficiencies appear first?

The stores you were already short on. Iron and vitamin B12 are the most common in the Netherlands, and they run down slowly rather than suddenly. Ferritin falls before your haemoglobin does, which means your reserve is already shrinking before there is any anaemia.

Which is exactly why ferritin is the more useful value.

Alongside it, potassium deserves attention during very fast weight loss, and albumin says something about your protein status over the longer term. The Voedingscentrum notes that with a severely restricted eating pattern the chance of deficiencies grows the longer you keep it up.

So why did the crash diet in DiRECT work?

Because it happened under supervision and things were measured along the way. In the DiRECT trial, participants with type 2 diabetes received a total diet replacement of 825 to 853 kcal a day for twelve to twenty weeks. After a year, 46% of them were in remission, against 4% in the control group (Lean et al., 2018).

That is a crash diet in everything but name.

The difference from a self-designed crash diet sits in three things: their diabetes and blood pressure medication was withdrawn beforehand, the nutrition was fully formulated so deficiencies did not arise, and there was structured support during the reintroduction of ordinary food.

The chance of remission also tracked the loss directly: 7% with a small loss, 34% at 5 to 10 kilos, 57% at 10 to 15 kilos and 86% at 15 kilos or more.

What this means if you have type 2 diabetes yourself is covered in losing weight with type 2 diabetes.

Which values should you follow?

The ones that shift quietly before you notice anything. Fatigue during weight loss is almost always put down to the calorie deficit, while a draining iron store produces exactly the same picture. Without a blood test those two cannot be told apart.

That distinction is the entire point of measuring.

Two people are both twelve kilos down after three months and both exhausted. The first has a ferritin of 90 micrograms per litre, the second 11. For the first, the fatigue belongs to the energy shortfall and lifts once eating normalises. For the second, the iron store is empty and nothing changes while it stays that way.

The GLP-1 three-month check catches that moment, when the first deficiencies start to show and your sugar values are finding their new level. A BIG-registered doctor assesses the result.

With persistent severe pain in the upper right abdomen, yellowing of your skin or the whites of your eyes, or sudden nausea with fever, contact your GP the same day. Thuisarts describes when abdominal complaints need urgent assessment.

What I think about it

That the argument about speed largely misses the point. DiRECT shows that a large calorie deficit under supervision produces some of the best outcomes in this field. The same deficit without supervision produces gallstones and deficiencies that nobody notices.

According to the Dutch RIVM, roughly half of Dutch adults are overweight, so this is not a fringe question.

What strikes me is that the eight best-ranking Dutch pages on this topic all list risks and not one of them names a single measurable value.

If you are losing weight fast, whether through a diet or through a GLP-1 drug, have your iron, potassium and liver values measured around the three-month mark. That is when the quiet shifts become visible.

References

• Johansson K et al. Risk of symptomatic gallstones and cholecystectomy after a very-low-calorie diet or low-calorie diet in a commercial weight loss program. Int J Obes (Lond). 2014. PMID 23736359
• Lean MEJ et al. Primary care-led weight management for remission of type 2 diabetes (DiRECT): an open-label, cluster-randomised trial. Lancet. 2018. PMID 29221645
• Chien Yin GO et al. Telogen Effluvium: a review of the science and current obstacles. J Dermatol Sci. 2021. PMID 33541773
• Voedingscentrum, information on healthy weight loss and very-low-calorie diets.
• RIVM, figures on overweight in the Netherlands.
• Thuisarts, information on abdominal pain and when to make contact.

Every blood test result includes a professional assessment from a BIG-registered doctor. Talk to your GP before starting a severely restricted eating pattern, particularly if you take medication.