Weight loss is big business, and looking at health trends, it will continue to be in the future too. People want weight loss fast, and people are more than happy to promise it to others. We had a reader contact us a few weeks ago asking how much weight loss is realistically possible in a week after reading an article in The Sun.

The article claimed that someone was able to lose 1 stone (6.35kg) in a week by only eating 400kcals/ day. Fortunately our reader wasn’t the typical ‘The Sun reader’, and questioned it. So, for a bit of fun, I thought I’d do some quick calculations to see how plausible this weight loss is, and then have a look at what is actually a realistic expectation for weight loss.


1 stone in 1 week

Lets start with the assumption that all the weight lost is pure fat, and not muscle (or brain matter…). Fat contains about 10% water by weight, so of the 6.35kg she lost, 5.72kg was fat and 0.64kg was water. Fat contains roughly 9 calories per gram, so the amount of calories she lost is weight was 51,480 calories, which represents 7,254 calories a day!

This person is only eating 400kcals a day, and the guideline requirements are 2000kcals for women. This 2000kcals represents the amount of calories the average woman needs to maintain healthy weight, so she is living on a deficit of around 1600kcals as day. This represents just under 200g of fat tissue (including water weight), over a week, this would represent 1.4kg of weight. Some how she has lost an extra 5kg of fat. She somehow needs to burn an extra 5,645kcals a day to reach the kind of weight loss being claimed.

This is the equivalent of cycling 192 miles in 12 hours EVERY DAY. That is like cycling the height of Wales everyday (which Google says would take 17 hours).

Hats off to her if she did manage to do that everyday for a week, and not die. But I have a feeling that she is not the cycling type.

I know I have made some assumptions in this calculation (e.g just losing fat) and done a little bit of rounding, but even if I was out by 50%, I still don’t think she would have cycled 96 miles every day for a week. In fact, I doubt this woman would even give cycling 1 mile a go, but that is beyond the point.

The article claims that by just eating 400kcals a day you will lose 1 stone (6.35kg) of weight in a week, which is impossible. The worry is that some people will believe this and give it a go. Not only is eating 400kcals a day not very healthy, but it won’t work!

At best you can expect to lose in the region of 1.4kg of fat mass if only eating 400kcals a day. If you did this kind of 400kcals diet though you will definitely lose some muscle mass too. So you may well lose more weight, but muscle mass is not the aim of weight loss.


How much weight loss is realistic?

A reasonable calorie deficit to work with is about 300kcals a day, meaning the average woman would eat 1,700kcals. This represents about 37g of fat tissue a day, which over a week is 259g. Over a month you can expect to lose around 1kg of fat tissue just from a 300kcal deficit. Chances are, you won’t just burn fat, and are also likely to lose muscle too, so you probably would lose more weight than this. If you have a greater calorie deficit then you will lose more weight (and more muscle mass).

If you are exercising too, it is a slightly different story because you might be losing more fat, but adding muscle too (or at least not losing as much). Muscle weighs more than fat, so weight change might not be as noticeable as much as body shape/ composition.

These calculations are pretty close to the Mayo Clinics guidelines of losing 0.5kg a week at 500kcal deficit, which are also the same as the NHS’s guidelines. Considering my calculations are assuming 100% of the weight loss is fat, when in reality muscle mass will also be lost, I’m pretty happy with the accuracy.



The media will always make outlandish weight loss claims. These should always be taken with a large pinch of salt, and a healthy dose of scepticism. Realistic and healthy weight loss goals are to lose around 0.25kg -0.5kg of fat mass a week. This is healthy, sustainable, and will help to minimise muscle loss.

Image courtesy of Chris Jones.

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