Low-carb diet? Why your week 1 weight loss is meaningless

by Mary C. Weaver, CSCS on May 4, 2012

Protein? Great! But you need carbs too.

Diet gurus love-carb diets–and dieters love them–because going on one usually causes a dramatic loss of weight the first week. Notice that I didn’t say loss of fat.

People get all excited about that first week’s weight loss of four, five, or six pounds or more. Well, there’s an easy explanation for what’s going on, and it has nothing to do with fat loss.

So what’s happening when you go on the Atkins, Dukan, or other low-carb diet and see a significant weight loss almost immediately?

In order to explain, I want to give you a little bit of background on carbohydrates first.

Fast and slow fuels

Carbs are your body’s preferred fuel source. No matter what you’re doing–whether you’re vegging out on the couch or running a marathon–you’re always burning a combination of carbohydrate and fat as fuel.

Depending on the activity, your body consumes more or less fat and more or less carbohydrate.

Carbohydrate is a very fast fuel–one your body can break down extremely quickly. So when you sprint, you’re using carbohydrate almost exclusively.

Breaking down fat is a slower process and involves more biological work. When you’re sitting on the couch or sleeping, you’re burning proportionately more fat.

Your brain runs on sugar–the glucose in your blood. Because your brain must have sugar and the rest of your body must have carbohydrate as a fast fuel, your body employs elaborate processes to make sure carbohydrate will always be available.

The body stores carbohydrate in your muscles and your liver in a form called glycogen. You can think of glycogen as fuel in the tank.

OK, stick with me–I’m about to explain how this relates to low-carbohydrate diets.

If you stop eating carbs, your body still absolutely has to have them. It immediately begins consuming the glycogen stored in your tank before moving on to the more challenging task of converting fats and proteins into usable carbohydrate.

Every gram of that stored glycogen in your liver or muscles is accompanied by four to five grams of water in your body. If your body uses up one gram of glycogen, it sheds those four to five grams of water.

What do you think happens if your body burns up one pound of glycogen? It sheds four to five pounds of water.

Bingo: you’ve got a five or six-pound “weight” loss: one pound of glycogen plus four or five pounds of water.

Let’s say you also lose one pound of fat in your first week on a low-carb diet because you are eating fewer calories than before.

By the way, there are only two reasons you lose fat: because you’re consuming fewer calories than your body needs or burning more calories through increased activity. The ratio of protein, carbs, and fats you’re consuming has nothing to do with it.

You could end up with a seven-pound weight loss during your first week on a low-carb diet, and you’d probably think, Hey, I’m making great progress!

Well, in this example, it turns out that only one pound of that loss was fat.

As soon as you start eating carbohydrates again–and you have to eventually–you begin storing glycogen, and the water you lost will come right back in proportion to the stored glycogen.

No minimum daily requirement?

Some rather uninformed people have argued that since there is no “minimum daily requirement” for carbs, that means you can do without them quite nicely.

First, there certainly are minimum daily requirements for vitamins and minerals, and if our diet consisted only of meat, dairy products, and fats, we couldn’t possibly get enough of them.

Vegetables and fruits–our primary source of many essential vitamins and minerals, as well as antioxidants–consist mostly of . . . carbs.

Second, and more to the point, because you would die without blood sugar, your body has failsafe mechanisms to turn fat and protein into sugar within your body. But these processes are far less efficient than turning dietary carbohydrate into sugar.

Anyone who tries to exercise vigorously while on a low-carb diet is going to feel like crap. The more active you are, the more you need carbohydrates. They are your body’s preferred fuel source, so how sensible do you think it is to restrict them unduly?

Granted, some carbs provide more nutrition than others. Fruit, whole grains, fibrous vegetables (e.g., broccoli and green beans), starchy vegetables (e.g., carrots and potatoes), and legumes are much better choices than Twinkies and Coke.

So please don’t get the idea that carbohydrates are making you fat–and thus that a low-carb diet will somehow magically make you lean. And if you go on a low-carb diet and see a dramatic early weight loss, don’t be too quick to assume the loss is primarily fat.

Bottom line: It’s the overconsumption of calories–no matter where they come from–that adds jiggle to our thighs and waistlines. Only eating less and moving more can change the equation.


{ 9 comments… read them below or add one }

1 Suzan St Maur May 4, 2012 at 12:30 pm

I’ve heard this before Mary, but never quite understood how it worked out – thanks so much, as I understand the low-carb issue totally now!


2 Jane Hatton
Twitter: evenbreak
May 4, 2012 at 12:46 pm

Thanks for this Mary. I recently lost 6 lbs in 4 days on Atkins (and I find it hard to lose weight). I was ecstatic! When, two weeks later, I had failed to lose any more and was really struggling as I hadn’t been to the loo in a fortnight (sorry – too much information!) I had to come off the diet and eat normally. The 6 lbs went straight back on of course. What a waste of two or three weeks (and quite uncomfortable!). Healthy eating for me from now on.


3 Sarah Arrow
Twitter: saraharrow
May 4, 2012 at 1:47 pm

I have to say, I have never done a carb diet where they were banned. Not even Atkins bans them, just requests you lower the intake until you get your eating under control.
That said, I can’t do any diet that does’t include rice…


4 Pamela Hernandez May 4, 2012 at 3:15 pm

I knew about the water and glycogen but never heard the lbs ratio. More information to share with my clients to help them understand low carb diets may cause weight loss but not fat loss.


5 Suzanne
Twitter: WorkoutNirvana
May 6, 2012 at 9:39 pm

Great info! I so agree with this – carbs have been designated as the devil in our diets and so wrongly. Carbs are energy! I do occasionally track macros with a weight-loss client who is having trouble. I like to know where exactly their calories are coming from so we can make healthier choices.


6 Shira May 7, 2012 at 1:10 pm

Carbs have gotten such a bad reputation because people associate them with donuts and bagels instead of apples and broccoli. People often forget they are a necessary part of a healthy diet. And when I’ve tried to exercise in the past while following a low carb regime, my energy level hovers around zero. Great post!


7 Beverly May 29, 2012 at 5:18 pm

Um. low carb diets don’t exclude vegetables, and include fruits quite frequently.
So that’s a *myth*, isn’t it that I can bust straight away.


8 Victoria Morrison June 14, 2012 at 8:49 pm

More sanity from you, Mary. I *tell* my clients this information all the time. Now I have an article to direct them to. Thank you :-)


9 Mary C. Weaver, CSCS
Twitter: themusclediva
June 14, 2012 at 10:00 pm

Victoria–thanks! One sane person to another, I really appreciate that!


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