South Beach Diet – Science of Carbs

In honor of one of my favorite shows on the radio, called Science Friday, I decided to do a little research today, Friday, into my diet in a scientific direction. Yeah it’s not Friday, but I don’t plan on being around tomorrow. So enjoy it a day early.

I wanted to look into the science behind the south beach diet. The post got so long I broke it up into 2 parts. In this post I will discuss Carbs. (part 1 on Fats)

South Beach Diet

The goal of the diet as it was developed as reported by Dr. Agatston, in his book, was to create a diet that had as few rules as possible and was flexible and simple. It is supposed to allow people to eat the way they actually want to eat while improving their blood chemistry and lose weight over the long run.  

The basic science of the diet on a macro-nutrient level is that there is good fat and bad fat. Eat the good stuff, and limit the bad stuff, the diet is that simple. You can get a list of what is good and bad but behind these lists is a bit of science. The Fats are all about specific details of the cholesterol and whether or not the fat is saturated or not. Things you can easily find on the nutrition labels.

All of these topics are covered in the first section of the South Beach Diet book. There is also a new book out.

Now let’s dive a bit deeper into the details of how the diet works.

Carbohydrates – Glycemic Index

The goal of the south beach diet regarding carbohydrates is to stop overeating by using three strategies. 

  1. Eat foods in combinations that raise the blood sugar level slowly. Then when the blood sugar drops it is also slower.
  2. Anticipate when the blood sugar will drop and have a snack to keep it from dropping too much before the next meal.
  3. Learn the glycemic index of foods you like to eat so that you can try and eat the lower indexed foods.

So what exactly is the glycemic index anyway? It is a crude measurement (how else can one describe taking a graph and boiling it down to 1 number) of how fast and to what degree a fixed portion of food (usually 50 grams of available carbohydrate) is absorbed into the blood versus a standard carb source (glucose usually or sometimes white bread). The blood sugar is measured for 2 hours and the area under the curve plotted from the results is compared to the curve for the standard. The result is a percentage and it is multiplied by 100 so people dont have to think about fractions. Here is the test in more detail. So 55 means the area under the curve is 55% as compared to white bread or glucose which would be considered a low glycemic index food.

Why is the Glycemic Index Important?
Your body performs best when your blood sugar is kept relatively constant. If your blood sugar drops too low, you become lethargic and/or experience increased hunger. And if it goes too high, your brain signals your pancreas to secrete more insulin. Insulin brings your blood sugar back down, but primarily by converting the excess sugar to stored fat. Also, the greater the rate of increase in your blood sugar, the more chance that your body will release an excess amount of insulin, and drive your blood sugar back down too low. 

Glycemic Load 

Now it is still possible to eat foods that have a higer glycemic index, just take care to eat them with other lower glycemic index foods and foods that slow down the digestion process like fiber and fats. Fiber and fats get in the way of digestion and absorbtion of sugars slowing the process down. An example of how to do this is eat a whole grain pancake with syrup instead of one made with white flour. It takes longer to break out the carbohydrates from the fiber. As for fats, they just slow things down cause they get in the way. The High GI food is the syrup, you are helping to slow down the process by eating whole grains and say butter or some other oil spread.

Here is an example to help understand digestion. Think back to the last time you drank alcohol on an empty stomach. It didn’t take quite so much to get a buzz as when you drank just after a meal now did it? When you had it with a meal all the food in the stomach slowed the absorbtion of the alcohol. The same goes for sugars. Eat a couple of eggo waffles smothered in syrup without any butter and drink a huge glass of pulp-free orange juice for breakfast and you will get sugar in your bloodstream faster than just about any other method short of a needle. If instead you ate an egg with some canadian bacon then had a waffle made with whole grain flour and a small amount of real maple syrup and an orange, you can bet you will absorb this sugar much more slowly. You might have just as much sugar in the entire meal, but it will digest much more slowly.

The important part of the diet is not to have a steep rise in blood sugar. As the insulin comes out too fast and drops things off too fast giving you the cravings too soon. Now the glycemic index of a food cannot tell you all there is to know. First of all you need to measure how you react to foods, not how the average of 10 people reacted. Also you need to understand that these numbers are not set in stone. If you cook or break it down by ripening for example, this is like a head start on the digestion process. Same thing as processing foods to remove the fiber. Boil your carrots until they are mush and you will have a much more easily digestible food compared to pulling one out of the ground, dusting it off, and popping it in your mouth. It is just not possible to know all the measurements and how they affect you unless you decide to get a blood sugar level tester and start experimenting. Those familiar with the cravings of a low carb and then back to high carb diet can probably correctly assume that they can sense if they over did it. These are the feelings that the South Beach Diet is suggesting you listen to when you start to test how your body reacts.

Now to muddy the waters a bit more. If you just look at the glycemic index of a food it tells you how much of the carbohydrates your body will digest and deposit into the blood stream in the first two hours. So if you eat 50 grams of available carbohydrates worth of twinkies, and 50 grams of available carbohydrates of cooked carrots and you will find that the area under both curves are about equal (give or take). So one would think according to the diet they are equal. Well, not by a long shot. First off there is more to diet than carbohydrates. I can’t imagine there are too many micronutrients in twinkies while I bet you get some goodies in carrots. That is one less vitamin pill you will have to take as a supplement. There is also the fat which has nothing to do with the glycemic index. Then there is fiber, again the twinkie comes up short. And fourth, how many carrots does one have to eat to get 50 grams of available carbs versus twinkie? This is how the test is performed. That would be very close to 2 twinkies versus about 9 cups of cut up carrots or about 18 medium sized carrots. Yikes!

So the amount of sugar that gets in your blood stream from eating 18 cooked carrots is the same as eating 2 twinkies. So the foods have the same glycemic index. The problem is obvious, I bet I could binge and eat 6 twinkies, but I would find it all but impossible to eat 54 carrots, I don’t know if I could cut that many up without getting blisters. So what good is the glycemic index then? Well it is a measure of how fast the sugar will be digested once you chew it up and swallow it. This is the measurement if it is all you eat. It is a crude measurement of how much blood sugar you get in two hours. If you eat a meal with other things it will all change. If you only eat a couple carrots and a thick slice of cheese you will not get as much sugar in your blood total, and you will slow it down a bit as compared to the twinkie. If you eat a twinkie with a bunch of fiber and a supplement you might start to be on a level playing field with the carrots, but then there is a lot of fiber and you will likely have a terrible stomach ache. So when you chop it down to a reasonable serving rather than eating a sliver of twinkie with your fiber and vitamin, you would probably decide to just go for the cooked carrot or two anyway.

So the glycemic index is not useful to compare which foods to eat, it just helps you learn how fast the foods you are thinking about eating will digest and then you can work on pairing them up or substituting with other stuff that will slow it down if you get a high number on something. If you have a set portion you want to eat you will find the glycemic load is a more useful number because it adjusts the food by portion size. So if you compare the two cooked carrots to the twinkies you will see a dramaticly lower number for the carrots cause you get to divide it by 5 since your portion is only around 5g of available carbohydrates versus 50 for two twinkies. This blood sugar will digest into the blood just as fast as the twinkie (give or take) but it will not be nearly as much.

There is another concern that these numbers do not take into account which is how steep is the rise, although you can get an idea from the number because a less steep curve means there is less is area under the curve in general. The key is not to use these GI and GL numbers to trick yourself. If you think that you can have a peanut butter cup instead of a banana because you saw a chart that said the GI was lower, then you have missed the point of the diet completely. Use the glycemic index to see if you should eat carrots or brocolli with your other items. Don’t try and substitue crap in just because some area under some curve says you will digest the sugar at slightly slower rate.


I got your comment and started doing a little research on the history of the glycemic index and it seems that the stuff was started by Dr Jenkins and others in Canada. This link is a research project completed in 1985. Dr Sears wrote a book called the Zone in 1995 and now has a website and a science website for it. The diet, from what I can tell is similar although how you apply it seems a bit different. You are to build you meals up following the rules to end up with something healthy. This is much like I was explaining above for how to properly apply the South Beach Diet. So although they sound a bit different they are aiming for the same goal. The major difference is that the Zone diet puts an emphasis on polyphenols which are found in fruits and vegetables. The diets of course are different based on their points of view, but I believe they attempt to reach the same goal through differeing explinations. For example, you get to eat all you want versus portions, but that is just a marketing ploy for the SBD. You eat from that list and you will get full and not want to eat more than you should. So although it sounds different it is similar in my opinion.


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6 Responses to “South Beach Diet – Science of Carbs”

  1.   South Beach Diet - Science of Carbs by Says:

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  2. South Beach Diet - Science of Carbs Says:

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  3. Healthy Blood Sugar Says:

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  4. DR Says:

    Nice synopsis of South Beach and the GI.

    Dr Jeannie Brand-Miller is currently the scientist doing the most with GI and GL (glycemic load)

    David Mendosa also has a great site –

  5. MizFit Says:


    this is such a FANTASTIC post.
    Im printing and waving in front of my husbands eyes (as he tries valiantly to avert them:))


  6. Most Popular Posts « Brianthinagain’s Weblog Says:

    […] Science of Carbs – 306 […]

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