There are more diets out there than anyone can name. Most of them are ineffective, either because they are based on flawed science, or because they are unsustainable. However, there are a few that are built on sound principles. These principles can be made use of when trying to get a grip on being overweight. One of these principles is to lower blood sugar levels, and a diet that achieves that is the low GI diet.
What is a low GI diet?
A low GI diet reduces the glycaemic load of the foods that you eat, and the glycaemic load of your meals depends on the GI or glycaemic index of the foods in it. The glycaemic index is a measure of a foods ability to raise your blood sugar levels. Glucose is used as the benchmark. The effect of glucose on blood sugar is measured over a 2 hour period. The total blood sugar over that period is counted as 100. Other foods can then be tested against it, and they get given scores which range form 0 to over 100.
Bear in mind that we often refer to blood glucose and blood sugar interchangeably, although table sugar is actually a compound of glucose and fructose. So when we say blood sugar, we actually mean blood glucose. If we eat carbohydrate containing foods, such as bread, jam, milk, pasta or potato, our digestive system breaks down the carbohydrates into glucose, fructose or galactose. These components are all absorbed directly into our bloodstream. The glucose is then what counts as blood sugar.
Different foods have different effects on our blood sugar levels. There are at least 4 reasons for these differences.
- They contain different amounts of glucose. An egg contains very little glucose and unsurprisingly has a low GI. Bread is made up of starch, which gets broken down into maltose. Maltose is two glucose molecules bound together. The maltose is then broken down into 2 glucose molecules. Bread as a result has a high GI.
- They affect how much glucose our cells can absorb out of the blood by stimulating our pancreas to release different amounts of insulin. Most foods produce an insulin response proportionate to the amount of glucose they contain, however there are some exceptions. High protein foods such as meat and fish can stimulate insulin release despite containing very little glucose. This makes them useful foods for reducing blood sugar.1
- They are digested at different rates. For instance carbohydrates are absorbed quickly, protein less quickly and fats slowest of all. Fibre is also a factor when it comes to digestion. Very little of it is absorbed, but it does slow down the absorption of other nutrients, including the starch that is broken down into glucose.
- The glucose in foods can be joined up in ways that slows our digestion of them. So for instance potato contains starch, which is glucose molecules joined up in a tree like structure. Your intestines break it down, and then your blood gets hit with a shed load of glucose. That is why baked potato can come out close to 100 on a measurement of its GI. In contrast pasta, which also contains starch, has a much lower GI. Why is this? Well the starch in pasta is bound up quite tightly and cooking it still leaves a lot of the glucose difficult for your digestive enzymes to release quickly. The glucose is delivered to your bloodstream more slowly, so slow in fact that your body is able to remove quite a lot of sugar from your blood and into the cells before you've digested even half of the pasta.
Why eat a low GI diet?
We want to keep our blood sugar levels steady and low, as glucose is effectively a poison. There are three undesirable effects of note:
- Weight gain is much easier if you have unstable blood sugar levels. The periods when blood sugar is high encourage insulin release, and insulin helps the body to turn sugar into fat. Why does it do this? Well, blood sugar in excess is dangerous and our body will do what it can to reduce it. One of the easiest ways of doing this is by converting the sugar to fat and storing it around the middle of our body. This is why a low GI diet is effective at reducing your weight as long as you can stick to it.
- For healthy people, eating a meal containing large amounts of high GI foods, raises blood sugar. This then stimulates a large release of insulin that causes the sugar to be taken out of the blood, by the cells. This leads to rebound hypoglycaemia in which too much sugar is removed from the blood. This is characterised by low energy, sleepiness and sometimes irritability.
- If a person has poor blood sugar control, then not enough insulin is released to reduce blood sugar to safe levels. The excess blood sugar then works as a poison that can lead ultimately to the complications of diabetes. These are kidney failure, blindness, heart failure and loss of the feet due to foot ulceration.
One of the main causes of diabetes, and the possibility of the more serious complications above, is a high GI diet. The constant demands on your body to reduce blood sugar levels can become too much. The pancreas can become less efficient at producing insulin or the cells can become less responsive to it. Of course, the diabetes resulting from this, known as type 2 diabetes, can normally be controlled with correct exercise and diet. However, staying healthy can become a lot harder.
Do foods always affect blood sugar levels the same way?
The answer to the question above is, no. There are a number of things to remember when considering what foods to choose, if you want to keep blood glucose levels even and low.
- You can combine foods. By mixing a high GI food with a low GI food you are effectively lowering the effect on your blood sugar levels.
- Some foods change their GI as they ripen. Bananas for instance start out as low GI and can be quite high when fully ripe.
- Food processing can also affect GI. Highly processed foods are easier to digest, and therefore their glucose gets absorbed into the bloodstream more quickly.
- Cooking increases the GI of most foods.
- Gluten free products tend to be higher GI, with a typical increase of 10 points over their gluten containing equivalent.
- By including fats and proteins with your meals you effectively lower the effect of a meal on your blood sugar levels. Protein based foods are a particularly effective.
What are the best foods for blood sugar levels?
In general, most people need to eat fewer high carbohydrate foods and more foods containing fats and proteins. Let's look at the different types of foods in turn.
Carbohydrates come in different forms. As mentioned above, highly processed foods are likely to contain carbohydrates that raise blood sugar quickly. Fruits and vegetables on the other hand generally contain plenty of water and fibre that reduce their effects on blood glucose. When it comes to wholegrain foods, these generally contain more fibre, and therefore are better for blood sugar. Wholemeal is more easy for the body to break down however, as it made from wholegrains that are then ground further. As such wholegrain breads such as granary have a fairly low GI of about 50, while wholemeal and white bread are fairly similar, both with a GI around 702. Pasta is the lowest GI of the main carb sources at 41-60. Pasta is low GI because it is produced from durum wheat a particularly "hard" type of wheat, which is harder for the body to digest. Rice has a highly variable GI depending on type and make. GI can be as low as 40 or as high as 90. It is not possible to say that brown rice, japonica or basmati are any lower than plain white rice as all types of rice have high variability in GI . Bread 50-95 and potatoes 56-85 also have highly variable GIs. The wide variation with bread and potatoes represents how they are baked or cooked.
Fats almost always have a very small effect on blood sugar levels. This is generally because they contain no glucose, although watch out when you eat dairy produce as a lot of it contains lactose (milk sugar), which is 50% glucose. Fat is the slowest of the major nutrients to be digested, and this in itself lowers the GI of any meal containing it.
Proteins seem to be the most effective at keeping blood sugar levels low and even. While protein can be converted into glucose in the body, this is only done when the body has a clear need for the glucose. Also the energy cost of converting it is high, so protein is unlikely to contribute to high blood sugar levels.
Alcohol is basically bad for blood sugar control. It can mess up the hormones that control blood glucose levels in the body. Notably the insulin that lowers blood glucose and glucagon that raises them can be affected. In the short term hypoglycaemia (low blood sugar) can occur after binge drinking, leading to some of the symptoms of hangover. Longer term chronic overuse of alcoholic drinks leads to raised blood sugar levels, some of whose symptoms include erectile dysfunction, fatigue, blurred vision and poor wound healing.
Is there ever a good time to consume loads of carbohydrates?
Of course there are times when eating a load of carbohydrate is beneficial for health. This is when we are using glucose faster than we can replenish it. In this state any carbohydrates we consume are likely to be used, either to produce energy or to replenish our glycogen stores. Glycogen is our way of storing glucose in a condensed mass in the liver and muscle. So if you've just run 6 miles hard then you will be fine eating a bit more bread, rice or pasta than you would normally.
Beware, however of taking on sports drinks during training, and then loading up with carbohydrate afterwards. It is not a carte blanche situation where you can eat as many carbs as you like so long as you exercise. I've come across many sportspeople who take on the recommendations of sports drink companies to replenish themselves while exercising. These same athletes then eat high carbohydrate foods after finishing exercise, as they've been told that for the first hour after completion of exercise they won't store any carbs as fat. The truth is, that once you've replenished yourself your carbs will get stored as fat. This situation will be quickly achieved if you consume sports drinks throughout low or medium intensity exercise.
So if we are exercising or recovering from exercise then carbohydrates can be added to our diet proportionately to the amount and intensity of our exercise. This also applies to concentrated mental tasks that also make use of plenty of blood sugar. An exam, or the nervous anticipation before an event such an exam, race or other event can use up enough carbs to increase our requirements for carbohydrates.
How do I find out the GI of the foods I eat?
The following online sites are useful.
- http://www.glycemicindex.com/ is an Australian site that explains how GI is measured and allows you to search a database to find out the GI values of different foods.
- http://www.the-gi-diet.org/lowgifoods/ is an easy ready reference.