Can you eat fruit on a diet?

This is a question I get asked all the time.  I speak with clients every day that have stopped eating fruit because they think it will stop them losing weight.  Hopefully with this article I can clarify some common mis-conceptions on fruit intake.

 

First, what is fruit and what does it contain?  Fruit is part of a flowering plant.  Usually it is the mature ovary of one or more flowers.  Fruits contain the sugars glucose, fructose and sucrose.  They also contain fiber and many vitamins.

 

Glucose, fructose and sucrose are simple carbohydrates.  A simple carbohydrate basically contains a single sugar unit or two sugar units.  A complex carbohydrate contains three or more sugar units in a bond. Glucose and fructose are monosaccharides, meaning they are the simplest form of carbohydrate in that they cannot be reduced in size to smaller carbohydrate units by hydrolysis.  And sucrose is a disaccharide, which consists of two monosaccharide units joined together (1).  Almost all carbohydrates are broken down to their constituent monosaccharide units by the body to be absorbed.  The difference is mearly in the speed of digestion.

 

The reason I am explaining the above is that the main reason given as to a person not wanting to eat fruit is because it contains sugar. So the question now becomes does eating simple sugar halt your weight loss pursuit?

In order to lose weight you must be in an energy deficit.  This is the first law of thermodynamics (2). Fundamentally, where this deficit comes from doesn’t matter and the size of the deficit determines the weight lost.  There is of course many other factors to take into account, including diet and training quality which determines if your body will change the way you want it to.  However, this is for another article. A number of studies have shown that when a diet containing the same macronutrient (protein/carbs/fat) totals are given to overweight adults but only the source of carbohydrate is changed, there is no difference in the amount of fat lost (3,4).  In simple terms the subjects that ate mostly sugar for their carbohydrate source saw just as much weight lost as the group following a low sugar diet.  This research has been replicated in 398 moderately obese adults eating simple carbohydrates such as fructose over 6 months.  Again with no significant affect on fat loss (5).  So you do not need to be stuck with brown rice and oatmeal for your only carb source for maximum fat loss.

 

“What about the sugar in fruit giving me a quick rise in energy then a crash, leaving me hungry?”.  How quickly sugar rises in the blood stream is measured using the Glycemic Index (GI).  The definition of GI given by The University of Sydney is, ‘The GI is a ranking of carbohydrates on a scale from 0 to 100 according to the extent to which they raise blood sugar levels after eating.  Foods with a high GI are those which are rapidly digested and absorbed and result in marked fluctuations in blood sugar levels.  Low-GI foods, by virtue of their slow digestion and absorption, produce gradual rises in blood sugar and insulin levels...’ (6).  So, what are the GI ratings of fruit and does it matter?

 

Most fruit is actually classed as low GI (7), with apples scoring 36, oranges 43 and bananas 51. Compare this to instant oat porridge at 79 and boiled potatoes at 78, you can see it is not as easy as simple carbs raise blood sugar and complex carbs keep it steady.  However, in terms of weight loss the GI total of a food has been shown not to matter.  In a 10-week study on overweight women consuming ad libitum (at liberty, or in even simpler terms, as much as they want) food intake, eating either low or high GI foods made no difference in the amount of fat lost, calorie intake or hunger (8).

 

If you are not counting your macronutrients and are looking to lose weight it is a good idea to pay attention to the Satiety Index (9) of a food.  This is basically how full eating a food makes you feel relative to how much energy is contained in the food.  Many fruits score well in satiety, such as oranges and apples.  So, it is fair to say that it is a myth that eating fruit will give you a rise and crash in energy and/or leave you hungry.  In fact, because of the high Satiety Index of many fruits as well as the fiber and vitamin content I would highly recommend consuming fruit as part of a healthy diet, whether you are trying to lose weight or not.

 

This brings me to my last point, health.  Most people know it is important to consume a high number of vitamins and antioxidants to help remain in optimal health.  Fruit has an abundance of both.  In fact research has shown that fruit can help lower risks of cancer and cardiovascular disease (10,11). Blueberries, for example, have a very high amount of antioxidants per weight.  Coming in at 9.24mmol/100g compared to 1.63mmol/100g in a kiwi fruit (12).  Blueberries have been shown to improve memory in older adults (13) and increase lifespan (14), by the way, so get them munched.  This is not to say that you should consume only blueberries over, say kiwi fruit, though.  Kiwi fruit has 105mg/100g of vitamin C compared to 9.7mg/100g in blueberries (15).  A wide variety of fruits are recommended to get the most health benefits.

In conclusion, there is no need to avoid fruit when you are trying to lose weight.  And, as you can see, there are many benefits to keeping or adding fruit to your diet. 

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Jan 2016

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