Saturday, 8 December 2012

Weight Gain on a High Carbohydrate Diet

There's a claim going around that you can't gain weight (fat)on a high carbohydrate, low fat diet. As far as I can tell, this belief is based on things said by Colin Campbell, Neal Barnard and/or personal experiences.

Whilst observing the diets of Americans and rural Chinese, Colin Campbell noticed Chinese people ate more calories than Americans but were slimmer. He put this partly down to them being more active, but also felt that their low protein, not low fat, diet may have played a part, as rodents, and pigs, fed low, sometimes deficient, protein diets gained less weight than those fed high protein diets. It was even found that high fat diets worked better.

I've only seen one study done on humans showing similar effects. They were given high and low protein diets. Overall, people gained less weight on the low protein diet, but for some people the effects of the two diets weren't significantly different. Some even did better on the high protein diet [1]. Again these weren't low fat diets always.

Some say Thermogenisis mops up any excess carbohydrate calories. A few studies have found that Thermogenisis is higher on high protein diets [2][3]. This makes sense as you'll see.

When you eat your body uses energy to digest the food. This produces heat. The harder a food is to digest, the higher the thermic effect and the more energy you burn. Out of the three macro nutrients, protein has the highest thermic effect. The thermic effect of protein is 10 – 20% of calories. The thermic effect of carbohydrate is 5 – 10%, and the thermic effect of fat 0 – 5% [4].

This means a high carbohydrate diet will have a higher thermic effect than a high fat diet with the same protein level. However, a lot of high fat diets are higher in protein than many low fat diets. This might make the difference, in thermogenisis, less significant.

In a study comparing the thermic effect of different types of carbohydrates, sucrose had the highest thermic effect. 10 healthy males were given 75g,  or 300 calories, of each carbohydrate type. Over a six hour period, they lost 33 calories, to thermogenisis, with sucrose [5].

If you think higher fibre sugar sources will lead to higher levels of thermogenisis, you're not right. Lower fibre meals induce thermogenisis more than higher fibre meals [6].

What about fat storage?

As with digesting, energy is needed to store energy as fat. Storing fat as fat doesn't require much energy. However, carbohydrate uses 23% of the energy [7]. With this in mind, you should gain less weight when over consuming on carbohydrate compared to fat. Instead of storing 975 out of 1000 excess calories, you would store 770.

Assuming you didn't exercise more to burn it off, if you consumed an excess of 500 carbohydrate calories a day for 1 month, you would theoretically gain over 1kg of fat.

500 – 50 (energy used digesting) = 450 – 104 (energy used converting to fat) = 346 divided by 9 (calories per gram of fat) = 38.4 (grams of fat) x 28 (4 weeks) = 1076g = 1.076kg

A study done on overfeeding fat and carbohydrate supports this. Fat overfeeding led to 90 – 95% of the excess energy being stored. Carbohydrate overfeeding led to 75 – 85% of the excess energy being stored [8].

Unlimited calories and weight loss
What about Neal Barnard's study where they put people on low fat vegan diets, and were told they could eat as many calories as they wanted. They lost weight and their diabetes improved. Does this mean you can eat unlimited calories and not gain weight, or even lose weight? No. They were allowed to eat as much as they wanted, but they averaged about 1400 calories a day [9].


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