Jan 25th, 2010
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How Sugars Make Us Fat

Fructose is the natural sugar present in fruits. During the last 30 years, large amounts have been incorporated into processed foods and beverages, including low-fat alternatives. It is found in large amounts in anything containing high fructose corn syrup, but this is not the only form. Evidence that fructose causes obesity is piling up. Dr. Robert Lustig, a Professor of paediatric endocrinology at UCSF presents a summary of the proof in a compelling 90 minute lecture.

Metabolic and hormonal pathways talk to each other to maintain an energy balance in our bodies. When a large amount of a single substance is consumed, it can perturb this balance. Fructose is one of these substances. It bypasses the systems that tell us that we are full, and ultimately results in the production of excess fat. Dr. Lustig does a great job explaining this process and I highly recommend that you watch this lecture if you want clarification on any of the points that I mention.

A Calorie is NOT a Calorie

Not all sugars are metabolized with the same efficiency. Glucose, the energy that every cell in your body requires, is metabolized most efficiently with only 1-2% of calories being converted to fat. Fructose (like other toxins) can only be metabolized by the liver and in the process 30% of those calories are directed toward lipogenesis (production of new fat). Dr. Lustig discusses this biochemical process in great detail beginning at approximately the 45 minute mark in his lecture.

Fructose Does Not Satisfy Hunger

Normally when food enters the small intestine following a meal, the hormone ghrelin (which tells your brain that you’re hungry) is suppressed and you will stop eating. Leptin is also released and signals satiety. As food is digested and blood glucose levels rise, beta cells in the pancreas produce insulin, further reducing appetite. Fructose does not reduce ghrelin levels nor increase leptin and insulin levels, therefore your appetite is not suppressed. If you are eating salty processed foods or sodas with a lot of fructose a nasty positive feedback loop will keep you eating long after you should. From fruits, we get about 15 grams of fructose/day. Today the average adolescent consumes 75 grams of fructose/day without the fibre that is present in fresh fruits and vegetables.

How Did Fructose End Up in Our Foods?

It’s cheap. During the eighties everyone was being told to eat low-fat foods to reduce heart disease. Taking the fat out of foods reduced the flavour, so the food industry replaced fat with sugar and salt. Since fructose was half the price of sugar (table sugar is sucrose; 1 part glucose + 1 part fructose), fructose found it’s way into everything from bread and cereal to soda pop and baby formula. Yes, baby formula and Coke both contain 10% fructose. This is how 6 months olds are becoming obese.

Tips for Reducing Obesity

Dr. Lustig closes his talk with four tips that form the basis of his patient’s treatment plan.

  • Eliminate sugar containing beverages; Drink only milk and water.

  • Eat carbohydrates alongside fibre; Fruits, vegetables and whole grains.

  • Wait a full 20 minutes before having another serving or dessert; It takes about 20 minutes for the stomach to tell your brain that you’ve had enough to eat.

  • Swap minutes for minutes; Minutes spent on sedentary activity (TV, video games) must be balanced with minutes spent doing physical activity.

I found Dr. Lustig’s lecture fascinating and I hope you did too. Hopefully his hard work will lead to better regulation of the food industry. By not buying refined and sweetened foods that pack grocery store shelves we can make our stand too.