Sugar, Fructose and Weight Gain

Fructose is a carbohydrate typically found in fruit sugars, table sugar, lollies and soft drinks. Glucose is found in complex carbohydrates which are located in grains, most vegetables and legumes. Fructose is metabolised differently to that of Glucose. Fructose when ingested enters the liver were it can be converted to Glucose or into Triglycerides. Triglycerides contain three free fatty acid molecules and are essential a transportation device for fats. Triglyceride levels have been shown to increase on high fructose diets (1,2,3,4). This increase in triglyceride levels indicate higher circulating levels of fats which is a sign that either fat synthesis (production) has increased or fat uptake has decreased.  Fructose fails to stimulate insulin production and insulin is the key hormone regulating fat uptake by fat cells. This may sound great that fructose or sugar prevents fat uptake by cells, but this can lead to increased blood triglyceride which leads to insulin resistance and Type 2 diabetes. Studies show increased abdominal fat distribution on high fructose diets (1). Abdominal adiposity has been linked with insulin resistance and the likelihood of developing both Diabetes and Heart Disease.

Fructose consumption fails to produce rises in the hormones insulin and leptin both of which may be involved in appetite regulation (2,4). Research on whether high sugar or fructose intake result results in increased energy intake is unclear.

Animal studies show links between fructose intake and obesity (5)  Human data is less clear due to differences in human and animal metabolism. Humans also tend not eat fructose at the levels seen in animal studies. A study examining sugared beverage intake in children showed a small correlation with sugar intake and weight gain (6). A study using middle aged women found no relationship between habitual sugar intake in the form of drinks but a relationship was found between weight gain in those who had increased their consumption of sugared drinks from the previous year (7). Sugar though is found in many foods other than soft drinks, and monitoring the entire food intake for sugar consumption would be required to further access the result.

A study where by a diet high in simple sugar was compared to one of high complex carbohydrates showed weight loss in the complex carbohydrate group compared to the simple group (8)

1. Stanhope, Kimber L., et al. "Consuming fructose-sweetened, not glucose-sweetened, beverages increases visceral adiposity and lipids and decreases insulin sensitivity in overweight/obese humans." The Journal of clinical investigation 119.5 (2009): 1322.
2. Teff, Karen L., et al. "Dietary fructose reduces circulating insulin and leptin, attenuates postprandial suppression of ghrelin, and increases triglycerides in women." Journal of Clinical Endocrinology & Metabolism 89.6 (2004): 2963-2972.
3. Lê, Kim-Anne, et al. "Fructose overconsumption causes dyslipidemia and ectopic lipid deposition in healthy subjects with and without a family history of type 2 diabetes." The American journal of clinical nutrition 89.6 (2009): 1760-1765.
4. Teff, Karen L., et al. "Endocrine and metabolic effects of consuming fructose-and glucose-sweetened beverages with meals in obese men and women: influence of insulin resistance on plasma triglyceride responses." Journal of Clinical Endocrinology & Metabolism 94.5 (2009): 1562-1569.
5. Bocarsly, Miriam E., et al. "High-fructose corn syrup causes characteristics of obesity in rats: Increased body weight, body fat and triglyceride levels." Pharmacology Biochemistry and Behavior 97.1 (2010): 101-106.
6. Ludwig, David S., Karen E. Peterson, and Steven L. Gortmaker. "Relation between consumption of sugar-sweetened drinks and childhood obesity: a prospective, observational analysis." The Lancet 357.9255 (2001): 505-508.
7. Schulze, Matthias B., et al. "Sugar-sweetened beverages, weight gain, and incidence of type 2 diabetes in young and middle-aged women." JAMA: the journal of the American Medical Association 292.8 (2004): 927-934.
8. Poppitt, Sally D., et al. "Long-term effects of ad libitum low-fat, high-carbohydrate diets on body weight and serum lipids in overweight subjects with metabolic syndrome." The American journal of clinical nutrition 75.1 (2002): 11-20.