There has been an increasing amount of media attention on the role that sugar plays in overall human health. FFHI-affiliated researchers at UC Davis are at the forefront of these important questions about the metabolic effects of sugar – glucose, fructose, sucrose, etc – in our diets. Research carried out in FFHI faculty member Dr. Peter Havel’s lab was featured prominently in Gary Taubes’ widely-read New York Times Magazine article, “Is Sugar Toxic?” Last week, Havel’s research group and collaborators were interviewed by 60 Minutes as part of an in-depth investigative report on sugar consumption and its deleterious health effects.
Dr. Kimber Stanhope, director of clinical research for Dr. Havel’s lab, was able to provide an interesting background about the research history of the lab, which has been investigating the importance of sugars in our diets since 1997. Dr. Stanhope explained that the research referenced in the New York Times Magazine article really began to take shape in 1996 when one of the hottest research topics was leptin. When fat cells are “full,” they secrete a hormone called leptin which tells the brain to stop eating and to burn more energy. But when fat cells are deprived of energy and start shrinking – for example, when a person severely reduces their caloric intake – the fat cells stop making leptin. This sends the opposite message to the brain – start eating and burn less energy. The Havel group conducted studies on isolated cultured fats cell and found that fat cell produced more leptin in response to insulin stimulated glucose metabolism.
This led Dr. Havel to hypothesize that eating fructose, which does not increase glucose or insulin levels, could result in decreased leptin levels. The group tested this hypothesis in 24-hour studies and found that day long leptin levels were reduced by 20-30% when people consumed meals accompanied with fructose- compared with glucose-sweetened beverages. They then hypothesized that a reduction of circulating leptin concentrations during prolonged consumption of diets high in energy from fructose could lead to increased energy intake and/or decreased energy expenditure and weight gain.
They tested this hypothesis during a diet intervention study in which two groups of subjects drank either glucose- or fructose-sweetened beverages that provided 25% of energy requirement. For an 8-week period, these subjects lived at home and were allowed to drink the beverages while eating as little or much of the regular diet as they wished. Despite the initial hypothesis that fructose-drinkers would be hungrier and thus have increased weight gain as compared to the glucose-drinkers, the data showed that both groups had similar weight gain. However, the fructose-drinkers accumulated more of the extra weight in the fat that surrounds the internal organs, and the glucose-drinkers gained subcutaneous fat. The importance of central adiposity, or belly weight, in chronic diseases such as type II diabetes and insulin resistance is now well known.
The Havel group is able to see differences between glucose and fructose by examining processes in the liver – the place where the human body turns sugar into fat. The fructose-drinkers had an increased rate at which the liver was turning sugar to fat, had increased lipid and lipoprotein levels and had decreased insulin sensitivity. None of these detrimental changes were observed in the glucose-drinkers.
There are a number of research papers reporting that people who eat high-sugar diets are more likely to develop cardiovascular disease and diabetes, and results from this study suggests that fructose can explain the connection between high-sugar diets and these diseases. Importantly, their results also showed that the two diets they tested had identical effects on weight gain, but different effects on risk factors for cardiovascular disease or diabetes. This suggests that these diseases are not simply the effect of how much weight a person gains or how many excess calories are consumed, but that the source of those calories is important as well.
Surely, the future promises more compelling research from the Havel group that will continue to address the effects of high-fructose diets on human health. Their current clinical study is testing the hypothesis that high-fructose diets contribute to cardiovascular disease and diabetes by examining the effects of fructose in the form it is usually consumed – high fructose corn syrup (HFCS) and sucrose. Early results were published and showed that risk factors for cardiovascular disease were increased in men and women who drank fructose or HFCS beverages, and not in those who drank glucose beverages.