Highlight HEALTH

A recent study evaluating the effects of fast-food-based overeating on liver enzymes and liver triglyceride content has been making the news this week. However, most media sources have been incorrectly interpreting the results. The Swedish study, published in the British Medical Association journal Gut, suggests that eating too much fast food can cause liver damage [1].

The goal of the study was to examine the potential link between changes in serum alanine aminotransferase (gene symbol ALT) to the amount of fatty infiltration in the liver of healthy non-obese subjects. ALT is an enzyme that, when present at high levels in the blood, is a diagnostic indicator of nonalcoholic fatty liver disease [2]. A high concentration of ALT in the blood is also a marker of risk for type 2 diabetes [3].

The Swedish investigation assessed the effects of four weeks of fast-food-based hyper-alimentation (meaning overeating) on the levels of serum ALT in 18 young, lean individuals (12 men, 6 women). The participants increased their caloric intake by eating two fast-food-based meals a day while minimizing their physical activity. Over the course of the study, seventeen of the 18 participants increased their body weight by 5 –15%. At the end of four weeks, 13 of the 18 subjects had developed pathological serum ALT concentrations (meaning ALT levels observed in diseased liver). Surprisingly, pathological levels of ALT were observed in most patients as early as one week after the study began, and were more than four times normal on average by the end of the study. Only two of the 18 individuals developed liver steatosis or fatty liver, a benign, non-progressive condition, whereby fat accumulates in liver cells.

The authors of the study conclude that chronically or intermittently elevated ALT can be caused by food alone. Lead researcher Fredrik Nystrom, M.D., Ph.D., at the University Hospital of Linkoping, said a key finding of the study was that signs of liver damage were linked to carbohydrates [4]:

It was not the fat in the hamburgers, it was rather the sugar in the coke.

Indeed, the authors specifically indicate in the study’s discussion section that [1]:

… when examining the relationship of the increase in ALT to intake of different nutrients, fat intake was unrelated increase in ALT while sugar and carbohydrate intake at week 3 clearly related to the ALT increase. This is in accordance with earlier findings by Solga et al who demonstrated that higher carbohydrate intake was significantly associated with an increased risk of biopsy-proven hepatic inflammation in morbidly obese patients undergoing bariatric surgery.

Most media sources, however, are focusing on the fat in fast food, not the carbohydrates. This is in sharp contrast to the study results, which paradoxically found a health benefit, apparently from fat. HDL cholesterol levels (the good cholesterol) increased over the four-week period, correlating with the increase in saturated fat [4]. Although the cholesterol findings have yet to be published, Dr. Nystrom indicated they were consistent with the French Paradox, the observation that the French, despite intake of a high-fat diet, suffer low incidence of coronary heart disease [4].

The data from this study indicates that, although the liver can regenerate itself, a continuous long-term fast food diet may cause irreversible damage. We’ve talked previously about the effects of healthy fast food on endothelial function. This latest study demonstrates yet another negative consequence of fast food on our health.

References

1. Kechagias et al. Fast food based hyper-alimentation can induce rapid and profound elevation of serum alanine aminotransferase in healthy subjects. Gut. 2008 Feb 14 [Epub ahead of print]. DOI: 10.1136/gut.2007.131797
   View abstract
2. Clark et al. The prevalence and etiology of elevated aminotransferase levels in the United States. Am J Gastroenterol. 2003 May;98(5):960-7.
   View abstract
3. Vozarova et al. High alanine aminotransferase is associated with decreased hepatic insulin sensitivity and predicts the development of type 2 diabetes. Diabetes. 2002 Jun;51(6):1889-95.
   View abstract
4. Fast-food binge harms liver, but boosts good cholesterol: study. Yahoo News. 2008 Feb 13.

The Glycemic Index (GI) is a system for rating carbohydrates, or saccharides, based on their immediate effect on the blood glucose level. An organic compound, glucose is a simple sugar or monosaccharide. Glucose is the principle sugar circulating in the blood; all cells use glucose as a source of energy and as a metabolic intermediate. Found in plants, glucose is the product of photosynthesis (the process of converting light energy to chemical energy and storing it in the bonds of a glucose molecule). Biologically active glucose is often referred to as dextrose. Glucose is often incorrectly referred to as table sugar - table sugar is a disaccharide composed of glucose and fructose.

When food containing carbohydrate is consumed, glucose from the food is digested and transported throughout the body in the blood. The speed with which the food is able to increase the blood glucose level is referred to as the glycemic response. The glycemic index of a food is defined as the blood glucose level measured over a 2 hour period after ingestion of a fixed portion of carbohydrate and expressed as a percentage of the area under the glucose response curve when the same amount of carbohydrate is consumed as glucose [(1].

Thus, carbohydrates that break down rapidly during digestion raise the blood glucose level quickly have a higher GI rating than foods that break down more slowly and raise the blood glucose level less. The glycemic index is influenced by many factors, including how much food is consumed, how the food is prepared and what other types of food were consumed along with the carbohydrate. For example, the ingestion of fiber with carbohydrate tends to flatten the glycemic response, likely due to the fiber reducing the absorption rate of the carbohydrate [(2]. The glycemic load is another rating system that takes into account both the glycemic index and the amount of carbohydrate consumed.

Why all the fuss? Low GI foods are usually lower in calories and fat, and high in fiber and nutrients. Low GI diets have been associated with increased HDL cholesterol levels (the good cholesterol) and a decreased risk of developing diabetes and cardiovascular disease [(3]. The blood glucose level can also affect how hungry and energetic we feel and whether our bodies burn fat or store it. The pancreas synthesizes a hormone called insulin that is responsible for transporting blood glucose into cells. When insulin is synthesized, our bodies stop burning fat and start storing it. Consumption of high GI foods cause the pancreas to produce a large amount of insulin - this insulin spike results in the transport of too much blood glucose out of the blood. As a consequence, the blood glucose level drops below normal, causing us to feel tired and hungry for something with a high sugar content, starting the cycle all over again. In contrast, consumption of low GI foods results in lower but more sustained increases in blood glucose and reduced insulin release.

More information can be found at the official website of the Glycemic Index and GI database.

References

1. Jenkins et al. Glycemic index of foods: a physiological basis for carbohydrate exchange. Am J Clin Nutr. 1981 Mar;34(3):362-6.
   View abstract
2. Jenkins and Jenkins. Dietary fiber and the glycemic response. Proc Soc Exp Biol Med. 1985 Dec;180(3):422-31.
   View abstract
3. Jenkins et al. Glycemic index: overview of implications in health and disease. Am J Clin Nutr. 2002 Jul;76(1):266S-73S.
   View abstract