Definition:
The term "high fructose corn syrup" is not a good descriptor of its
composition, but the term was mandated to distinguish the newly
developed fructose-containing corn syrup from traditional all-glucose
corn syrups. Factors that may account for the different effects of
fructose alone or a mixture of fructose and glucose could be its
gastrointestinal effects and absorption characteristics. (5)
The Problem
Today we have too much access to energy that we don't need. This leads to the problem of obesity when people consume too many calories, pointing towards higher risks of cardiovascular diseases.
The Possible Culprit
High Fructose Corn Syrup (HFCS) contributes to this problem because when consumed, it does not stimulate the pancreas to produce insulin. In animal models, it even induces insulin resistance, leading to diabetes (1).
"the long-term consumption of diets high in fat and fructose is likely
to lead to increased energy intake, weight gain, and obesity" (1).
Additionally, when ingested by itself, fructose is poorly absorbed
from the gastrointestinal tract, and it is almost entirely cleared by
the liver since it's absorbed through a different system than glucose.
This puts more work on the liver. (2)
Furthermore, HFCS
"did not suppress circulating ghrelin, a major appetite-stimulating
hormone" - (2)
So even though you're taking in a massive amount of energy and overworking your liver, you don't feel full, which causes you to keep eating and drinking.
For runners and other endurance athletes, this is ideal. They can store up massive amounts of energy without the need to stuff themselves until their stomachs are nearly exploding. For people who burn a massive amount of energy on a regular basis, HFCS comes as a good source for replenishing and preparing that energy for usage, but for everyone else who doesn't burn high levels of calories, this indicates a high influx of potential energy without anywhere to go.
The Counter
This short-term study notes,
"There were no differences in energy or macronutrient intake on day 2.
The only appetite variable that differed between sweeteners was desire
to eat" (4)
Another study notes that most of the testing has been done on rats, whereas in humans,
for people with insulin resistance, diets
with 50 grams or more per day (high consumption) may result in
elevated triglycerides, but there is no effect with normal levels of
fructose consumption. (5)
The Caution
Fructose is poorly absorbed from the digestive tract when it is
consumed alone, but absorption improves when fructose is consumed in
combination with glucose and amino acids. In addition, the principal
sweetener in soft drinks in the US, HFCS, is not pure fructose but a
mixture of fructose (55%) and glucose (45%). HFCS is predominately
present as HFCS-55 (55% fructose, 41% glucose, and 4% glucose
polymers) or HFCS-42 (42% fructose, 53% glucose and 5% glucose
polymers) (5).
Studies on humans hasn't been substantial enough to develop hard evidence. In fact, one of the studies indicates that fructose increased appetites while another claimed it inhibited appetite!
If there's one thing that seems highly possible right now, it's that high fructose corn syrup gives energy, whether you need it or not. Since most people don't, that contributes to possible obesity (3).
Sources:
(1) Fructose, weight gain, and the insulin resistance syndrome
(2) How bad is Fructose?
(3) A sweet problem: Princeton researchers find that high-fructose corn syrup prompts considerably more weight gain
(4) Effects of high-fructose corn syrup and sucrose consumption on circulating glucose, insulin, leptin, and ghrelin and on appetite in normal-weight women.
(5) Health implications of fructose consumption: A review of recent data
