For people with obesity, bariatric surgery is by far the most effective way to lose weight and keep it off, albeit also the most invasive.

While diets have been shown scientifically to not work long-term for the vast majority of people, making the stomach much smaller via surgical means is incredibly effective – one study found that, on average, those that undergo the procedure lost over a quarter of their body weight.

Furthermore, the surgery doesn't just cause weight loss – it seems to also significantly lower the risk of some cancers, according to recent data.

One study using data from the SPLENDID (Surgical Procedures and Long-term Effectiveness in Neoplastic Disease Incidence and Death) matched cohort found that the weight loss surgery was associated with a 32 percent relative risk reduction in obesity related cancers and a 48 percent lower relative risk of cancer related death.

Another study, presented at the American Society for Metabolic and Bariatric Surgery (ASMBS) Annual Meeting found something similar, highlighting that the incidence of breast, gynecologic, kidney, brain, lung and thyroid cancer were all lowered for those who had the surgery.The researchers note that participants with obesity who hadn't had the surgery were 3.5 times more likely to die from cancer.

While these numbers sound really impressive, it's important to look at the absolute risk of these cancers as well.

In the study on the SPLENDID data, the team looked at 30,318 patients – 5,053 who had bariatric surgery and 25,265 matched controls who did not.

After 10 years, 2.9 percent of the bariatric surgery group and 4.9 percent of the control group had gotten a cancer related to obesity. This makes the absolute risk increase of these cancers 2 percent. The death by these cancers was even lower – an absolute risk difference of 0.6 percent.

The second study involved a smaller group, but it looked into specific cancer types; it involved 1,620 patients who either had a gastric bypass or a sleeve gastrectomy, and 2,156 controls. They found that the 10-year incidence of cancer for the weight loss surgery group was 5.2 percent, compared to 12.2 percent in the controls.

The survival rate was 92.9 percent for the weight loss group, and 78.9 percent for the controls. Although most cancer types showed a small increase, gynecologic cancers were at 0.4 percent in the surgery group and 2.6 percent in the control group. Thyroid cancer also showed a large difference, 0.10 vs 0.70 percent in surgery vs control.

"According to the American Cancer Society, obesity is second only to tobacco as a preventable cause of cancer in the United States," said the SPLENDID study's senior author, Cleveland Clinic cardiovascular doctor Steven Nissen.

"This study provides the best possible evidence on the value of intentional weight loss to reduce cancer risk and mortality."

Cancer occurs when cells undergo several genetic mutations, including some that specifically turn off repair or quality control checks that would usually go as far as killing faulty, mutated cells to prevent further problems. Without control mechanisms, damaged cells grow uncontrollably and lead to tumors, including malignant ones like cancer.

The mutations that cause this can occur from a number of sources. Some mutations have been passed down from your parents (think the BRCA1 gene), some are just unlucky random mutations, while others can be traced to UV exposure or chemicals in cigarettes.

Unfortunately, the mechanisms of why obesity increases the risk of cancer are not yet ironed out. However, scientists think that high levels of fat cells can cause low-level inflammation in the body, which over time causes DNA mutations, and therefore gradually increases the risk of cancer.

It's important to emphasize that despite this increased risk, bariatric surgery isn't for everyone. People can have both positive and negative responses to the surgery, side effects can be intense, and significantly it doesn't change any psychological problems a person might have with food – it just limits the amount you can eat.

However, these results indicate that weight loss surgery can genuinely affect more than just a person's body mass, and gives those who want to lose weight an additional health benefit to look forward to.

The SPLENDID study was published in JAMA; the second study has been presented at the ASMBS Annual Meeting ahead of peer review.