Researchers compared three ways to lose weight: eating very little food, eating very little plus stomach surgery (sleeve), or eating very little plus a different stomach surgery (bypass). They measured special hormones that control hunger and how the body uses energy. They found that bypass surgery changed these hormones differently than the other methods. These hormone changes might help explain why some people keep weight off better after surgery. The study included 44 people with severe obesity who were followed for 10 weeks.

The Quick Take

  • What they studied: Whether different weight loss methods (strict dieting alone, dieting plus sleeve gastrectomy surgery, or dieting plus bypass surgery) change the levels of two hormones that affect hunger and metabolism
  • Who participated: 44 adults with severe obesity (15 in diet-only group, 15 in sleeve surgery group, 14 in bypass surgery group) who were followed for 10 weeks
  • Key finding: Bypass surgery created bigger changes in hunger hormones compared to the other methods. Specifically, bypass surgery lowered one hunger hormone (GIP) and raised another (glucagon) more than sleeve surgery or diet alone did
  • What it means for you: These hormone changes from bypass surgery may help explain why some people maintain weight loss better after this type of surgery, but more research is needed to confirm this works long-term in larger groups of people

The Research Details

This was a controlled study where researchers assigned people to three different weight loss approaches and measured what happened over 10 weeks. All groups followed a very strict, low-calorie diet for 10 weeks. Two groups also had stomach surgery during this time—one group had their stomach made smaller (sleeve gastrectomy), and another group had their stomach rerouted (gastric bypass). The researchers measured hormone levels before and after the 10 weeks, both when people were fasting and while they were eating a meal. They also measured how well insulin was working and how many calories people burned at rest.

Understanding how different weight loss methods change hormones is important because these hormones control hunger, fullness, and how fast your body burns calories. If we know which methods create the best hormone changes, doctors might be able to predict who will successfully keep weight off long-term. This helps explain the ‘why’ behind weight loss success, not just whether weight comes off.

This study was a controlled trial, which is a strong research design. However, the sample size was relatively small (44 people total), which means results might not apply to everyone. The study only lasted 10 weeks, so we don’t know if these hormone changes continue long-term. The study was registered on ClinicalTrials.gov, which suggests it followed proper research standards. The journal Obesity is a respected publication in this field.

What the Results Show

The bypass surgery group showed the biggest changes in hunger hormones. Specifically, their GIP hormone (which normally increases when you eat and helps control blood sugar) dropped significantly after eating, while their glucagon hormone (which normally rises when you haven’t eaten) increased more than in the other groups. The sleeve surgery group and diet-only group had much smaller changes in these hormones. These differences were measured both when people were fasting and during the first hour after eating a meal. The hormone changes appeared to be directly related to the type of weight loss method used, with bypass surgery creating the most dramatic shifts.

When glucagon increased, people’s bodies burned more calories at rest (higher resting energy expenditure). However, in the bypass surgery group, higher glucagon was also linked to worse insulin sensitivity, meaning their bodies had a harder time controlling blood sugar. When fasting GIP increased, it was associated with higher insulin resistance (measured by HOMA-IR), suggesting this hormone change might not be beneficial. These secondary findings suggest the hormone changes have complex effects on metabolism—some helpful and some potentially unhelpful.

Previous research suggested that bypass surgery works partly through hormone changes, and this study provides more specific evidence about which hormones change and how much. The finding that bypass surgery lowers GIP aligns with some earlier studies, though the clinical significance of this change is still being debated. The study adds to growing evidence that different weight loss methods create different hormonal environments in the body, which may explain why some people maintain weight loss better with surgery than with diet alone.

The study only included 44 people, which is a small group, so results might not apply to everyone. The study only lasted 10 weeks, so we don’t know if these hormone changes continue for months or years. The study didn’t follow people long-term to see who actually kept the weight off. All participants were on a very strict diet, so results might be different for people following less restrictive diets. The study didn’t include a control group that didn’t lose weight, so we can’t compare these changes to what happens without weight loss.

The Bottom Line

If you have severe obesity and are considering weight loss options, discuss these findings with your doctor. Bypass surgery appears to create stronger hormone changes that might support long-term weight maintenance, but this doesn’t mean it’s right for everyone. Diet alone can work for weight loss, but the hormone changes are less dramatic. The evidence is moderate—these findings are promising but need confirmation in larger, longer studies before making major decisions. Anyone considering surgery should weigh benefits against surgical risks with their healthcare team.

People with severe obesity considering weight loss surgery should know about this research. People who have already had bypass surgery may find it interesting to understand how their surgery affects their body’s hormones. Healthcare providers treating obesity should consider these hormone changes when counseling patients. People trying to lose weight through diet alone should know that their hormone changes may be different from those who have surgery. People with diabetes or insulin resistance should discuss these findings with their doctor, as the hormone changes affect blood sugar control.

The hormone changes happened within 10 weeks in this study. However, it’s unclear how long these changes last or when they reach their maximum effect. Weight loss maintenance typically becomes a challenge after 6-12 months, so the real test of whether these hormone changes help would be seen over a year or more. Don’t expect immediate results—hormonal changes take time to influence weight maintenance.

Want to Apply This Research?

  • Track hunger levels (1-10 scale) before and after meals daily, and note energy levels throughout the day. Compare patterns week-to-week to see if hunger hormones are stabilizing. This provides indirect measurement of hormone changes without blood tests.
  • Use the app to log meal timing and fullness after eating. Set reminders to eat at consistent times, as this helps regulate hunger hormones. Track which foods keep you feeling full longest—this helps identify which foods work best with your body’s hormone patterns.
  • Create a weekly summary comparing hunger patterns, energy levels, and fullness duration. Over months, look for trends in whether you’re feeling less hungry or more energetic. If using the app after weight loss surgery, track how your hunger changes over the first 3-6 months as hormones adjust.

This research describes what happened in a 10-week study with 44 people and should not be used to make personal medical decisions. Weight loss surgery carries real risks and should only be considered under medical supervision. The hormone changes described here are interesting but don’t yet prove that they guarantee long-term weight loss success. If you’re considering weight loss surgery or have questions about your hormones and metabolism, talk to your doctor or an obesity specialist. This information is educational and not a substitute for professional medical advice.