Friday, February 8, 2013

Science Friday: Hunger Games

The link between exercise and weight loss is controversial. Lots of studies and personal testimonies (yours truly included) indicate that exercise leads to weight loss, but there are other studies that swear it doesn't. (Some studies just throw everything you thought you knew on its head.) I've talked before about the idea of "fit and fat;" it seems that even people who may not experience weight loss still experience many healthy benefits of exercise. But what's the deal with the lack of weight loss? Of course exercise burns calories; one worry is that people over estimate what they burned and overeat, consuming more than they lost. Also, exercise makes us hungry. (See my current amount of food consumption, which has sky rocketed with marathon mileage.) 

An interesting recent study investigated the link between hormones released before and after an exercise intervention and the amount of food eaten. The hormones studied (CCK, obestatin, GIP, and leptin) help to regulate your eating; CCK and leptin tell you to stop eating, GIP tells you to eat more. (Obestatin's role remains controversial.) Obesity may disrupt these hormones' ability to accurately regulate food intake. For example, some obese people have high levels of leptin but seem resistant to its effects. (Leptin should decrease eating.) Some evidence suggests that exercise can help get these hormones back in check, which should help prevent overeating and improve weight loss.

In this study, both male and female overweight participants underwent a 12 week exercise program. Before and after the program, hormones levels were measured after fasting overnight and then directly after breakfast. Other days, participants were given a milkshake and then treated to a pasta lunch. (Milkshakes and pasta? Count me in!) The amount of pasta eaten and the amount of food consumed for the next 24 hours was assessed. The trick was the milkshake: one day it was filled with malodextrin, a mostly flavorless substance that more than doubled the calories. Another day malodextrin wasn't added. The researchers asked how the milkshake affected the rest of the day's eating. Will the participants' bodies sense the extra calories and eat less after the megacalorie milkshake? Will an exercise program help?

First of all, the participants lost weight in the exercise program. That's an important note because it's more evidence that exercise can lead to weight loss, even though the participants were told to maintain their normal diet throughout. After the exercise intervention, leptin levels were decreased during fasting and remained decreased after a meal. This may seem counterintuitive, but leptin decreasing with exercise may help the body become more responsive to its effects. (Remember, obese people may have too much leptin and be insensitive.) The milkshake and pasta lunch combination didn't change after the exercise program. Both before and after the exercise program, the participants ate less pasta after the megacalorie milkshake than after a normal milkshake. However, their food intake over the course of the day changed: after the exercise intervention, the mega milkshake led to less food eaten the rest of the day. (Prior to the intervention, the mega milkshake actually led to more food being eaten the rest of the day, see graph below.) This indicates that the exercise program helped to regulate the body's ability to count calories. It's not that exercise makes you eat less (the days with a normal milkshake had increased food intake compared to days before the exercising, compare hatched bars below), but that your body is better able to judge how much you're eating and when you've had all you need.

 Before the exercise program ("All-Baseline," at left) the participants ate more after the mega milkshake (HEP, solid bars) than after the normal milkshake (LEP, hatched bars). After the exercise program ("All-End", at right), they ate less all day following the mega shake.
It's important to note this study looked at participants who were overweight to start with. Previous studies on this subject tested normal weight people, which isn't as relevant to the population of dieters these studies aim to help. The good news for people trying to lose weight with exercise is that exercise may get your body to help out, by teaching it to better regulate eating.

Lastly, the study is an important reminder to know what you're eating. Restaurants (and researchers!) can trick you into consuming way more calories than you may be aware of. This study suggests your body might be able to compensate to some degree, but do yourself and your body a favor and eat fresh and homemade whenever possible!

Dream big, 

1 comment :

  1. Runner Teal...

    Interesting how complex this diet/exercise science is! No wonder so many investigators disagree with each other -- both scientists and non-.

    It seems something like the psychology of human learning...the variables of individuals make it very tricky to make grand general statements.

    Thank you for the lesson!