A low-calorie diet is not a foreign idea to any American. Many people restrict their diets in order to trim their waistlines or maintain a particular physique. And despite being the subject of much controversy, low-calorie diets are now linked to longer lifespan. How exactly this happens, however, is attributed to multiple things.

A new study indicates that life-long calorie restriction promotes the growth of 'good' bacteria in the digestive system that increase life span. Previous studies have indicated that calorie restriction, in general, has the capacity to expand lifespan, but has not explained how it happens. This study compared the lifespans and changes to digestive bacteria colonies of mice on a low-calorie diet their entire lives, instead of for a short span of time (as other studies have).

Researchers chose to test the effect of a life-long calorie restricted diet because, in a previous study by the same group, mice were put on either high- or low-fat diets. The study was carried out for four years and the researchers found that the mice on low-fat diets lived 25 percent longer than the mice on fatty diets. However, the researchers were unable to identify the mechanism through which their life spans were improved.

In this study, the researchers were able to identify the changes to digestive bacteria that occur during a low-calorie diet that, in turn, extend life. Measurements of digestive bacteria were taken in these mice on high- and low-calorie diets at the ages of 62, 81, and 144 weeks. While few differences in digestive bacteria existed for the earlier time points, many differences existed for later time points.

They found that at 144 weeks, many mice had elevated levels of the bacteria, Lactobacillus. But, all of the mice who were on high-calorie diets had died by the 144-week mark and showed significantly lower levels of Lactobacillus bacteria. The mice who who lived to, and past, 144 weeks had an average of 12 percent more of these bacteria populating their digestive systems than the mice who had died before the 144-week mark.

Lactobacillus bacteria are benign bacteria that are a major part of healthy digestion. The bacteria are associated with weight management and loss, as found in a 2008 study, and are often taken as a supplement. In a previous study, Lactobacillus was responsible for a 24-percent decrease in body weight within 48 hours in mice. As a result, the bacteria is revered for its weight loss possibilities and even hunger reduction.

Changes to other bacterial populations appeared to become characteristic of the mice living longer and were attributed to the changes to their diet. Firmicutes, a bacteria associated with higher calorie intake and obesity, decreased from 28 percent at the 62-week mark to less than one percent at the 144-week mark. Conversely, the mice with higher calorie intake had elevated levels of Firmicutes until death. It is important to note that Firmicutes is negatively correlated with lifespan. In a 2012 study, the bacteria was found to increase fat intake in zebrafish. As a result, a low-calorie diet reduces Firmicutes's abundance in the digestive system, further improving lifespan.

Since the mice that lived the longest were on a low-calorie diet and had elevated Lactobacillus levels, the researchers looked at whether the changes to bacteria altered immunity. Bacteria exist in the digestive system, partly, to aid the body's immunity. And, the researchers found that Lactobacillus did exactly that. Lactobacillus was found to stop disease-causing agents from binding to the walls of the digestive tract, hindering them from causing any sort of illness, as they could not affect the body when Lactobacillus was around. Similarly, without any infections, thanks to Lactobacillus, the bodily inflammation levels decreased. This is important because inflammation, while a part of the immune response that allows infections to be fought, can make the body vulnerable to other infections. When the digestive system has fewer stressors like infection and inflammation, it can work optimally, and therefore, promote longer life.

So as calorie restriction has proven to elongate life in mice, its implications for humans and their digestive systems could be just around the corner. Dietary changes have long been a part of fitness, but they may soon become a major part of anti-aging regimens in a world where we're terrified of looking older and dying young.

Source: Zhang C, Li S, Yang L, et al. Structural modulation of guy microbiota in life-long calorie-restricted mice. Nature Communications. 2013.