With sufficient sample size for the specific strain of mice at a fixed age, one can generate a normal distribution of weights and then estimate obesity with the distribution. One might also use some sort of body condition score, such as that used in dairy cattle to estimate obesity in the strain or population. You would need some "standards" by which to assess the condition of animals. Obese is really a relative term and there is not any standard guide for what is "obese". For example, seals and polar bears may be obese during some parts of the year and lean during other parts of the year as their seasonal diet and reproductive status dictates.

With sufficient sample size for the specific strain of mice at a fixed age, one can generate a normal distribution of weights and then estimate obesity with the distribution. One might also use some sort of body condition score, such as that used in dairy cattle to estimate obesity in the strain or population. You would need some "standards" by which to assess the condition of animals. Obese is really a relative term and there is not any standard guide for what is "obese". For example, seals and polar bears may be obese during some parts of the year and lean during other parts of the year as their seasonal diet and reproductive status dictates.

There is large variation in susceptibility to obesity in mouse strains (see http://www.ncbi.nlm.nih.gov/pubmed/23423668). For example, BALB are almost completely resistant to obesity, whereas C57BL6 become fat; hence, the latter are generally used in food intake/diabetes studies. Females are generally more prone, due to anabolic effects of testosterone in males. Nonetheless, to indirectly measure obesity in mice, we would generally follow weight gain versus standard chow-fed animals, before determining whether glucose intolerance or defective insulin signaling is present (both indicative of insulin resistance) +/- inguinal fat pad mass. Or conversely, you could compare growth curves of GM (e.g. Ob/Ob mutants) and WT littermates. As Jack rightly mentions, obesity is really a human-centric term which has connotations with disease. However, Siberian Hamsters become 'physiologically obese' during long days- these rodents deposit huge amounts of fat, yet are apparently normal from a biochemical standpoint. The same is true for Grizzly Bears, which are obese prior to hibernation, but display normal insulin sensitivity due to altered PTEN-signalling (http://www.cell.com/cell-metabolism/abstract/S1550-4131%2814%2900316-7). Likewise, some humans with high BMI (> 30) are not clinically obese e.g. those with high muscle mass, for example rugby players.

I agree with Jack and David. In our experience, working with C57BL/J (~ 600 mice); at 20 weeks, mice fed HFD had a wide distribution in body weight gain showing bimodal characteristics.

To consistently choose the “obese” mice we define a number (average ± 3 standard deviations of the control group).

The heavier group on the HFD was named ‘‘diet-induced obese’’ (DIO) mice. Others on HFD remained almost as lean as controls, those fed regular chow. We defined these as ‘‘diet-induced obese resistant’’ (DIO-R) mice, those with body weights ranging between the average ± 3 standard deviations of the control group. The small number of mice (5%) that overlapped in the bimodal distribution of body weight was excluded from experiments. Over a period of 20 weeks, the DIO-R group became only 6.7% heavier than the control group, while the DIO group became 32.9% heavier. You can get more information from my paper published in 2007 (Enriori PJ et al, Cell Metab, 5, 181–194).

Importantly, no all obese mice show alterations in glucose homeostasis

Hello Xiaoqiu,

If you want to determine, if your transgenic (or other wise treated) animal is fat compared to a wild type mouse, you might take several measurements: 1) body weight, 2) body size 3) body composition (lean versus fat mass).

If your mice weigh the same, they might not have the same body composition or size. Similarly, if they are heavier, they might not necessarily be obese, but just larger.

Cheers, Laura

yes, laura is correct. All my consideration are based in the same size of the mice, for example, the MC4RK) mice are not just heavier, also longer, so chek this parameter befor compare

I would caution using the previously referenced difference in mouse strains (see http://www.ncbi.nlm.nih.gov/pubmed/23423668) that publication only looks at male mice and in my experience BALB/c female mice are not resistant to increased weight gain on high-fat diet.