The principle findings of this study demonstrate that a 40% restriction on the amount of feed offered to the rats did not cause malnutrition in adult Wistar rats over a four-week period. In addition, the caloric difference between the two control diets used (Purina®: 3028.0 Kcal/kg and AIN-93 M: 3802.7 Kcal/kg) did not cause changes in the levels of muscle and liver glycogen, whereas the way in which the diets were administered resulted in increased levels of these substrates in the animals in the RAP and RAD groups. Additionally, the American Institute of Nutrition diet (AIN-93 M) that was administered ad libitum improved the aerobic and anaerobic capacity of the ALD group, probably due to the lower density of these animals in water.
Malnutrition in animals is often characterised by low serum albumin and total protein concentrations and high levels of liver lipids [18, 25]. In the present study, the animals that had restricted access to feed (RAP and RAD) did not show these characteristics, confirming previous research . In addition, studies have shown that dietary restriction (80 to 60% of ad libitum intake) decreases the risk of chronic degenerative diseases such as cancer, type-2 diabetes and kidney disease, prolonging the life span of laboratory rats and mice by up to 40% without causing malnutrition [5–7].
Comparing the effects of a standard diet (Purina®) to those of a freely administered high calorie diet, Chun, Lee, Kim, et al.  showed that animals on a high calorie diet have higher levels of body fat. These findings are consistent with the present study, where the ALD group, which was fed a higher caloric diet American Institute of Nutrition diet (AIN-93 M), showed more weight gain than the ALP group. According to Silva, Marcondes and Mello , animals that are subjected to high-fat diets tend to accumulate more fat than control animals.
The RAP and RAD groups showed higher glycogen values, primarily in the soleus muscle and liver, than those fed ad libitum. Corroborating these findings, Pedrosa, Tirapegui, Rogero, et al. , when comparing sedentary and trained animals, both with and without feed restriction (25 and 50% of ad libitum intake), observed higher muscle and liver glycogen values in the animals in the restricted groups. In addition, Wetter, Gazdag, Dean, et al.  observed similar or higher muscle glycogen values in rats subjected to feed restriction (60% of consumed by the ad libitum group), as has been demonstrated by other studies [30, 31]. Conversely, these authors found higher liver glycogen values in animals fed ad libitum, suggesting that the influence of dietary restriction on the content of this substrate is dependent on the tissue analysed. In this regard, further studies are needed to determine the changes caused by dietary restriction on the mechanisms of glycogen synthesis and utilisation in different tissues.
The differences in the levels of muscular glycogen could influence aerobic and anaerobic capacity in animals, as determined using the lactate minimum test. However, there were no significant differences in the anaerobic threshold values between the groups, demonstrating that the diets and their form of administration did not influence the aerobic capacity of the animals. In addition, the loads corresponding to the anaerobic threshold in relation to body weight (4.5%) are similar to those reported by previous studies that used eutrophic rats [18, 32, 33] ARAÚJO et al., 2007).
However, the animals in the ALD group showed lower lactate concentrations values. This finding, together with the lower quantities of glycogen in the ad libitum groups, is consistent with those reported by Voltarelli, Gobatto and Mello , who observed lower lactate concentrations values in glycogen-depleted animals when comparing anaerobic threshold determined using lactate minimum test in a group of fed animals and a group of animals subjected to glycogen depletion.
The animals in the ALD group showed the same characteristics observed in humans subjects during a lactate minimum test after glycogen depletion, i.e., the intensity corresponding to the minimum lactate concentration was not influenced by a reduction in glycogen stores; however, the lactate concentrations were significantly lower upon depletion . Further, the lactate concentrations and time to exhaustion values may have been influenced by the density of the animals in the ALD group, since these animals had an increase in body weight and body fat.
Araújo, Araújo, Dangelo, et al.  demonstrated that the anaerobic threshold in obese animals, as determined using maximal steady state lactate levels, may be higher than that in well-nourished animals, attributing these findings to the lower density of these animals in an aquatic environment. Thus, in our study, the intensity at the same workload may have been underestimated for animals that have higher levels of fat , resulting in the lower lactate concentrations values and higher time to exhaustion values seen in the ALD group. Therefore, more studies are needed to normalise the variables related to the increased loads used in lactateminimum test as a function of the body density of the animals.
The results of this study suggest that the caloric differences between the two diets did not noticeably influence the levels of muscle and liver glycogen, whereas these levels could be influenced by the form in which the diets were administered. However, the higher levels of glycogen seen in the RAP and RAD groups did not influence the aerobic and anaerobic capacity as determined using the lactate minimum test. In addition, the lower lactate concentrations and higher time to exhaustion values seen for the ALD group may be explained by the lower density of the animals in this group. Thus, one limitation of this study was the lack of quantification of the density of the animals and the use of loads that did not consider this variable in water.
In summary, feed restriction induced changes in energetic substrates, and ad libitum intake of a semi-purified American Institute of Nutrition diet (AIN-93 M) resulted in increased adipose tissue, which likely reduced the density of the animals in water and favoured their performance in the swimming exercise.