The major finding of the present investigation was the significantly higher dietary NEAP in the elderly than in the young (P
= 0.0112). This is possibly the result of age-associated dietary changes. A positive correlation between age and NEAP (P
= 0.0159; ) further supports this notion. These findings are in close agreement with the results of Berkemeyer,30
who reported that 24-hour urinary pH and net acid excretion (5.94, 60.03 mEq/d) in the elderly were significantly lower (P
< 0.05) and higher (P
< 0.05), respectively, compared to the younger age groups.
In the present study, a wide range of acid–base values was found using both the Remer2
methods, which provided NEAP and protein/potassium ratios, respectively. For the young and the elderly, ranges of NEAP were 12.1–67.8 mEq/d and 2.0–78.3 mEq/d, respectively. Regardless of the method used, the mean dietary acid–base balance (NEAP) was significantly higher for the elderly than the young; P
= 0.0035 for NEAP (elderly, 44.1 mEq/d versus young, 40.1 mEq/d) and P
= 0.0035 for the protein:potassium ratio (elderly, 1.4 mEq/d versus young, 1.1 mEq/d) ().
The values for NEAP (elderly, 44.1 mEq/d versus young 40.1 mEq/d) reported in this study are much lower than those reported by Prynne et al (67.8 mEq/d).31
However, since Prynne et al31
only reported acid loads for teenagers in their study, an age effect might be the reason for this difference. Another study on 400 elderly (aged 55–75 years) reported a mean NEAP value of 60 (SD 27.0) mEq/d,32
a value almost identical to that reported in the elderly by Remer and Manz;13
however, this value was larger than the one we found in the present study (elderly, 44.1 mEq/d versus young, 40.1 mEq/d). Given that these other studies were conducted in developed countries (England and the UK), where differences in dietary practices include more acid-forming diets (large amounts of meat, fish, and poultry), this might explain the higher dietary acid loads.33
Another plausible reason for the lower value of NEAP in our study may be that the basic mineral, calcium, has not been excluded from the NEAP equation in our study. Removing calcium from the formula would have resulted in relatively higher NEAP values (elderly, 52.0 mEq/d versus young, 47.4 mEq/d), as also argued by Gannon et al.27
Interestingly, however, Lemann34
estimated that the average diet consumed by healthy American adults generates approximately 50 mEq/d, a value very similar to ours. Similarly, our results are also in close agreement with the findings of Gannon et al,27
who reported a median value for an acid load of 47.9 mEq/d in an elderly population living in different geographical areas of the UK.
In our study, most of the results from the correlation analysis between NEAP and both the anthropometric characteristics and nutrients ( and ) are in close agreement with the findings of the above-mentioned study by Gannon et al,27
as well as with a number of other studies (reviewed in Berkemeyer35
). The relationships between NEAP and macro- and micronutrients were to some extent, but not entirely, in accordance with what we initially expected, demonstrating the importance of examining how these nutrients may have an impact on dietary acid load. Gannon et al27
reported a significantly positive association between acid load and energy, protein, and phosphorus intake. In the current study, we also noted essentially the same trends insofar as, with an increase in energy intake, there was also an increase in NEAP. Another factor is that the OA component of the equation, which is a major contributor to overall acidity, is predicted from body size, and the data from the current study vary positively in association with energy intake, albeit to a much lesser extent than protein or phosphorus.
In terms of dietary habits, what might explain the relatively higher acid loads noted in the elderly as compared to the young in the present study? There are a number of possibilities. As an example, our dietary survey showed that potatoes were largely consumed by the young, and in a variety of different ways (roasted, baked, boiled, and so on) (data not shown). Potatoes are rich in potassium content,27
which may help to reduce the dietary acid load, as possibly also seen in the present study (lower acid loads among the young). Similarly, young subjects in the present study seemed to consume larger quantities of fruits and vegetables, which the elderly did not (primarily due to chewing and dental problems), and hence this could be another reason for the higher dietary acid loads observed in the elderly as compared to the young. The intake of potassium, magnesium, fruit, and vegetables has been associated with a more alkaline environment in the human body.11
Almost all vegetables and fruits are rich sources of potassium, as previously reported by others.13
One epidemiological study of 384 healthy men and women aged 65 years and above found an association between a higher intake of foods rich in potassium (fruits and vegetables) and greater lean muscle mass.10
The authors of this study speculated that “this association is likely to result from the fact that the ingestion of potassium-rich alkaline foods such as fruit and vegetables relieves the mild metabolic acidosis that occurs with the ingestion of a typical American diet,” and suggested that it is plausible that age-related muscle mass decline and sarcopenia may be prevented by the appropriate intake of alkaline potassium salts.
It has been suggested and argued that food combinations are also very important for the acid–base balance.27
We noted in our study that the elderly had a tendency to eat vegetables (including potatoes, green leafy vegetables, pumpkin, etc), which are alkali-generating, only when cooked and served with meat, fish, and eggs, which may be one of the reasons for the increased dietary acidity. In contrast, the young ate vegetables alone, which were mostly uncooked, in the form of salad, for example. This unique dietary pattern in the elderly is of vital importance as far as their acid–base homeostasis is concerned. Animal protein, grain, and high amounts of milk increases the acidity of the body, whereas foods rich in minerals such as green vegetables and fruit increase the alkalinity.38
A typical Western diet (with high levels of animal protein and low amounts of vegetables) is likely to induce a chronic, low-grade metabolic acidosis.33
Age-associated decline in renal function implies that the capacity of the kidneys to excrete acid is decreased,8
which leads to a lower blood pH in addition to a reduced plasma bicarbonate concentration.7
As the elderly are generally apt to consume lower amounts of fruits and vegetables,8
they constitute a risk group for acid–base disturbances and related conditions, and hence they are likely to experience an increased secretion of calcium and possibly magnesium.
Although we did not consider any health consequences in relation to higher or lower dietary acid loads in the present study, overwhelming evidence supports the negative effects of imbalances in acid–base homeostasis across a number of health disorders (for example, bone loss, skeletal muscle atrophy, and nephrolithiasis) in the aging population.1
Interestingly, Wachman and Bernstein hypothesized decades ago that consuming a typical Western diet (high in animal protein, low in vegetables, highly acidic) may result in the lifelong utilization of the buffer salts found in bone, leading to bone fragility and osteoporosis.19
Furthermore, Frassetto et al40
have recently demonstrated that plasma bicarbonate concentration and glomerular filtration rate associated with increasing age causes these parameters to fall, whereas blood hydrogen ion concentrations increase with age. This, in turn, leads to an increasingly worsening low grade metabolic acidosis and is reflective of increases in dietary acidity. If the acid–base/skeleton theory is to be believed,41
the findings of the present study have important implications with respect to advocating for dietary acid–base advice in the aging population, particularly in elderly women. This could include programs to increase fruit and vegetable consumption in those participants with the highest estimates of NEAP.
There are some methodological issues relevant to the present work that are worth discussing here. Acid–base effects of foods are determined by a number of methods. Measuring 24 hour urine pH is one way, but it is not practical for large populations. A model based on only five nutrients has been shown to predict renal acid load in children.14
Protein, phosphorus, potassium, magnesium, and calcium were used to calculate PRAL, and this correlated well with urine pH. Remer and Manz13
tested foods and their effect on urine pH, and listed foods according to their PRAL. Accordingly, fats and oils are neutral; fish, meats, eggs, grains, nuts, and dairy products have positive PRALs; and fruits, vegetables, and, to a lesser extent, beverages, have negative PRALs. Another, simpler method has been proposed to estimate dietary acid load by measuring the ratio of the protein:potassium content of food eaten. A study by Frassetto et al16
analyzed 20 diets with protein content ranging from 39 to 193 g per day, and potassium content from 40 to133 mEq/d. The ratio of protein:potassium content varied over a fivefold range, from 0.45 to 2.21, and these ratios correlated very well with the data presented by Remer and Manz.14
These data suggest that acid load can be predicted by simply determining the protein and potassium content of the diet.16
This trend towards close agreement between the two methods has previously been observed by other investigators as well.27
In the current study, we used both methods and found a strong correlation between them (r = 0.764; ), suggesting that these methods can be used alternatively.
Figure 4 Comparison of estimates of acid–base balance using NEAP13 and protein:potassium (K) ratio.16
A limitation of the present study is the lack of information on female participants, who are likely to be influenced differently by acid–base balances.27
Also, in our study we could not differentiate between the acidity of the same food types that were prepared differently. It would be interesting to look at the effects of both food preparation and wholegrain versus refined grains on NEAP. This is certainly an area deserving further research, as also suggested by other investigators.27
Body shape, overall size, and body composition are likely to change with age,23
and future work to confirm the predictive value for OA from surface area in different population groups is needed to assess changes in body shape, size, and composition. Nevertheless, we conclude that these analyses provide an insight into the acid generating potential of the diet of a relatively small sample of Pakistani elderly people, showing the diet to be rich in dietary acid precursors, which are higher in the elderly than in the young, and increasing with age. Such trends may also be found in other developing societies with similar diets and, in turn, may have important consequences for this vulnerable group. Although it has been suggested that merely reducing dietary acid load is not sufficient in preventing the age-related worsening of acidosis and the resulting muscle breakdown, the intake of lower acid-producing diets could help slow down this age-related acidogenesis.11